ovsdb-idl.c

/* Copyright (c) 2009, 2010, 2011, 2012, 2013, 2014, 2015 Nicira, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at:
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#include <config.h>

#include "ovsdb-idl.h"

#include <errno.h>
#include <inttypes.h>
#include <limits.h>
#include <stdlib.h>

#include "bitmap.h"
#include "coverage.h"
#include "dynamic-string.h"
#include "fatal-signal.h"
#include "json.h"
#include "jsonrpc.h"
#include "ovsdb/ovsdb.h"
#include "ovsdb/table.h"
#include "ovsdb-data.h"
#include "ovsdb-error.h"
#include "ovsdb-idl-provider.h"
#include "ovsdb-parser.h"
#include "poll-loop.h"
#include "shash.h"
#include "sset.h"
#include "util.h"
#include "openvswitch/vlog.h"

VLOG_DEFINE_THIS_MODULE(ovsdb_idl);

COVERAGE_DEFINE(txn_uncommitted);
COVERAGE_DEFINE(txn_unchanged);
COVERAGE_DEFINE(txn_incomplete);
COVERAGE_DEFINE(txn_aborted);
COVERAGE_DEFINE(txn_success);
COVERAGE_DEFINE(txn_try_again);
COVERAGE_DEFINE(txn_not_locked);
COVERAGE_DEFINE(txn_error);

/* An arc from one idl_row to another.  When row A contains a UUID that
 * references row B, this is represented by an arc from A (the source) to B
 * (the destination).
 *
 * Arcs from a row to itself are omitted, that is, src and dst are always
 * different.
 *
 * Arcs are never duplicated, that is, even if there are multiple references
 * from A to B, there is only a single arc from A to B.
 *
 * Arcs are directed: an arc from A to B is the converse of an an arc from B to
 * A.  Both an arc and its converse may both be present, if each row refers
 * to the other circularly.
 *
 * The source and destination row may be in the same table or in different
 * tables.
 */
struct ovsdb_idl_arc {
    struct ovs_list src_node;   /* In src->src_arcs list. */
    struct ovs_list dst_node;   /* In dst->dst_arcs list. */
    struct ovsdb_idl_row *src;  /* Source row. */
    struct ovsdb_idl_row *dst;  /* Destination row. */
};

enum ovsdb_idl_state {
    IDL_S_SCHEMA_REQUESTED,
    IDL_S_MONITOR_REQUESTED,
    IDL_S_MONITORING
};

struct ovsdb_idl {
    const struct ovsdb_idl_class *class;
    struct jsonrpc_session *session;
    struct shash table_by_name;
    struct ovsdb_idl_table *tables; /* Contains "struct ovsdb_idl_table *"s.*/
    unsigned int change_seqno;
    bool verify_write_only;

    /* Session state. */
    unsigned int state_seqno;
    enum ovsdb_idl_state state;
    struct json *request_id;

    /* Database locking. */
    char *lock_name;            /* Name of lock we need, NULL if none. */
    bool has_lock;              /* Has db server told us we have the lock? */
    bool is_lock_contended;     /* Has db server told us we can't get lock? */
    struct json *lock_request_id; /* JSON-RPC ID of in-flight lock request. */

    /* Transaction support. */
    struct ovsdb_idl_txn *txn;
    struct hmap outstanding_txns;
};

struct ovsdb_idl_txn {
    struct hmap_node hmap_node;
    struct json *request_id;
    struct ovsdb_idl *idl;
    struct hmap txn_rows;
    enum ovsdb_idl_txn_status status;
    char *error;
    bool dry_run;
    struct ds comment;

    /* Increments. */
    const char *inc_table;
    const char *inc_column;
    struct uuid inc_row;
    unsigned int inc_index;
    int64_t inc_new_value;

    /* Inserted rows. */
    struct hmap inserted_rows;  /* Contains "struct ovsdb_idl_txn_insert"s. */
};

struct ovsdb_idl_txn_insert {
    struct hmap_node hmap_node; /* In struct ovsdb_idl_txn's inserted_rows. */
    struct uuid dummy;          /* Dummy UUID used locally. */
    int op_index;               /* Index into transaction's operation array. */
    struct uuid real;           /* Real UUID used by database server. */
};

static struct vlog_rate_limit syntax_rl = VLOG_RATE_LIMIT_INIT(1, 5);
static struct vlog_rate_limit semantic_rl = VLOG_RATE_LIMIT_INIT(1, 5);

static void ovsdb_idl_clear(struct ovsdb_idl *);
static void ovsdb_idl_send_schema_request(struct ovsdb_idl *);
static void ovsdb_idl_send_monitor_request(struct ovsdb_idl *,
                                           const struct json *schema_json);
static void ovsdb_idl_parse_update(struct ovsdb_idl *, const struct json *);
static struct ovsdb_error *ovsdb_idl_parse_update__(struct ovsdb_idl *,
                                                    const struct json *);
static bool ovsdb_idl_process_update(struct ovsdb_idl_table *,
                                     const struct uuid *,
                                     const struct json *old,
                                     const struct json *new);
static void ovsdb_idl_insert_row(struct ovsdb_idl_row *, const struct json *);
static void ovsdb_idl_delete_row(struct ovsdb_idl_row *);
static bool ovsdb_idl_modify_row(struct ovsdb_idl_row *, const struct json *);

static bool ovsdb_idl_row_is_orphan(const struct ovsdb_idl_row *);
static struct ovsdb_idl_row *ovsdb_idl_row_create__(
    const struct ovsdb_idl_table_class *);
static struct ovsdb_idl_row *ovsdb_idl_row_create(struct ovsdb_idl_table *,
                                                  const struct uuid *);
static void ovsdb_idl_row_destroy(struct ovsdb_idl_row *);

static void ovsdb_idl_row_parse(struct ovsdb_idl_row *);
static void ovsdb_idl_row_unparse(struct ovsdb_idl_row *);
static void ovsdb_idl_row_clear_old(struct ovsdb_idl_row *);
static void ovsdb_idl_row_clear_new(struct ovsdb_idl_row *);

static void ovsdb_idl_txn_abort_all(struct ovsdb_idl *);
static bool ovsdb_idl_txn_process_reply(struct ovsdb_idl *,
                                        const struct jsonrpc_msg *msg);

static void ovsdb_idl_send_lock_request(struct ovsdb_idl *);
static void ovsdb_idl_send_unlock_request(struct ovsdb_idl *);
static void ovsdb_idl_parse_lock_reply(struct ovsdb_idl *,
                                       const struct json *);
static void ovsdb_idl_parse_lock_notify(struct ovsdb_idl *,
                                        const struct json *params,
                                        bool new_has_lock);

/* Creates and returns a connection to database 'remote', which should be in a
 * form acceptable to jsonrpc_session_open().  The connection will maintain an
 * in-memory replica of the remote database whose schema is described by
 * 'class'.  (Ordinarily 'class' is compiled from an OVSDB schema automatically
 * by ovsdb-idlc.)
 *
 * Passes 'retry' to jsonrpc_session_open().  See that function for
 * documentation.
 *
 * If 'monitor_everything_by_default' is true, then everything in the remote
 * database will be replicated by default.  ovsdb_idl_omit() and
 * ovsdb_idl_omit_alert() may be used to selectively drop some columns from
 * monitoring.
 *
 * If 'monitor_everything_by_default' is false, then no columns or tables will
 * be replicated by default.  ovsdb_idl_add_column() and ovsdb_idl_add_table()
 * must be used to choose some columns or tables to replicate.
 */
struct ovsdb_idl *
ovsdb_idl_create(const char *remote, const struct ovsdb_idl_class *class,
                 bool monitor_everything_by_default, bool retry)
{
    struct ovsdb_idl *idl;
    uint8_t default_mode;
    size_t i;

    default_mode = (monitor_everything_by_default
                    ? OVSDB_IDL_MONITOR | OVSDB_IDL_ALERT
                    : 0);

    idl = xzalloc(sizeof *idl);
    idl->class = class;
    idl->session = jsonrpc_session_open(remote, retry);
    shash_init(&idl->table_by_name);
    idl->tables = xmalloc(class->n_tables * sizeof *idl->tables);
    for (i = 0; i < class->n_tables; i++) {
        const struct ovsdb_idl_table_class *tc = &class->tables[i];
        struct ovsdb_idl_table *table = &idl->tables[i];
        size_t j;

        shash_add_assert(&idl->table_by_name, tc->name, table);
        table->class = tc;
        table->modes = xmalloc(tc->n_columns);
        memset(table->modes, default_mode, tc->n_columns);
        table->need_table = false;
        shash_init(&table->columns);
        for (j = 0; j < tc->n_columns; j++) {
            const struct ovsdb_idl_column *column = &tc->columns[j];

            shash_add_assert(&table->columns, column->name, column);
        }
        hmap_init(&table->rows);
        table->idl = idl;
    }

    idl->state_seqno = UINT_MAX;
    idl->request_id = NULL;

    hmap_init(&idl->outstanding_txns);

    return idl;
}

/* Destroys 'idl' and all of the data structures that it manages. */
void
ovsdb_idl_destroy(struct ovsdb_idl *idl)
{
    if (idl) {
        size_t i;

        ovs_assert(!idl->txn);
        ovsdb_idl_clear(idl);
        jsonrpc_session_close(idl->session);

        for (i = 0; i < idl->class->n_tables; i++) {
            struct ovsdb_idl_table *table = &idl->tables[i];
            shash_destroy(&table->columns);
            hmap_destroy(&table->rows);
            free(table->modes);
        }
        shash_destroy(&idl->table_by_name);
        free(idl->tables);
        json_destroy(idl->request_id);
        free(idl->lock_name);
        json_destroy(idl->lock_request_id);
        hmap_destroy(&idl->outstanding_txns);
        free(idl);
    }
}

static void
ovsdb_idl_clear(struct ovsdb_idl *idl)
{
    bool changed = false;
    size_t i;

    for (i = 0; i < idl->class->n_tables; i++) {
        struct ovsdb_idl_table *table = &idl->tables[i];
        struct ovsdb_idl_row *row, *next_row;

        if (hmap_is_empty(&table->rows)) {
            continue;
        }

        changed = true;
        HMAP_FOR_EACH_SAFE (row, next_row, hmap_node, &table->rows) {
            struct ovsdb_idl_arc *arc, *next_arc;

            if (!ovsdb_idl_row_is_orphan(row)) {
                ovsdb_idl_row_unparse(row);
            }
            LIST_FOR_EACH_SAFE (arc, next_arc, src_node, &row->src_arcs) {
                free(arc);
            }
            /* No need to do anything with dst_arcs: some node has those arcs
             * as forward arcs and will destroy them itself. */

            ovsdb_idl_row_destroy(row);
        }
    }

    if (changed) {
        idl->change_seqno++;
    }
}

/* Processes a batch of messages from the database server on 'idl'.  This may
 * cause the IDL's contents to change.  The client may check for that with
 * ovsdb_idl_get_seqno(). */
void
ovsdb_idl_run(struct ovsdb_idl *idl)
{
    int i;

    ovs_assert(!idl->txn);
    jsonrpc_session_run(idl->session);
    for (i = 0; jsonrpc_session_is_connected(idl->session) && i < 50; i++) {
        struct jsonrpc_msg *msg;
        unsigned int seqno;

        seqno = jsonrpc_session_get_seqno(idl->session);
        if (idl->state_seqno != seqno) {
            idl->state_seqno = seqno;
            json_destroy(idl->request_id);
            idl->request_id = NULL;
            ovsdb_idl_txn_abort_all(idl);

            ovsdb_idl_send_schema_request(idl);
            idl->state = IDL_S_SCHEMA_REQUESTED;
            if (idl->lock_name) {
                ovsdb_idl_send_lock_request(idl);
            }
        }

        msg = jsonrpc_session_recv(idl->session);
        if (!msg) {
            break;
        }

