ovsdb-idl: Tolerate initialization races for singleton tables.

By verifying that singleton tables (that is, tables that should have exactly one row) are empty when they emit transactions that insert into