        if (msg->type == JSONRPC_NOTIFY
            && !strcmp(msg->method, "update")
            && msg->params->type == JSON_ARRAY
            && msg->params->u.array.n == 2
            && msg->params->u.array.elems[0]->type == JSON_NULL) {
            /* Database contents changed. */
            ovsdb_idl_parse_update(idl, msg->params->u.array.elems[1]);
        } else if (msg->type == JSONRPC_REPLY
                   && idl->request_id
                   && json_equal(idl->request_id, msg->id)) {
            switch (idl->state) {
            case IDL_S_SCHEMA_REQUESTED:
                /* Reply to our "get_schema" request. */
                json_destroy(idl->request_id);
                idl->request_id = NULL;
                ovsdb_idl_send_monitor_request(idl, msg->result);
                idl->state = IDL_S_MONITOR_REQUESTED;
                break;

            case IDL_S_MONITOR_REQUESTED:
                /* Reply to our "monitor" request. */
                idl->change_seqno++;
                json_destroy(idl->request_id);
                idl->request_id = NULL;
                idl->state = IDL_S_MONITORING;
                ovsdb_idl_clear(idl);
                ovsdb_idl_parse_update(idl, msg->result);
                break;

            case IDL_S_MONITORING:
            default:
                OVS_NOT_REACHED();
            }
        } else if (msg->type == JSONRPC_REPLY
                   && idl->lock_request_id
                   && json_equal(idl->lock_request_id, msg->id)) {
            /* Reply to our "lock" request. */
            ovsdb_idl_parse_lock_reply(idl, msg->result);
        } else if (msg->type == JSONRPC_NOTIFY
                   && !strcmp(msg->method, "locked")) {
            /* We got our lock. */
            ovsdb_idl_parse_lock_notify(idl, msg->params, true);
        } else if (msg->type == JSONRPC_NOTIFY
                   && !strcmp(msg->method, "stolen")) {
            /* Someone else stole our lock. */
            ovsdb_idl_parse_lock_notify(idl, msg->params, false);
        } else if ((msg->type == JSONRPC_ERROR
                    || msg->type == JSONRPC_REPLY)
                   && ovsdb_idl_txn_process_reply(idl, msg)) {
            /* ovsdb_idl_txn_process_reply() did everything needful. */
        } else {
            /* This can happen if ovsdb_idl_txn_destroy() is called to destroy
             * a transaction before we receive the reply, so keep the log level
             * low. */
            VLOG_DBG("%s: received unexpected %s message",
                     jsonrpc_session_get_name(idl->session),
                     jsonrpc_msg_type_to_string(msg->type));
        }
        jsonrpc_msg_destroy(msg);
    }
}

/* Arranges for poll_block() to wake up when ovsdb_idl_run() has something to
 * do or when activity occurs on a transaction on 'idl'. */
void
ovsdb_idl_wait(struct ovsdb_idl *idl)
{
    jsonrpc_session_wait(idl->session);
    jsonrpc_session_recv_wait(idl->session);
}

/* Returns a "sequence number" that represents the state of 'idl'.  When
 * ovsdb_idl_run() changes the database, the sequence number changes.  The
 * initial fetch of the entire contents of the remote database is considered to
 * be one kind of change.  Successfully acquiring a lock, if one has been
 * configured with ovsdb_idl_set_lock(), is also considered to be a change.
 *
 * As long as the sequence number does not change, the client may continue to
 * use any data structures it obtains from 'idl'.  But when it changes, the
 * client must not access any of these data structures again, because they
 * could have freed or reused for other purposes.
 *
 * The sequence number can occasionally change even if the database does not.
 * This happens if the connection to the database drops and reconnects, which
 * causes the database contents to be reloaded even if they didn't change.  (It
 * could also happen if the database server sends out a "change" that reflects
 * what the IDL already thought was in the database.  The database server is
 * not supposed to do that, but bugs could in theory cause it to do so.) */
unsigned int
ovsdb_idl_get_seqno(const struct ovsdb_idl *idl)
{
    return idl->change_seqno;
}

/* Returns true if 'idl' successfully connected to the remote database and
 * retrieved its contents (even if the connection subsequently dropped and is
 * in the process of reconnecting).  If so, then 'idl' contains an atomic
 * snapshot of the database's contents (but it might be arbitrarily old if the
 * connection dropped).
 *
 * Returns false if 'idl' has never connected or retrieved the database's
 * contents.  If so, 'idl' is empty. */
bool
ovsdb_idl_has_ever_connected(const struct ovsdb_idl *idl)
{
    return ovsdb_idl_get_seqno(idl) != 0;
}

/* Reconfigures 'idl' so that it would reconnect to the database, if
 * connection was dropped. */
void
ovsdb_idl_enable_reconnect(struct ovsdb_idl *idl)
{
    jsonrpc_session_enable_reconnect(idl->session);
}

/* Forces 'idl' to drop its connection to the database and reconnect.  In the
 * meantime, the contents of 'idl' will not change. */
void
ovsdb_idl_force_reconnect(struct ovsdb_idl *idl)
{
    jsonrpc_session_force_reconnect(idl->session);
}

/* Some IDL users should only write to write-only columns.  Furthermore,
 * writing to a column which is not write-only can cause serious performance
 * degradations for these users.  This function causes 'idl' to reject writes
 * to columns which are not marked write only using ovsdb_idl_omit_alert(). */
void
ovsdb_idl_verify_write_only(struct ovsdb_idl *idl)
{
    idl->verify_write_only = true;
}

/* Returns true if 'idl' is currently connected or trying to connect. */
bool
ovsdb_idl_is_alive(const struct ovsdb_idl *idl)
{
    return jsonrpc_session_is_alive(idl->session);
}

/* Returns the last error reported on a connection by 'idl'.  The return value
 * is 0 only if no connection made by 'idl' has ever encountered an error.  See
 * jsonrpc_get_status() for return value interpretation. */
int
ovsdb_idl_get_last_error(const struct ovsdb_idl *idl)
{
    return jsonrpc_session_get_last_error(idl->session);
}

static unsigned char *
ovsdb_idl_get_mode(struct ovsdb_idl *idl,
                   const struct ovsdb_idl_column *column)
{
    size_t i;

    ovs_assert(!idl->change_seqno);

    for (i = 0; i < idl->class->n_tables; i++) {
        const struct ovsdb_idl_table *table = &idl->tables[i];
        const struct ovsdb_idl_table_class *tc = table->class;

        if (column >= tc->columns && column < &tc->columns[tc->n_columns]) {
            return &table->modes[column - tc->columns];
        }
    }

    OVS_NOT_REACHED();
}

static void
add_ref_table(struct ovsdb_idl *idl, const struct ovsdb_base_type *base)
{
    if (base->type == OVSDB_TYPE_UUID && base->u.uuid.refTableName) {
        struct ovsdb_idl_table *table;

        table = shash_find_data(&idl->table_by_name,
                                base->u.uuid.refTableName);
        if (table) {
            table->need_table = true;
        } else {
            VLOG_WARN("%s IDL class missing referenced table %s",
                      idl->class->database, base->u.uuid.refTableName);
        }
    }
}

/* Turns on OVSDB_IDL_MONITOR and OVSDB_IDL_ALERT for 'column' in 'idl'.  Also
 * ensures that any tables referenced by 'column' will be replicated, even if
 * no columns in that table are selected for replication (see
 * ovsdb_idl_add_table() for more information).
 *
 * This function is only useful if 'monitor_everything_by_default' was false in
 * the call to ovsdb_idl_create().  This function should be called between
 * ovsdb_idl_create() and the first call to ovsdb_idl_run().
 */
void
ovsdb_idl_add_column(struct ovsdb_idl *idl,
                     const struct ovsdb_idl_column *column)
{
    *ovsdb_idl_get_mode(idl, column) = OVSDB_IDL_MONITOR | OVSDB_IDL_ALERT;
    add_ref_table(idl, &column->type.key);
    add_ref_table(idl, &column->type.value);
}

/* Ensures that the table with class 'tc' will be replicated on 'idl' even if
 * no columns are selected for replication.  This can be useful because it
 * allows 'idl' to keep track of what rows in the table actually exist, which
 * in turn allows columns that reference the table to have accurate contents.
 * (The IDL presents the database with references to rows that do not exist
 * removed.)
 *
 * This function is only useful if 'monitor_everything_by_default' was false in
 * the call to ovsdb_idl_create().  This function should be called between
 * ovsdb_idl_create() and the first call to ovsdb_idl_run().
 */
void
ovsdb_idl_add_table(struct ovsdb_idl *idl,
                    const struct ovsdb_idl_table_class *tc)
{
    size_t i;

    for (i = 0; i < idl->class->n_tables; i++) {
        struct ovsdb_idl_table *table = &idl->tables[i];

        if (table->class == tc) {
            table->need_table = true;
            return;
        }
    }

    OVS_NOT_REACHED();
}

/* Turns off OVSDB_IDL_ALERT for 'column' in 'idl'.
 *
 * This function should be called between ovsdb_idl_create() and the first call
 * to ovsdb_idl_run().
 */
void
ovsdb_idl_omit_alert(struct ovsdb_idl *idl,
                     const struct ovsdb_idl_column *column)
{
    *ovsdb_idl_get_mode(idl, column) &= ~OVSDB_IDL_ALERT;
}

/* Sets the mode for 'column' in 'idl' to 0.  See the big comment above
 * OVSDB_IDL_MONITOR for details.
 *
 * This function should be called between ovsdb_idl_create() and the first call
 * to ovsdb_idl_run().
 */
void
ovsdb_idl_omit(struct ovsdb_idl *idl, const struct ovsdb_idl_column *column)
{
    *ovsdb_idl_get_mode(idl, column) = 0;
}

static void
ovsdb_idl_send_schema_request(struct ovsdb_idl *idl)
{
    struct jsonrpc_msg *msg;

    json_destroy(idl->request_id);
    msg = jsonrpc_create_request(
        "get_schema",
        json_array_create_1(json_string_create(idl->class->database)),
        &idl->request_id);
    jsonrpc_session_send(idl->session, msg);
}

static void
log_error(struct ovsdb_error *error)
{
    char *s = ovsdb_error_to_string(error);
    VLOG_WARN("error parsing database schema: %s", s);
    free(s);
    ovsdb_error_destroy(error);
}

/* Frees 'schema', which is in the format returned by parse_schema(). */
static void
free_schema(struct shash *schema)
{
    if (schema) {
        struct shash_node *node, *next;

        SHASH_FOR_EACH_SAFE (node, next, schema) {
            struct sset *sset = node->data;
            sset_destroy(sset);
            free(sset);
            shash_delete(schema, node);
        }
        shash_destroy(schema);
        free(schema);
    }
}

/* Parses 'schema_json', an OVSDB schema in JSON format as described in RFC
 * 7047, to obtain the names of its rows and columns.  If successful, returns
 * an shash whose keys are table names and whose values are ssets, where each
 * sset contains the names of its table's columns.  On failure (due to a parse
 * error), returns NULL.
 *
 * It would also be possible to use the general-purpose OVSDB schema parser in
 * ovsdb-server, but that's overkill, possibly too strict for the current use
 * case, and would require restructuring ovsdb-server to separate the schema
 * code from the rest. */
static struct shash *
parse_schema(const struct json *schema_json)
{
    struct ovsdb_parser parser;
    const struct json *tables_json;
    struct ovsdb_error *error;
    struct shash_node *node;
    struct shash *schema;

    ovsdb_parser_init(&parser, schema_json, "database schema");
    tables_json = ovsdb_parser_member(&parser, "tables", OP_OBJECT);
    error = ovsdb_parser_destroy(&parser);
    if (error) {
        log_error(error);
        return NULL;
    }

    schema = xmalloc(sizeof *schema);
    shash_init(schema);
    SHASH_FOR_EACH (node, json_object(tables_json)) {
        const char *table_name = node->name;
        const struct json *json = node->data;
        const struct json *columns_json;

        ovsdb_parser_init(&parser, json, "table schema for table %s",
                          table_name);
        columns_json = ovsdb_parser_member(&parser, "columns", OP_OBJECT);
        error = ovsdb_parser_destroy(&parser);
        if (error) {
            log_error(error);
            free_schema(schema);
            return NULL;
        }

        struct sset *columns = xmalloc(sizeof *columns);
        sset_init(columns);

        struct shash_node *node2;
        SHASH_FOR_EACH (node2, json_object(columns_json)) {
            const char *column_name = node2->name;
            sset_add(columns, column_name);
        }
        shash_add(schema, table_name, columns);
    }
    return schema;
}

static void
ovsdb_idl_send_monitor_request(struct ovsdb_idl *idl,
                               const struct json *schema_json)
{
    struct shash *schema = parse_schema(schema_json);
    struct json *monitor_requests;
    struct jsonrpc_msg *msg;
    size_t i;

    monitor_requests = json_object_create();
    for (i = 0; i < idl->class->n_tables; i++) {
        const struct ovsdb_idl_table *table = &idl->tables[i];
        const struct ovsdb_idl_table_class *tc = table->class;
        struct json *monitor_request, *columns;
        const struct sset *table_schema;
        size_t j;

        table_schema = (schema
                        ? shash_find_data(schema, table->class->name)
                        : NULL);

        columns = table->need_table ? json_array_create_empty() : NULL;
        for (j = 0; j < tc->n_columns; j++) {
            const struct ovsdb_idl_column *column = &tc->columns[j];
            if (table->modes[j] & OVSDB_IDL_MONITOR) {
                if (table_schema
                    && !sset_contains(table_schema, column->name)) {
                    VLOG_WARN("%s table in %s database lacks %s column "
                              "(database needs upgrade?)",
                              table->class->name, idl->class->database,
                              column->name);
                    continue;
                }
                if (!columns) {
                    columns = json_array_create_empty();
                }
                json_array_add(columns, json_string_create(column->name));
            }
        }

        if (columns) {
            if (schema && !table_schema) {
                VLOG_WARN("%s database lacks %s table "
                          "(database needs upgrade?)",
                          idl->class->database, table->class->name);
                json_destroy(columns);
                continue;
            }

            monitor_request = json_object_create();
            json_object_put(monitor_request, "columns", columns);
            json_object_put(monitor_requests, tc->name, monitor_request);
        }
    }
    free_schema(schema);

    json_destroy(idl->request_id);
    msg = jsonrpc_create_request(
        "monitor",
        json_array_create_3(json_string_create(idl->class->database),
                            json_null_create(), monitor_requests),
        &idl->request_id);
    jsonrpc_session_send(idl->session, msg);
}

static void
ovsdb_idl_parse_update(struct ovsdb_idl *idl, const struct json *table_updates)
{
    struct ovsdb_error *error = ovsdb_idl_parse_update__(idl, table_updates);
    if (error) {
        if (!VLOG_DROP_WARN(&syntax_rl)) {
            char *s = ovsdb_error_to_string(error);
            VLOG_WARN_RL(&syntax_rl, "%s", s);
            free(s);
        }
        ovsdb_error_destroy(error);
    }
}

static struct ovsdb_error *
ovsdb_idl_parse_update__(struct ovsdb_idl *idl,
                         const struct json *table_updates)
{
    const struct shash_node *tables_node;

    if (table_updates->type != JSON_OBJECT) {
        return ovsdb_syntax_error(table_updates, NULL,
                                  "<table-updates> is not an object");
    }
    SHASH_FOR_EACH (tables_node, json_object(table_updates)) {
        const struct json *table_update = tables_node->data;
        const struct shash_node *table_node;
        struct ovsdb_idl_table *table;

        table = shash_find_data(&idl->table_by_name, tables_node->name);
        if (!table) {
            return ovsdb_syntax_error(
                table_updates, NULL,
                "<table-updates> includes unknown table \"%s\"",
                tables_node->name);
        }

        if (table_update->type != JSON_OBJECT) {
            return ovsdb_syntax_error(table_update, NULL,
                                      "<table-update> for table \"%s\" is "
                                      "not an object", table->class->name);
        }
        SHASH_FOR_EACH (table_node, json_object(table_update)) {
            const struct json *row_update = table_node->data;
            const struct json *old_json, *new_json;
            struct uuid uuid;

            if (!uuid_from_string(&uuid, table_node->name)) {
                return ovsdb_syntax_error(table_update, NULL,
                                          "<table-update> for table \"%s\" "
                                          "contains bad UUID "
                                          "\"%s\" as member name",
                                          table->class->name,
                                          table_node->name);
            }
            if (row_update->type != JSON_OBJECT) {
                return ovsdb_syntax_error(row_update, NULL,
                                          "<table-update> for table \"%s\" "
                                          "contains <row-update> for %s that "
                                          "is not an object",
                                          table->class->name,
                                          table_node->name);
            }

            old_json = shash_find_data(json_object(row_update), "old");
            new_json = shash_find_data(json_object(row_update), "new");
            if (old_json && old_json->type != JSON_OBJECT) {
                return ovsdb_syntax_error(old_json, NULL,
                                          "\"old\" <row> is not object");
            } else if (new_json && new_json->type != JSON_OBJECT) {
                return ovsdb_syntax_error(new_json, NULL,
                                          "\"new\" <row> is not object");
            } else if ((old_json != NULL) + (new_json != NULL)
                       != shash_count(json_object(row_update))) {
                return ovsdb_syntax_error(row_update, NULL,
                                          "<row-update> contains unexpected "
                                          "member");
            } else if (!old_json && !new_json) {
                return ovsdb_syntax_error(row_update, NULL,
                                          "<row-update> missing \"old\" "
                                          "and \"new\" members");
            }

            if (ovsdb_idl_process_update(table, &uuid, old_json, new_json)) {
                idl->change_seqno++;
            }
        }
    }

    return NULL;
}

static struct ovsdb_idl_row *
ovsdb_idl_get_row(struct ovsdb_idl_table *table, const struct uuid *uuid)
{
    struct ovsdb_idl_row *row;

    HMAP_FOR_EACH_WITH_HASH (row, hmap_node, uuid_hash(uuid), &table->rows) {
        if (uuid_equals(&row->uuid, uuid)) {
            return row;
        }
    }
    return NULL;
}

/* Returns true if a column with mode OVSDB_IDL_MODE_RW changed, false
 * otherwise. */
static bool
ovsdb_idl_process_update(struct ovsdb_idl_table *table,
                         const struct uuid *uuid, const struct json *old,
                         const struct json *new)
{
    struct ovsdb_idl_row *row;

    row = ovsdb_idl_get_row(table, uuid);
    if (!new) {
        /* Delete row. */
        if (row && !ovsdb_idl_row_is_orphan(row)) {
            /* XXX perhaps we should check the 'old' values? */
            ovsdb_idl_delete_row(row);
        } else {
            VLOG_WARN_RL(&semantic_rl, "cannot delete missing row "UUID_FMT" "
                         "from table %s",
                         UUID_ARGS(uuid), table->class->name);
            return false;
        }
    } else if (!old) {
        /* Insert row. */
        if (!row) {
            ovsdb_idl_insert_row(ovsdb_idl_row_create(table, uuid), new);
        } else if (ovsdb_idl_row_is_orphan(row)) {
            ovsdb_idl_insert_row(row, new);
        } else {
            VLOG_WARN_RL(&semantic_rl, "cannot add existing row "UUID_FMT" to "
                         "table %s", UUID_ARGS(uuid), table->class->name);
            return ovsdb_idl_modify_row(row, new);
        }
    } else {
        /* Modify row. */
        if (row) {
            /* XXX perhaps we should check the 'old' values? */
            if (!ovsdb_idl_row_is_orphan(row)) {
                return ovsdb_idl_modify_row(row, new);
            } else {
                VLOG_WARN_RL(&semantic_rl, "cannot modify missing but "
                             "referenced row "UUID_FMT" in table %s",
                             UUID_ARGS(uuid), table->class->name);
                ovsdb_idl_insert_row(row, new);
            }
        } else {
            VLOG_WARN_RL(&semantic_rl, "cannot modify missing row "UUID_FMT" "
                         "in table %s", UUID_ARGS(uuid), table->class->name);
            ovsdb_idl_insert_row(ovsdb_idl_row_create(table, uuid), new);
        }
    }

    return true;
}

/* Returns true if a column with mode OVSDB_IDL_MODE_RW changed, false
 * otherwise. */
static bool
ovsdb_idl_row_update(struct ovsdb_idl_row *row, const struct json *row_json)
{
    struct ovsdb_idl_table *table = row->table;
    struct shash_node *node;
    bool changed = false;

    SHASH_FOR_EACH (node, json_object(row_json)) {
        const char *column_name = node->name;
        const struct ovsdb_idl_column *column;
        struct ovsdb_datum datum;
        struct ovsdb_error *error;

        column = shash_find_data(&table->columns, column_name);
        if (!column) {
            VLOG_WARN_RL(&syntax_rl, "unknown column %s updating row "UUID_FMT,
                         column_name, UUID_ARGS(&row->uuid));
            continue;
        }

        error = ovsdb_datum_from_json(&datum, &column->type, node->data, NULL);
        if (!error) {
            unsigned int column_idx = column - table->class->columns;
            struct ovsdb_datum *old = &row->old[column_idx];

            if (!ovsdb_datum_equals(old, &datum, &column->type)) {
                ovsdb_datum_swap(old, &datum);
                if (table->modes[column_idx] & OVSDB_IDL_ALERT) {
                    changed = true;
                }
            } else {
                /* Didn't really change but the OVSDB monitor protocol always
                 * includes every value in a row. */
            }

            ovsdb_datum_destroy(&datum, &column->type);
        } else {
            char *s = ovsdb_error_to_string(error);
            VLOG_WARN_RL(&syntax_rl, "error parsing column %s in row "UUID_FMT
                         " in table %s: %s", column_name,
                         UUID_ARGS(&row->uuid), table->class->name, s);
            free(s);
            ovsdb_error_destroy(error);
        }
    }
    return changed;
}

/* When a row A refers to row B through a column with a "refTable" constraint,
 * but row B does not exist, row B is called an "orphan row".  Orphan rows
 * should not persist, because the database enforces referential integrity, but
 * they can appear transiently as changes from the database are received (the
 * database doesn't try to topologically sort them and circular references mean
 * it isn't always possible anyhow).
 *
 * This function returns true if 'row' is an orphan row, otherwise false.
 */
static bool
ovsdb_idl_row_is_orphan(const struct ovsdb_idl_row *row)
{
    return !row->old && !row->new;
}

/* Returns true if 'row' is conceptually part of the database as modified by
 * the current transaction (if any), false otherwise.
 *
 * This function will return true if 'row' is not an orphan (see the comment on
 * ovsdb_idl_row_is_orphan()) and:
 *
 *   - 'row' exists in the database and has not been deleted within the
 *     current transaction (if any).
 *
 *   - 'row' was inserted within the current transaction and has not been
 *     deleted.  (In the latter case you should not have passed 'row' in at
 *     all, because ovsdb_idl_txn_delete() freed it.)
 *
 * This function will return false if 'row' is an orphan or if 'row' was
 * deleted within the current transaction.
 */
static bool
ovsdb_idl_row_exists(const struct ovsdb_idl_row *row)
{
    return row->new != NULL;
}

static void
ovsdb_idl_row_parse(struct ovsdb_idl_row *row)
{
    const struct ovsdb_idl_table_class *class = row->table->class;
    size_t i;

    for (i = 0; i < class->n_columns; i++) {
        const struct ovsdb_idl_column *c = &class->columns[i];
        (c->parse)(row, &row->old[i]);
    }
}

static void
ovsdb_idl_row_unparse(struct ovsdb_idl_row *row)
{
    const struct ovsdb_idl_table_class *class = row->table->class;
    size_t i;

    for (i = 0; i < class->n_columns; i++) {
        const struct ovsdb_idl_column *c = &class->columns[i];
        (c->unparse)(row);
    }
}

static void
ovsdb_idl_row_clear_old(struct ovsdb_idl_row *row)
{
    ovs_assert(row->old == row->new);
    if (!ovsdb_idl_row_is_orphan(row)) {
        const struct ovsdb_idl_table_class *class = row->table->class;
        size_t i;

        for (i = 0; i < class->n_columns; i++) {
            ovsdb_datum_destroy(&row->old[i], &class->columns[i].type);
        }
        free(row->old);
        row->old = row->new = NULL;
    }
}

static void
ovsdb_idl_row_clear_new(struct ovsdb_idl_row *row)
{
    if (row->old != row->new) {
        if (row->new) {
            const struct ovsdb_idl_table_class *class = row->table->class;
            size_t i;

            if (row->written) {
                BITMAP_FOR_EACH_1 (i, class->n_columns, row->written) {
                    ovsdb_datum_destroy(&row->new[i], &class->columns[i].type);
                }
            }
            free(row->new);
            free(row->written);
            row->written = NULL;
        }
        row->new = row->old;
    }
}

static void
ovsdb_idl_row_clear_arcs(struct ovsdb_idl_row *row, bool destroy_dsts)
{
    struct ovsdb_idl_arc *arc, *next;

    /* Delete all forward arcs.  If 'destroy_dsts', destroy any orphaned rows
     * that this causes to be unreferenced. */
    LIST_FOR_EACH_SAFE (arc, next, src_node, &row->src_arcs) {
        list_remove(&arc->dst_node);
        if (destroy_dsts
            && ovsdb_idl_row_is_orphan(arc->dst)
            && list_is_empty(&arc->dst->dst_arcs)) {
            ovsdb_idl_row_destroy(arc->dst);
        }
        free(arc);
    }
    list_init(&row->src_arcs);
}

/* Force nodes that reference 'row' to reparse. */
static void
ovsdb_idl_row_reparse_backrefs(struct ovsdb_idl_row *row)
{
    struct ovsdb_idl_arc *arc, *next;

    /* This is trickier than it looks.  ovsdb_idl_row_clear_arcs() will destroy
     * 'arc', so we need to use the "safe" variant of list traversal.  However,
     * calling an ovsdb_idl_column's 'parse' function will add an arc
     * equivalent to 'arc' to row->arcs.  That could be a problem for
     * traversal, but it adds it at the beginning of the list to prevent us
     * from stumbling upon it again.
     *
     * (If duplicate arcs were possible then we would need to make sure that
     * 'next' didn't also point into 'arc''s destination, but we forbid
     * duplicate arcs.) */
    LIST_FOR_EACH_SAFE (arc, next, dst_node, &row->dst_arcs) {
        struct ovsdb_idl_row *ref = arc->src;

        ovsdb_idl_row_unparse(ref);
        ovsdb_idl_row_clear_arcs(ref, false);
        ovsdb_idl_row_parse(ref);
    }
}

static struct ovsdb_idl_row *
ovsdb_idl_row_create__(const struct ovsdb_idl_table_class *class)
{
    struct ovsdb_idl_row *row = xzalloc(class->allocation_size);
    class->row_init(row);
    list_init(&row->src_arcs);
    list_init(&row->dst_arcs);
    hmap_node_nullify(&row->txn_node);
    return row;
}

static struct ovsdb_idl_row *
ovsdb_idl_row_create(struct ovsdb_idl_table *table, const struct uuid *uuid)
{
    struct ovsdb_idl_row *row = ovsdb_idl_row_create__(table->class);
    hmap_insert(&table->rows, &row->hmap_node, uuid_hash(uuid));
    row->uuid = *uuid;
    row->table = table;
    return row;
}

static void
ovsdb_idl_row_destroy(struct ovsdb_idl_row *row)
{
    if (row) {
        ovsdb_idl_row_clear_old(row);
        hmap_remove(&row->table->rows, &row->hmap_node);
        free(row);
    }
}

static void
ovsdb_idl_insert_row(struct ovsdb_idl_row *row, const struct json *row_json)
{
    const struct ovsdb_idl_table_class *class = row->table->class;
    size_t i;

    ovs_assert(!row->old && !row->new);
    row->old = row->new = xmalloc(class->n_columns * sizeof *row->old);
    for (i = 0; i < class->n_columns; i++) {
        ovsdb_datum_init_default(&row->old[i], &class->columns[i].type);
    }
    ovsdb_idl_row_update(row, row_json);
    ovsdb_idl_row_parse(row);

    ovsdb_idl_row_reparse_backrefs(row);
}

static void
ovsdb_idl_delete_row(struct ovsdb_idl_row *row)
{
    ovsdb_idl_row_unparse(row);
    ovsdb_idl_row_clear_arcs(row, true);
    ovsdb_idl_row_clear_old(row);
    if (list_is_empty(&row->dst_arcs)) {
        ovsdb_idl_row_destroy(row);
    } else {
        ovsdb_idl_row_reparse_backrefs(row);
    }
}

/* Returns true if a column with mode OVSDB_IDL_MODE_RW changed, false
 * otherwise. */
static bool
ovsdb_idl_modify_row(struct ovsdb_idl_row *row, const struct json *row_json)
{
    bool changed;

    ovsdb_idl_row_unparse(row);
    ovsdb_idl_row_clear_arcs(row, true);
    changed = ovsdb_idl_row_update(row, row_json);
    ovsdb_idl_row_parse(row);

    return changed;
}

static bool
may_add_arc(const struct ovsdb_idl_row *src, const struct ovsdb_idl_row *dst)
{
    const struct ovsdb_idl_arc *arc;

    /* No self-arcs. */
    if (src == dst) {
        return false;
    }

    /* No duplicate arcs.
     *
     * We only need to test whether the first arc in dst->dst_arcs originates
     * at 'src', since we add all of the arcs from a given source in a clump
     * (in a single call to ovsdb_idl_row_parse()) and new arcs are always
     * added at the front of the dst_arcs list. */
    if (list_is_empty(&dst->dst_arcs)) {
        return true;
    }
    arc = CONTAINER_OF(dst->dst_arcs.next, struct ovsdb_idl_arc, dst_node);
    return arc->src != src;
}

static struct ovsdb_idl_table *
ovsdb_idl_table_from_class(const struct ovsdb_idl *idl,
                           const struct ovsdb_idl_table_class *table_class)
{
    return &idl->tables[table_class - idl->class->tables];
}

/* Called by ovsdb-idlc generated code. */
struct ovsdb_idl_row *
ovsdb_idl_get_row_arc(struct ovsdb_idl_row *src,
                      struct ovsdb_idl_table_class *dst_table_class,
                      const struct uuid *dst_uuid)
{
    struct ovsdb_idl *idl = src->table->idl;
    struct ovsdb_idl_table *dst_table;
    struct ovsdb_idl_arc *arc;
    struct ovsdb_idl_row *dst;

    dst_table = ovsdb_idl_table_from_class(idl, dst_table_class);
    dst = ovsdb_idl_get_row(dst_table, dst_uuid);
    if (idl->txn) {
        /* We're being called from ovsdb_idl_txn_write().  We must not update
         * any arcs, because the transaction will be backed out at commit or
         * abort time and we don't want our graph screwed up.
         *
         * Just return the destination row, if there is one and it has not been
         * deleted. */
        if (dst && (hmap_node_is_null(&dst->txn_node) || dst->new)) {
            return dst;
        }
        return NULL;
    } else {
        /* We're being called from some other context.  Update the graph. */
        if (!dst) {
            dst = ovsdb_idl_row_create(dst_table, dst_uuid);
        }

        /* Add a new arc, if it wouldn't be a self-arc or a duplicate arc. */
        if (may_add_arc(src, dst)) {
            /* The arc *must* be added at the front of the dst_arcs list.  See
             * ovsdb_idl_row_reparse_backrefs() for details. */
            arc = xmalloc(sizeof *arc);
            list_push_front(&src->src_arcs, &arc->src_node);
            list_push_front(&dst->dst_arcs, &arc->dst_node);
            arc->src = src;
            arc->dst = dst;
        }

        return !ovsdb_idl_row_is_orphan(dst) ? dst : NULL;
    }
}

/* Searches 'tc''s table in 'idl' for a row with UUID 'uuid'.  Returns a
 * pointer to the row if there is one, otherwise a null pointer.  */
const struct ovsdb_idl_row *
ovsdb_idl_get_row_for_uuid(const struct ovsdb_idl *idl,
                           const struct ovsdb_idl_table_class *tc,
                           const struct uuid *uuid)
{
    return ovsdb_idl_get_row(ovsdb_idl_table_from_class(idl, tc), uuid);
}

static struct ovsdb_idl_row *
next_real_row(struct ovsdb_idl_table *table, struct hmap_node *node)
{
    for (; node; node = hmap_next(&table->rows, node)) {
        struct ovsdb_idl_row *row;

        row = CONTAINER_OF(node, struct ovsdb_idl_row, hmap_node);
        if (ovsdb_idl_row_exists(row)) {
            return row;
        }
    }
    return NULL;
}

/* Returns a row in 'table_class''s table in 'idl', or a null pointer if that
 * table is empty.
 *
 * Database tables are internally maintained as hash tables, so adding or
 * removing rows while traversing the same table can cause some rows to be
 * visited twice or not at apply. */
const struct ovsdb_idl_row *
ovsdb_idl_first_row(const struct ovsdb_idl *idl,
                    const struct ovsdb_idl_table_class *table_class)
{
    struct ovsdb_idl_table *table
        = ovsdb_idl_table_from_class(idl, table_class);
    return next_real_row(table, hmap_first(&table->rows));
}

/* Returns a row following 'row' within its table, or a null pointer if 'row'
 * is the last row in its table. */
const struct ovsdb_idl_row *
ovsdb_idl_next_row(const struct ovsdb_idl_row *row)
{
    struct ovsdb_idl_table *table = row->table;

    return next_real_row(table, hmap_next(&table->rows, &row->hmap_node));
}

/* Reads and returns the value of 'column' within 'row'.  If an ongoing
 * transaction has changed 'column''s value, the modified value is returned.
 *
 * The caller must not modify or free the returned value.
 *
 * Various kinds of changes can invalidate the returned value: writing to the
 * same 'column' in 'row' (e.g. with ovsdb_idl_txn_write()), deleting 'row'
 * (e.g. with ovsdb_idl_txn_delete()), or completing an ongoing transaction
 * (e.g. with ovsdb_idl_txn_commit() or ovsdb_idl_txn_abort()).  If the
 * returned value is needed for a long time, it is best to make a copy of it
 * with ovsdb_datum_clone(). */
const struct ovsdb_datum *
ovsdb_idl_read(const struct ovsdb_idl_row *row,
               const struct ovsdb_idl_column *column)
{
    const struct ovsdb_idl_table_class *class;
    size_t column_idx;

    ovs_assert(!ovsdb_idl_row_is_synthetic(row));

    class = row->table->class;
    column_idx = column - class->columns;

    ovs_assert(row->new != NULL);
    ovs_assert(column_idx < class->n_columns);

    if (row->written && bitmap_is_set(row->written, column_idx)) {
        return &row->new[column_idx];
    } else if (row->old) {
        return &row->old[column_idx];
    } else {
        return ovsdb_datum_default(&column->type);
    }
}

/* Same as ovsdb_idl_read(), except that it also asserts that 'column' has key
 * type 'key_type' and value type 'value_type'.  (Scalar and set types will
 * have a value type of OVSDB_TYPE_VOID.)
 *
 * This is useful in code that "knows" that a particular column has a given
 * type, so that it will abort if someone changes the column's type without
 * updating the code that uses it. */
const struct ovsdb_datum *
ovsdb_idl_get(const struct ovsdb_idl_row *row,
              const struct ovsdb_idl_column *column,
              enum ovsdb_atomic_type key_type OVS_UNUSED,
              enum ovsdb_atomic_type value_type OVS_UNUSED)
{
    ovs_assert(column->type.key.type == key_type);
    ovs_assert(column->type.value.type == value_type);

    return ovsdb_idl_read(row, column);
}

/* Returns true if the field represented by 'column' in 'row' may be modified,
 * false if it is immutable.
 *
 * Normally, whether a field is mutable is controlled by its column's schema.
 * However, an immutable column can be set to any initial value at the time of
 * insertion, so if 'row' is a new row (one that is being added as part of the
 * current transaction, supposing that a transaction is in progress) then even
 * its "immutable" fields are actually mutable. */
bool
ovsdb_idl_is_mutable(const struct ovsdb_idl_row *row,
                     const struct ovsdb_idl_column *column)
{
    return column->mutable || (row->new && !row->old);
}

/* Returns false if 'row' was obtained from the IDL, true if it was initialized
 * to all-zero-bits by some other entity.  If 'row' was set up some other way
 * then the return value is indeterminate. */
bool
ovsdb_idl_row_is_synthetic(const struct ovsdb_idl_row *row)
{
    return row->table == NULL;
}

/* Transactions. */

static void ovsdb_idl_txn_complete(struct ovsdb_idl_txn *txn,
                                   enum ovsdb_idl_txn_status);

/* Returns a string representation of 'status'.  The caller must not modify or
 * free the returned string.
 *
 * The return value is probably useful only for debug log messages and unit
 * tests. */
const char *
ovsdb_idl_txn_status_to_string(enum ovsdb_idl_txn_status status)
{
    switch (status) {
    case TXN_UNCOMMITTED:
        return "uncommitted";
    case TXN_UNCHANGED:
        return "unchanged";
    case TXN_INCOMPLETE:
        return "incomplete";
    case TXN_ABORTED:
        return "aborted";
    case TXN_SUCCESS:
        return "success";
    case TXN_TRY_AGAIN:
        return "try again";
    case TXN_NOT_LOCKED:
        return "not locked";
    case TXN_ERROR:
        return "error";
    }
    return "<unknown>";
}

/* Starts a new transaction on 'idl'.  A given ovsdb_idl may only have a single
 * active transaction at a time.  See the large comment in ovsdb-idl.h for
 * general information on transactions. */
struct ovsdb_idl_txn *
ovsdb_idl_txn_create(struct ovsdb_idl *idl)
{
    struct ovsdb_idl_txn *txn;

    ovs_assert(!idl->txn);
    idl->txn = txn = xmalloc(sizeof *txn);
    txn->request_id = NULL;
    txn->idl = idl;
    hmap_init(&txn->txn_rows);
    txn->status = TXN_UNCOMMITTED;
    txn->error = NULL;
    txn->dry_run = false;
    ds_init(&txn->comment);

    txn->inc_table = NULL;
    txn->inc_column = NULL;

    hmap_init(&txn->inserted_rows);

    return txn;
}

/* Appends 's', which is treated as a printf()-type format string, to the
 * comments that will be passed to the OVSDB server when 'txn' is committed.
 * (The comment will be committed to the OVSDB log, which "ovsdb-tool
 * show-log" can print in a relatively human-readable form.) */
void
ovsdb_idl_txn_add_comment(struct ovsdb_idl_txn *txn, const char *s, ...)
{
    va_list args;

    if (txn->comment.length) {
        ds_put_char(&txn->comment, '\n');
    }

    va_start(args, s);
    ds_put_format_valist(&txn->comment, s, args);
    va_end(args);
}

/* Marks 'txn' as a transaction that will not actually modify the database.  In
 * almost every way, the transaction is treated like other transactions.  It
 * must be committed or aborted like other transactions, it will be sent to the
 * database server like other transactions, and so on.  The only difference is
 * that the operations sent to the database server will include, as the last
 * step, an "abort" operation, so that any changes made by the transaction will
 * not actually take effect. */
void
ovsdb_idl_txn_set_dry_run(struct ovsdb_idl_txn *txn)
{
    txn->dry_run = true;
}

/* Causes 'txn', when committed, to increment the value of 'column' within
 * 'row' by 1.  'column' must have an integer type.  After 'txn' commits
 * successfully, the client may retrieve the final (incremented) value of
 * 'column' with ovsdb_idl_txn_get_increment_new_value().
 *
 * The client could accomplish something similar with ovsdb_idl_read(),
 * ovsdb_idl_txn_verify() and ovsdb_idl_txn_write(), or with ovsdb-idlc
 * generated wrappers for these functions.  However, ovsdb_idl_txn_increment()
 * will never (by itself) fail because of a verify error.
 *
 * The intended use is for incrementing the "next_cfg" column in the
 * Open_vSwitch table. */
void
ovsdb_idl_txn_increment(struct ovsdb_idl_txn *txn,
                        const struct ovsdb_idl_row *row,
                        const struct ovsdb_idl_column *column)
{
    ovs_assert(!txn->inc_table);
    ovs_assert(column->type.key.type == OVSDB_TYPE_INTEGER);
    ovs_assert(column->type.value.type == OVSDB_TYPE_VOID);

    txn->inc_table = row->table->class->name;
    txn->inc_column = column->name;
    txn->inc_row = row->uuid;
}

/* Destroys 'txn' and frees all associated memory.  If ovsdb_idl_txn_commit()
 * has been called for 'txn' but the commit is still incomplete (that is, the
 * last call returned TXN_INCOMPLETE) then the transaction may or may not still
 * end up committing at the database server, but the client will not be able to
 * get any further status information back. */
void
ovsdb_idl_txn_destroy(struct ovsdb_idl_txn *txn)
{
    struct ovsdb_idl_txn_insert *insert, *next;

    json_destroy(txn->request_id);
    if (txn->status == TXN_INCOMPLETE) {
        hmap_remove(&txn->idl->outstanding_txns, &txn->hmap_node);
    }
    ovsdb_idl_txn_abort(txn);
    ds_destroy(&txn->comment);
    free(txn->error);
    HMAP_FOR_EACH_SAFE (insert, next, hmap_node, &txn->inserted_rows) {
        free(insert);
    }
    hmap_destroy(&txn->inserted_rows);
    free(txn);
}

/* Causes poll_block() to wake up if 'txn' has completed committing. */
void
ovsdb_idl_txn_wait(const struct ovsdb_idl_txn *txn)
{
    if (txn->status != TXN_UNCOMMITTED && txn->status != TXN_INCOMPLETE) {
        poll_immediate_wake();
    }
}

static struct json *
where_uuid_equals(const struct uuid *uuid)
{
    return
        json_array_create_1(
            json_array_create_3(
                json_string_create("_uuid"),
                json_string_create("=="),
                json_array_create_2(
                    json_string_create("uuid"),
                    json_string_create_nocopy(
                        xasprintf(UUID_FMT, UUID_ARGS(uuid))))));
}

static char *
uuid_name_from_uuid(const struct uuid *uuid)
{
    char *name;
    char *p;

    name = xasprintf("row"UUID_FMT, UUID_ARGS(uuid));
    for (p = name; *p != '\0'; p++) {
        if (*p == '-') {
            *p = '_';
        }
    }

    return name;
}

static const struct ovsdb_idl_row *
ovsdb_idl_txn_get_row(const struct ovsdb_idl_txn *txn, const struct uuid *uuid)
{
    const struct ovsdb_idl_row *row;

    HMAP_FOR_EACH_WITH_HASH (row, txn_node, uuid_hash(uuid), &txn->txn_rows) {
        if (uuid_equals(&row->uuid, uuid)) {
            return row;
        }
    }
    return NULL;
}

/* XXX there must be a cleaner way to do this */
static struct json *
substitute_uuids(struct json *json, const struct ovsdb_idl_txn *txn)
{
    if (json->type == JSON_ARRAY) {
        struct uuid uuid;
        size_t i;

        if (json->u.array.n == 2
            && json->u.array.elems[0]->type == JSON_STRING
            && json->u.array.elems[1]->type == JSON_STRING
            && !strcmp(json->u.array.elems[0]->u.string, "uuid")
            && uuid_from_string(&uuid, json->u.array.elems[1]->u.string)) {
            const struct ovsdb_idl_row *row;

            row = ovsdb_idl_txn_get_row(txn, &uuid);
            if (row && !row->old && row->new) {
                json_destroy(json);

                return json_array_create_2(
                    json_string_create("named-uuid"),
                    json_string_create_nocopy(uuid_name_from_uuid(&uuid)));
            }
        }

        for (i = 0; i < json->u.array.n; i++) {
            json->u.array.elems[i] = substitute_uuids(json->u.array.elems[i],
                                                      txn);
        }
    } else if (json->type == JSON_OBJECT) {
        struct shash_node *node;

        SHASH_FOR_EACH (node, json_object(json)) {
            node->data = substitute_uuids(node->data, txn);
        }
    }
    return json;
}

static void
ovsdb_idl_txn_disassemble(struct ovsdb_idl_txn *txn)
{
    struct ovsdb_idl_row *row, *next;

    /* This must happen early.  Otherwise, ovsdb_idl_row_parse() will call an
     * ovsdb_idl_column's 'parse' function, which will call
     * ovsdb_idl_get_row_arc(), which will seen that the IDL is in a
     * transaction and fail to update the graph.  */
    txn->idl->txn = NULL;

    HMAP_FOR_EACH_SAFE (row, next, txn_node, &txn->txn_rows) {
        if (row->old) {
            if (row->written) {
                ovsdb_idl_row_unparse(row);
                ovsdb_idl_row_clear_arcs(row, false);
                ovsdb_idl_row_parse(row);
            }
        } else {
            ovsdb_idl_row_unparse(row);
        }
        ovsdb_idl_row_clear_new(row);

        free(row->prereqs);
        row->prereqs = NULL;

        free(row->written);
        row->written = NULL;

        hmap_remove(&txn->txn_rows, &row->txn_node);
        hmap_node_nullify(&row->txn_node);
        if (!row->old) {
            hmap_remove(&row->table->rows, &row->hmap_node);
            free(row);
        }
    }
    hmap_destroy(&txn->txn_rows);
    hmap_init(&txn->txn_rows);
}

/* Attempts to commit 'txn'.  Returns the status of the commit operation, one
 * of the following TXN_* constants:
 *
 *   TXN_INCOMPLETE:
 *
 *       The transaction is in progress, but not yet complete.  The caller
 *       should call again later, after calling ovsdb_idl_run() to let the IDL
 *       do OVSDB protocol processing.
 *
 *   TXN_UNCHANGED:
 *
 *       The transaction is complete.  (It didn't actually change the database,
 *       so the IDL didn't send any request to the database server.)
 *
 *   TXN_ABORTED:
 *
 *       The caller previously called ovsdb_idl_txn_abort().
 *
 *   TXN_SUCCESS:
 *
 *       The transaction was successful.  The update made by the transaction
 *       (and possibly other changes made by other database clients) should
 *       already be visible in the IDL.
 *
 *   TXN_TRY_AGAIN:
 *
 *       The transaction failed for some transient reason, e.g. because a
 *       "verify" operation reported an inconsistency or due to a network
 *       problem.  The caller should wait for a change to the database, then
 *       compose a new transaction, and commit the new transaction.
 *
 *       Use the return value of ovsdb_idl_get_seqno() to wait for a change in
 *       the database.  It is important to use its return value *before* the
 *       initial call to ovsdb_idl_txn_commit() as the baseline for this
 *       purpose, because the change that one should wait for can happen after
 *       the initial call but before the call that returns TXN_TRY_AGAIN, and
 *       using some other baseline value in that situation could cause an
 *       indefinite wait if the database rarely changes.
 *
 *   TXN_NOT_LOCKED:
 *
 *       The transaction failed because the IDL has been configured to require
 *       a database lock (with ovsdb_idl_set_lock()) but didn't get it yet or
 *       has already lost it.
 *
 * Committing a transaction rolls back all of the changes that it made to the
 * IDL's copy of the database.  If the transaction commits successfully, then
 * the database server will send an update and, thus, the IDL will be updated
 * with the committed changes. */
enum ovsdb_idl_txn_status
ovsdb_idl_txn_commit(struct ovsdb_idl_txn *txn)
{
    struct ovsdb_idl_row *row;
    struct json *operations;
    bool any_updates;

    if (txn != txn->idl->txn) {
        goto coverage_out;
    }

    /* If we need a lock but don't have it, give up quickly. */
    if (txn->idl->lock_name && !ovsdb_idl_has_lock(txn->idl)) {
        txn->status = TXN_NOT_LOCKED;
        goto disassemble_out;
    }

    operations = json_array_create_1(
        json_string_create(txn->idl->class->database));

    /* Assert that we have the required lock (avoiding a race). */
    if (txn->idl->lock_name) {
        struct json *op = json_object_create();
        json_array_add(operations, op);
        json_object_put_string(op, "op", "assert");
        json_object_put_string(op, "lock", txn->idl->lock_name);
    }

    /* Add prerequisites and declarations of new rows. */
    HMAP_FOR_EACH (row, txn_node, &txn->txn_rows) {
        /* XXX check that deleted rows exist even if no prereqs? */
        if (row->prereqs) {
            const struct ovsdb_idl_table_class *class = row->table->class;
            size_t n_columns = class->n_columns;
            struct json *op, *columns, *row_json;
            size_t idx;

            op = json_object_create();
            json_array_add(operations, op);
            json_object_put_string(op, "op", "wait");
            json_object_put_string(op, "table", class->name);
            json_object_put(op, "timeout", json_integer_create(0));
            json_object_put(op, "where", where_uuid_equals(&row->uuid));
            json_object_put_string(op, "until", "==");
            columns = json_array_create_empty();
            json_object_put(op, "columns", columns);
            row_json = json_object_create();
            json_object_put(op, "rows", json_array_create_1(row_json));

            BITMAP_FOR_EACH_1 (idx, n_columns, row->prereqs) {
                const struct ovsdb_idl_column *column = &class->columns[idx];
                json_array_add(columns, json_string_create(column->name));
                json_object_put(row_json, column->name,
                                ovsdb_datum_to_json(&row->old[idx],
                                                    &column->type));
            }
        }
    }

    /* Add updates. */
    any_updates = false;
    HMAP_FOR_EACH (row, txn_node, &txn->txn_rows) {
        const struct ovsdb_idl_table_class *class = row->table->class;

        if (!row->new) {
            if (class->is_root) {
                struct json *op = json_object_create();
                json_object_put_string(op, "op", "delete");
                json_object_put_string(op, "table", class->name);
                json_object_put(op, "where", where_uuid_equals(&row->uuid));
                json_array_add(operations, op);
                any_updates = true;
            } else {
                /* Let ovsdb-server decide whether to really delete it. */
            }
        } else if (row->old != row->new) {
            struct json *row_json;
            struct json *op;
            size_t idx;

            op = json_object_create();
            json_object_put_string(op, "op", row->old ? "update" : "insert");
            json_object_put_string(op, "table", class->name);
            if (row->old) {
                json_object_put(op, "where", where_uuid_equals(&row->uuid));
            } else {
                struct ovsdb_idl_txn_insert *insert;

                any_updates = true;

                json_object_put(op, "uuid-name",
                                json_string_create_nocopy(
                                    uuid_name_from_uuid(&row->uuid)));

                insert = xmalloc(sizeof *insert);
                insert->dummy = row->uuid;
                insert->op_index = operations->u.array.n - 1;
                uuid_zero(&insert->real);
                hmap_insert(&txn->inserted_rows, &insert->hmap_node,
                            uuid_hash(&insert->dummy));
            }
            row_json = json_object_create();
            json_object_put(op, "row", row_json);

            if (row->written) {
                BITMAP_FOR_EACH_1 (idx, class->n_columns, row->written) {
                    const struct ovsdb_idl_column *column =
                                                        &class->columns[idx];

                    if (row->old
                        || !ovsdb_datum_is_default(&row->new[idx],
                                                  &column->type)) {
                        json_object_put(row_json, column->name,
                                        substitute_uuids(
                                            ovsdb_datum_to_json(&row->new[idx],
                                                                &column->type),
                                            txn));

                        /* If anything really changed, consider it an update.
                         * We can't suppress not-really-changed values earlier
                         * or transactions would become nonatomic (see the big
                         * comment inside ovsdb_idl_txn_write()). */
                        if (!any_updates && row->old &&
                            !ovsdb_datum_equals(&row->old[idx], &row->new[idx],
                                                &column->type)) {
                            any_updates = true;
                        }
                    }
                }
            }

            if (!row->old || !shash_is_empty(json_object(row_json))) {
                json_array_add(operations, op);
            } else {
                json_destroy(op);
            }
        }
    }

    /* Add increment. */
    if (txn->inc_table && any_updates) {
        struct json *op;

        txn->inc_index = operations->u.array.n - 1;

        op = json_object_create();
        json_object_put_string(op, "op", "mutate");
        json_object_put_string(op, "table", txn->inc_table);
        json_object_put(op, "where",
                        substitute_uuids(where_uuid_equals(&txn->inc_row),
                                         txn));
        json_object_put(op, "mutations",
                        json_array_create_1(
                            json_array_create_3(
                                json_string_create(txn->inc_column),
                                json_string_create("+="),
                                json_integer_create(1))));
        json_array_add(operations, op);

        op = json_object_create();
        json_object_put_string(op, "op", "select");
        json_object_put_string(op, "table", txn->inc_table);
        json_object_put(op, "where",
                        substitute_uuids(where_uuid_equals(&txn->inc_row),
                                         txn));
        json_object_put(op, "columns",
                        json_array_create_1(json_string_create(
                                                txn->inc_column)));
        json_array_add(operations, op);
    }

    if (txn->comment.length) {
        struct json *op = json_object_create();
        json_object_put_string(op, "op", "comment");
        json_object_put_string(op, "comment", ds_cstr(&txn->comment));
        json_array_add(operations, op);
    }

    if (txn->dry_run) {
        struct json *op = json_object_create();
        json_object_put_string(op, "op", "abort");
        json_array_add(operations, op);
    }

    if (!any_updates) {
        txn->status = TXN_UNCHANGED;
        json_destroy(operations);
    } else if (!jsonrpc_session_send(
                   txn->idl->session,
                   jsonrpc_create_request(
                       "transact", operations, &txn->request_id))) {
        hmap_insert(&txn->idl->outstanding_txns, &txn->hmap_node,
                    json_hash(txn->request_id, 0));
        txn->status = TXN_INCOMPLETE;
    } else {
        txn->status = TXN_TRY_AGAIN;
    }

disassemble_out:
    ovsdb_idl_txn_disassemble(txn);
coverage_out:
    switch (txn->status) {
    case TXN_UNCOMMITTED:   COVERAGE_INC(txn_uncommitted);    break;
    case TXN_UNCHANGED:     COVERAGE_INC(txn_unchanged);      break;
    case TXN_INCOMPLETE:    COVERAGE_INC(txn_incomplete);     break;
    case TXN_ABORTED:       COVERAGE_INC(txn_aborted);        break;
    case TXN_SUCCESS:       COVERAGE_INC(txn_success);        break;
    case TXN_TRY_AGAIN:     COVERAGE_INC(txn_try_again);      break;
    case TXN_NOT_LOCKED:    COVERAGE_INC(txn_not_locked);     break;
    case TXN_ERROR:         COVERAGE_INC(txn_error);          break;
    }

    return txn->status;
}

/* Attempts to commit 'txn', blocking until the commit either succeeds or
 * fails.  Returns the final commit status, which may be any TXN_* value other
 * than TXN_INCOMPLETE.
 *
 * This function calls ovsdb_idl_run() on 'txn''s IDL, so it may cause the
 * return value of ovsdb_idl_get_seqno() to change. */
enum ovsdb_idl_txn_status
ovsdb_idl_txn_commit_block(struct ovsdb_idl_txn *txn)
{
    enum ovsdb_idl_txn_status status;

    fatal_signal_run();
    while ((status = ovsdb_idl_txn_commit(txn)) == TXN_INCOMPLETE) {
        ovsdb_idl_run(txn->idl);
        ovsdb_idl_wait(txn->idl);
        ovsdb_idl_txn_wait(txn);
        poll_block();
    }
    return status;
}

/* Returns the final (incremented) value of the column in 'txn' that was set to
 * be incremented by ovsdb_idl_txn_increment().  'txn' must have committed
 * successfully. */
int64_t
ovsdb_idl_txn_get_increment_new_value(const struct ovsdb_idl_txn *txn)
{
    ovs_assert(txn->status == TXN_SUCCESS);
    return txn->inc_new_value;
}

/* Aborts 'txn' without sending it to the database server.  This is effective
 * only if ovsdb_idl_txn_commit() has not yet been called for 'txn'.
 * Otherwise, it has no effect.
 *
 * Aborting a transaction doesn't free its memory.  Use
 * ovsdb_idl_txn_destroy() to do that. */
void
ovsdb_idl_txn_abort(struct ovsdb_idl_txn *txn)
{
    ovsdb_idl_txn_disassemble(txn);
    if (txn->status == TXN_UNCOMMITTED || txn->status == TXN_INCOMPLETE) {
        txn->status = TXN_ABORTED;
    }
}

/* Returns a string that reports the error status for 'txn'.  The caller must
 * not modify or free the returned string.  A call to ovsdb_idl_txn_destroy()
 * for 'txn' may free the returned string.
 *
 * The return value is ordinarily one of the strings that
 * ovsdb_idl_txn_status_to_string() would return, but if the transaction failed
 * due to an error reported by the database server, the return value is that
 * error. */
const char *
ovsdb_idl_txn_get_error(const struct ovsdb_idl_txn *txn)
{
    if (txn->status != TXN_ERROR) {
        return ovsdb_idl_txn_status_to_string(txn->status);
    } else if (txn->error) {
        return txn->error;
    } else {
        return "no error details available";
    }
}

static void
ovsdb_idl_txn_set_error_json(struct ovsdb_idl_txn *txn,
                             const struct json *json)
{
    if (txn->error == NULL) {
        txn->error = json_to_string(json, JSSF_SORT);
    }
}

/* For transaction 'txn' that completed successfully, finds and returns the
 * permanent UUID that the database assigned to a newly inserted row, given the
 * 'uuid' that ovsdb_idl_txn_insert() assigned locally to that row.
 *
 * Returns NULL if 'uuid' is not a UUID assigned by ovsdb_idl_txn_insert() or
 * if it was assigned by that function and then deleted by
 * ovsdb_idl_txn_delete() within the same transaction.  (Rows that are inserted
 * and then deleted within a single transaction are never sent to the database
 * server, so it never assigns them a permanent UUID.) */
const struct uuid *
ovsdb_idl_txn_get_insert_uuid(const struct ovsdb_idl_txn *txn,
                              const struct uuid *uuid)
{
    const struct ovsdb_idl_txn_insert *insert;

    ovs_assert(txn->status == TXN_SUCCESS || txn->status == TXN_UNCHANGED);
    HMAP_FOR_EACH_IN_BUCKET (insert, hmap_node,
                             uuid_hash(uuid), &txn->inserted_rows) {
        if (uuid_equals(uuid, &insert->dummy)) {
            return &insert->real;
        }
    }
    return NULL;
}

static void
ovsdb_idl_txn_complete(struct ovsdb_idl_txn *txn,
                       enum ovsdb_idl_txn_status status)
{
    txn->status = status;
    hmap_remove(&txn->idl->outstanding_txns, &txn->hmap_node);
}

/* Writes 'datum' to the specified 'column' in 'row_'.  Updates both 'row_'
 * itself and the structs derived from it (e.g. the "struct ovsrec_*", for
 * ovs-vswitchd).
 *
 * 'datum' must have the correct type for its column.  The IDL does not check
 * that it meets schema constraints, but ovsdb-server will do so at commit time
 * so it had better be correct.
 *
 * A transaction must be in progress.  Replication of 'column' must not have
 * been disabled (by calling ovsdb_idl_omit()).
 *
 * Usually this function is used indirectly through one of the "set" functions
 * generated by ovsdb-idlc.
 *
 * Takes ownership of what 'datum' points to (and in some cases destroys that
 * data before returning) but makes a copy of 'datum' itself.  (Commonly
 * 'datum' is on the caller's stack.) */
static void
ovsdb_idl_txn_write__(const struct ovsdb_idl_row *row_,
                      const struct ovsdb_idl_column *column,
                      struct ovsdb_datum *datum, bool owns_datum)
{
    struct ovsdb_idl_row *row = CONST_CAST(struct ovsdb_idl_row *, row_);
    const struct ovsdb_idl_table_class *class;
    size_t column_idx;
    bool write_only;

    if (ovsdb_idl_row_is_synthetic(row)) {
        goto discard_datum;
    }

    class = row->table->class;
    column_idx = column - class->columns;
    write_only = row->table->modes[column_idx] == OVSDB_IDL_MONITOR;

    ovs_assert(row->new != NULL);
    ovs_assert(column_idx < class->n_columns);
    ovs_assert(row->old == NULL ||
               row->table->modes[column_idx] & OVSDB_IDL_MONITOR);

    if (row->table->idl->verify_write_only && !write_only) {
        VLOG_ERR("Bug: Attempt to write to a read/write column (%s:%s) when"
                 " explicitly configured not to.", class->name, column->name);
        goto discard_datum;
    }

    /* If this is a write-only column and the datum being written is the same
     * as the one already there, just skip the update entirely.  This is worth
     * optimizing because we have a lot of columns that get periodically
     * refreshed into the database but don't actually change that often.
     *
     * We don't do this for read/write columns because that would break
     * atomicity of transactions--some other client might have written a
     * different value in that column since we read it.  (But if a whole
     * transaction only does writes of existing values, without making any real
     * changes, we will drop the whole transaction later in
     * ovsdb_idl_txn_commit().) */
    if (write_only && ovsdb_datum_equals(ovsdb_idl_read(row, column),
                                         datum, &column->type)) {
        goto discard_datum;
    }

    if (hmap_node_is_null(&row->txn_node)) {
        hmap_insert(&row->table->idl->txn->txn_rows, &row->txn_node,
                    uuid_hash(&row->uuid));
    }
    if (row->old == row->new) {
        row->new = xmalloc(class->n_columns * sizeof *row->new);
    }
    if (!row->written) {
        row->written = bitmap_allocate(class->n_columns);
    }
    if (bitmap_is_set(row->written, column_idx)) {
        ovsdb_datum_destroy(&row->new[column_idx], &column->type);
    } else {
        bitmap_set1(row->written, column_idx);
    }
    if (owns_datum) {
        row->new[column_idx] = *datum;
    } else {
        ovsdb_datum_clone(&row->new[column_idx], datum, &column->type);
    }
    (column->unparse)(row);
    (column->parse)(row, &row->new[column_idx]);
    return;

discard_datum:
    if (owns_datum) {
        ovsdb_datum_destroy(datum, &column->type);
    }
}

void
ovsdb_idl_txn_write(const struct ovsdb_idl_row *row,
                    const struct ovsdb_idl_column *column,
                    struct ovsdb_datum *datum)
{
    ovsdb_idl_txn_write__(row, column, datum, true);
}

void
ovsdb_idl_txn_write_clone(const struct ovsdb_idl_row *row,
                          const struct ovsdb_idl_column *column,
                          const struct ovsdb_datum *datum)
{
    ovsdb_idl_txn_write__(row, column,
                          CONST_CAST(struct ovsdb_datum *, datum), false);
}

/* Causes the original contents of 'column' in 'row_' to be verified as a
 * prerequisite to completing the transaction.  That is, if 'column' in 'row_'
 * changed (or if 'row_' was deleted) between the time that the IDL originally
 * read its contents and the time that the transaction commits, then the
 * transaction aborts and ovsdb_idl_txn_commit() returns TXN_AGAIN_WAIT or
 * TXN_AGAIN_NOW (depending on whether the database change has already been
 * received).
 *
 * The intention is that, to ensure that no transaction commits based on dirty
 * reads, an application should call ovsdb_idl_txn_verify() on each data item
 * read as part of a read-modify-write operation.
 *
 * In some cases ovsdb_idl_txn_verify() reduces to a no-op, because the current
 * value of 'column' is already known:
 *
 *   - If 'row_' is a row created by the current transaction (returned by
 *     ovsdb_idl_txn_insert()).
 *
 *   - If 'column' has already been modified (with ovsdb_idl_txn_write())
 *     within the current transaction.
 *
 * Because of the latter property, always call ovsdb_idl_txn_verify() *before*
 * ovsdb_idl_txn_write() for a given read-modify-write.
 *
 * A transaction must be in progress.
 *
 * Usually this function is used indirectly through one of the "verify"
 * functions generated by ovsdb-idlc. */
void
ovsdb_idl_txn_verify(const struct ovsdb_idl_row *row_,
                     const struct ovsdb_idl_column *column)
{
    struct ovsdb_idl_row *row = CONST_CAST(struct ovsdb_idl_row *, row_);
    const struct ovsdb_idl_table_class *class;
    size_t column_idx;

    if (ovsdb_idl_row_is_synthetic(row)) {
        return;
    }

    class = row->table->class;
    column_idx = column - class->columns;

    ovs_assert(row->new != NULL);
    ovs_assert(row->old == NULL ||
               row->table->modes[column_idx] & OVSDB_IDL_MONITOR);
    if (!row->old
        || (row->written && bitmap_is_set(row->written, column_idx))) {
        return;
    }

    if (hmap_node_is_null(&row->txn_node)) {
        hmap_insert(&row->table->idl->txn->txn_rows, &row->txn_node,
                    uuid_hash(&row->uuid));
    }
    if (!row->prereqs) {
        row->prereqs = bitmap_allocate(class->n_columns);
    }
    bitmap_set1(row->prereqs, column_idx);
}

/* Deletes 'row_' from its table.  May free 'row_', so it must not be
 * accessed afterward.
 *
 * A transaction must be in progress.
 *
 * Usually this function is used indirectly through one of the "delete"
 * functions generated by ovsdb-idlc. */
void
ovsdb_idl_txn_delete(const struct ovsdb_idl_row *row_)
{
    struct ovsdb_idl_row *row = CONST_CAST(struct ovsdb_idl_row *, row_);

    if (ovsdb_idl_row_is_synthetic(row)) {
        return;
    }

    ovs_assert(row->new != NULL);
    if (!row->old) {
        ovsdb_idl_row_unparse(row);
        ovsdb_idl_row_clear_new(row);
        ovs_assert(!row->prereqs);
        hmap_remove(&row->table->rows, &row->hmap_node);
        hmap_remove(&row->table->idl->txn->txn_rows, &row->txn_node);
        free(row);
        return;
    }
    if (hmap_node_is_null(&row->txn_node)) {
        hmap_insert(&row->table->idl->txn->txn_rows, &row->txn_node,
                    uuid_hash(&row->uuid));
    }
    ovsdb_idl_row_clear_new(row);
    row->new = NULL;
}

/* Inserts and returns a new row in the table with the specified 'class' in the
 * database with open transaction 'txn'.
 *
 * The new row is assigned a provisional UUID.  If 'uuid' is null then one is
 * randomly generated; otherwise 'uuid' should specify a randomly generated
 * UUID not otherwise in use.  ovsdb-server will assign a different UUID when
 * 'txn' is committed, but the IDL will replace any uses of the provisional
 * UUID in the data to be to be committed by the UUID assigned by
 * ovsdb-server.
 *
 * Usually this function is used indirectly through one of the "insert"
 * functions generated by ovsdb-idlc. */
const struct ovsdb_idl_row *
ovsdb_idl_txn_insert(struct ovsdb_idl_txn *txn,
                     const struct ovsdb_idl_table_class *class,
                     const struct uuid *uuid)
{
    struct ovsdb_idl_row *row = ovsdb_idl_row_create__(class);

    if (uuid) {
        ovs_assert(!ovsdb_idl_txn_get_row(txn, uuid));
        row->uuid = *uuid;
    } else {
        uuid_generate(&row->uuid);
    }

    row->table = ovsdb_idl_table_from_class(txn->idl, class);
    row->new = xmalloc(class->n_columns * sizeof *row->new);
    hmap_insert(&row->table->rows, &row->hmap_node, uuid_hash(&row->uuid));
    hmap_insert(&txn->txn_rows, &row->txn_node, uuid_hash(&row->uuid));
    return row;
}

static void
ovsdb_idl_txn_abort_all(struct ovsdb_idl *idl)
{
    struct ovsdb_idl_txn *txn;

    HMAP_FOR_EACH (txn, hmap_node, &idl->outstanding_txns) {
        ovsdb_idl_txn_complete(txn, TXN_TRY_AGAIN);
    }
}

static struct ovsdb_idl_txn *
ovsdb_idl_txn_find(struct ovsdb_idl *idl, const struct json *id)
{
    struct ovsdb_idl_txn *txn;

    HMAP_FOR_EACH_WITH_HASH (txn, hmap_node,
                             json_hash(id, 0), &idl->outstanding_txns) {
        if (json_equal(id, txn->request_id)) {
            return txn;
        }
    }
    return NULL;
}

static bool
check_json_type(const struct json *json, enum json_type type, const char *name)
{
    if (!json) {
        VLOG_WARN_RL(&syntax_rl, "%s is missing", name);
        return false;
    } else if (json->type != type) {
        VLOG_WARN_RL(&syntax_rl, "%s is %s instead of %s",
                     name, json_type_to_string(json->type),
                     json_type_to_string(type));
        return false;
    } else {
        return true;
    }
}

static bool
ovsdb_idl_txn_process_inc_reply(struct ovsdb_idl_txn *txn,
                                const struct json_array *results)
{
    struct json *count, *rows, *row, *column;
    struct shash *mutate, *select;

    if (txn->inc_index + 2 > results->n) {
        VLOG_WARN_RL(&syntax_rl, "reply does not contain enough operations "
                     "for increment (has %"PRIuSIZE", needs %u)",
                     results->n, txn->inc_index + 2);
        return false;
    }

    /* We know that this is a JSON object because the loop in
     * ovsdb_idl_txn_process_reply() checked. */
    mutate = json_object(results->elems[txn->inc_index]);
    count = shash_find_data(mutate, "count");
    if (!check_json_type(count, JSON_INTEGER, "\"mutate\" reply \"count\"")) {
        return false;
    }
    if (count->u.integer != 1) {
        VLOG_WARN_RL(&syntax_rl,
                     "\"mutate\" reply \"count\" is %lld instead of 1",
                     count->u.integer);
        return false;
    }

    select = json_object(results->elems[txn->inc_index + 1]);
    rows = shash_find_data(select, "rows");
    if (!check_json_type(rows, JSON_ARRAY, "\"select\" reply \"rows\"")) {
        return false;
    }
    if (rows->u.array.n != 1) {
        VLOG_WARN_RL(&syntax_rl, "\"select\" reply \"rows\" has %"PRIuSIZE" elements "
                     "instead of 1",
                     rows->u.array.n);
        return false;
    }
    row = rows->u.array.elems[0];
    if (!check_json_type(row, JSON_OBJECT, "\"select\" reply row")) {
        return false;
    }
    column = shash_find_data(json_object(row), txn->inc_column);
    if (!check_json_type(column, JSON_INTEGER,
                         "\"select\" reply inc column")) {
        return false;
    }
    txn->inc_new_value = column->u.integer;
    return true;
}

static bool
ovsdb_idl_txn_process_insert_reply(struct ovsdb_idl_txn_insert *insert,
                                   const struct json_array *results)
{
    static const struct ovsdb_base_type uuid_type = OVSDB_BASE_UUID_INIT;
    struct ovsdb_error *error;
    struct json *json_uuid;
    union ovsdb_atom uuid;
    struct shash *reply;

    if (insert->op_index >= results->n) {
        VLOG_WARN_RL(&syntax_rl, "reply does not contain enough operations "
                     "for insert (has %"PRIuSIZE", needs %u)",
                     results->n, insert->op_index);
        return false;
    }

    /* We know that this is a JSON object because the loop in
     * ovsdb_idl_txn_process_reply() checked. */
    reply = json_object(results->elems[insert->op_index]);
    json_uuid = shash_find_data(reply, "uuid");
    if (!check_json_type(json_uuid, JSON_ARRAY, "\"insert\" reply \"uuid\"")) {
        return false;
    }

    error = ovsdb_atom_from_json(&uuid, &uuid_type, json_uuid, NULL);
    if (error) {
        char *s = ovsdb_error_to_string(error);
        VLOG_WARN_RL(&syntax_rl, "\"insert\" reply \"uuid\" is not a JSON "
                     "UUID: %s", s);
        free(s);
        ovsdb_error_destroy(error);
        return false;
    }

    insert->real = uuid.uuid;

    return true;
}

static bool
ovsdb_idl_txn_process_reply(struct ovsdb_idl *idl,
                            const struct jsonrpc_msg *msg)
{
    struct ovsdb_idl_txn *txn;
    enum ovsdb_idl_txn_status status;

    txn = ovsdb_idl_txn_find(idl, msg->id);
    if (!txn) {
        return false;
    }

    if (msg->type == JSONRPC_ERROR) {
        status = TXN_ERROR;
    } else if (msg->result->type != JSON_ARRAY) {
        VLOG_WARN_RL(&syntax_rl, "reply to \"transact\" is not JSON array");
        status = TXN_ERROR;
    } else {
        struct json_array *ops = &msg->result->u.array;
        int hard_errors = 0;
        int soft_errors = 0;
        int lock_errors = 0;
        size_t i;

        for (i = 0; i < ops->n; i++) {
            struct json *op = ops->elems[i];

            if (op->type == JSON_NULL) {
                /* This isn't an error in itself but indicates that some prior
                 * operation failed, so make sure that we know about it. */
                soft_errors++;
            } else if (op->type == JSON_OBJECT) {
                struct json *error;

                error = shash_find_data(json_object(op), "error");
                if (error) {
                    if (error->type == JSON_STRING) {
                        if (!strcmp(error->u.string, "timed out")) {
                            soft_errors++;
                        } else if (!strcmp(error->u.string, "not owner")) {
                            lock_errors++;
                        } else if (strcmp(error->u.string, "aborted")) {
                            hard_errors++;
                            ovsdb_idl_txn_set_error_json(txn, op);
                        }
                    } else {
                        hard_errors++;
                        ovsdb_idl_txn_set_error_json(txn, op);
                        VLOG_WARN_RL(&syntax_rl,
                                     "\"error\" in reply is not JSON string");
                    }
                }
            } else {
                hard_errors++;
                ovsdb_idl_txn_set_error_json(txn, op);
                VLOG_WARN_RL(&syntax_rl,
                             "operation reply is not JSON null or object");
            }
        }

        if (!soft_errors && !hard_errors && !lock_errors) {
            struct ovsdb_idl_txn_insert *insert;

            if (txn->inc_table && !ovsdb_idl_txn_process_inc_reply(txn, ops)) {
                hard_errors++;
            }

            HMAP_FOR_EACH (insert, hmap_node, &txn->inserted_rows) {
                if (!ovsdb_idl_txn_process_insert_reply(insert, ops)) {
                    hard_errors++;
                }
            }
        }

        status = (hard_errors ? TXN_ERROR
                  : lock_errors ? TXN_NOT_LOCKED
                  : soft_errors ? TXN_TRY_AGAIN
                  : TXN_SUCCESS);
    }

    ovsdb_idl_txn_complete(txn, status);
    return true;
}

/* Returns the transaction currently active for 'row''s IDL.  A transaction
 * must currently be active. */
struct ovsdb_idl_txn *
ovsdb_idl_txn_get(const struct ovsdb_idl_row *row)
{
    struct ovsdb_idl_txn *txn = row->table->idl->txn;
    ovs_assert(txn != NULL);
    return txn;
}

/* Returns the IDL on which 'txn' acts. */
struct ovsdb_idl *
ovsdb_idl_txn_get_idl (struct ovsdb_idl_txn *txn)
{
    return txn->idl;
}

/* Blocks until 'idl' successfully connects to the remote database and
 * retrieves its contents. */
void
ovsdb_idl_get_initial_snapshot(struct ovsdb_idl *idl)
{
    while (1) {
        ovsdb_idl_run(idl);
        if (ovsdb_idl_has_ever_connected(idl)) {
            return;
        }
        ovsdb_idl_wait(idl);
        poll_block();
    }
}

/* If 'lock_name' is nonnull, configures 'idl' to obtain the named lock from
 * the database server and to avoid modifying the database when the lock cannot
 * be acquired (that is, when another client has the same lock).
 *
 * If 'lock_name' is NULL, drops the locking requirement and releases the
 * lock. */
void
ovsdb_idl_set_lock(struct ovsdb_idl *idl, const char *lock_name)
{
    ovs_assert(!idl->txn);
    ovs_assert(hmap_is_empty(&idl->outstanding_txns));

    if (idl->lock_name && (!lock_name || strcmp(lock_name, idl->lock_name))) {
        /* Release previous lock. */
        ovsdb_idl_send_unlock_request(idl);
        free(idl->lock_name);
        idl->lock_name = NULL;
        idl->is_lock_contended = false;
    }

    if (lock_name && !idl->lock_name) {
        /* Acquire new lock. */
        idl->lock_name = xstrdup(lock_name);
        ovsdb_idl_send_lock_request(idl);
    }
}

/* Returns true if 'idl' is configured to obtain a lock and owns that lock.
 *
 * Locking and unlocking happens asynchronously from the database client's
 * point of view, so the information is only useful for optimization (e.g. if
 * the client doesn't have the lock then there's no point in trying to write to
 * the database). */
bool
ovsdb_idl_has_lock(const struct ovsdb_idl *idl)
{
    return idl->has_lock;
}

/* Returns true if 'idl' is configured to obtain a lock but the database server
 * has indicated that some other client already owns the requested lock. */
bool
ovsdb_idl_is_lock_contended(const struct ovsdb_idl *idl)
{
    return idl->is_lock_contended;
}

static void
ovsdb_idl_update_has_lock(struct ovsdb_idl *idl, bool new_has_lock)
{
    if (new_has_lock && !idl->has_lock) {
        if (idl->state == IDL_S_MONITORING) {
            idl->change_seqno++;
        } else {
            /* We're setting up a session, so don't signal that the database
             * changed.  Finalizing the session will increment change_seqno
             * anyhow. */
        }
        idl->is_lock_contended = false;
    }
    idl->has_lock = new_has_lock;
}

static void
ovsdb_idl_send_lock_request__(struct ovsdb_idl *idl, const char *method,
                              struct json **idp)
{
    ovsdb_idl_update_has_lock(idl, false);

    json_destroy(idl->lock_request_id);
    idl->lock_request_id = NULL;

    if (jsonrpc_session_is_connected(idl->session)) {
        struct json *params;

        params = json_array_create_1(json_string_create(idl->lock_name));
        jsonrpc_session_send(idl->session,
                             jsonrpc_create_request(method, params, idp));
    }
}

static void
ovsdb_idl_send_lock_request(struct ovsdb_idl *idl)
{
    ovsdb_idl_send_lock_request__(idl, "lock", &idl->lock_request_id);
}

static void
ovsdb_idl_send_unlock_request(struct ovsdb_idl *idl)
{
    ovsdb_idl_send_lock_request__(idl, "unlock", NULL);
}

static void
ovsdb_idl_parse_lock_reply(struct ovsdb_idl *idl, const struct json *result)
{
    bool got_lock;

    json_destroy(idl->lock_request_id);
    idl->lock_request_id = NULL;

    if (result->type == JSON_OBJECT) {
        const struct json *locked;

        locked = shash_find_data(json_object(result), "locked");
        got_lock = locked && locked->type == JSON_TRUE;
    } else {
        got_lock = false;
    }

    ovsdb_idl_update_has_lock(idl, got_lock);
    if (!got_lock) {
        idl->is_lock_contended = true;
    }
}

static void
ovsdb_idl_parse_lock_notify(struct ovsdb_idl *idl,
                            const struct json *params,
                            bool new_has_lock)
{
    if (idl->lock_name
        && params->type == JSON_ARRAY
        && json_array(params)->n > 0
        && json_array(params)->elems[0]->type == JSON_STRING) {
        const char *lock_name = json_string(json_array(params)->elems[0]);

        if (!strcmp(idl->lock_name, lock_name)) {
            ovsdb_idl_update_has_lock(idl, new_has_lock);
            if (!new_has_lock) {
                idl->is_lock_contended = true;
            }
        }
    }
}

void
ovsdb_idl_loop_destroy(struct ovsdb_idl_loop *loop)
{
    if (loop) {
        ovsdb_idl_destroy(loop->idl);
    }
}

struct ovsdb_idl_txn *
ovsdb_idl_loop_run(struct ovsdb_idl_loop *loop)
{
    ovsdb_idl_run(loop->idl);
    loop->open_txn = (loop->committing_txn
                      || ovsdb_idl_get_seqno(loop->idl) == loop->skip_seqno
                      ? NULL
                      : ovsdb_idl_txn_create(loop->idl));
    return loop->open_txn;
}

void
ovsdb_idl_loop_commit_and_wait(struct ovsdb_idl_loop *loop)
{
    if (loop->open_txn) {
        loop->committing_txn = loop->open_txn;
        loop->open_txn = NULL;

        loop->precommit_seqno = ovsdb_idl_get_seqno(loop->idl);
    }

    struct ovsdb_idl_txn *txn = loop->committing_txn;
    if (txn) {
        enum ovsdb_idl_txn_status status = ovsdb_idl_txn_commit(txn);
        if (status != TXN_INCOMPLETE) {
            switch (status) {
            case TXN_TRY_AGAIN:
                /* We want to re-evaluate the database when it's changed from
                 * the contents that it had when we started the commit.  (That
                 * might have already happened.) */
                loop->skip_seqno = loop->precommit_seqno;
                if (ovsdb_idl_get_seqno(loop->idl) != loop->skip_seqno) {
                    poll_immediate_wake();
                }
                break;

            case TXN_SUCCESS:
                /* If the database has already changed since we started the
                 * commit, re-evaluate it immediately to avoid missing a change
                 * for a while. */
                if (ovsdb_idl_get_seqno(loop->idl) != loop->precommit_seqno) {
                    poll_immediate_wake();
                }
                break;

            case TXN_UNCHANGED:
            case TXN_ABORTED:
            case TXN_NOT_LOCKED:
            case TXN_ERROR:
                break;

            case TXN_UNCOMMITTED:
            case TXN_INCOMPLETE:
                OVS_NOT_REACHED();

            }
            ovsdb_idl_txn_destroy(txn);
            loop->committing_txn = NULL;
        }
    }

    ovsdb_idl_wait(loop->idl);
}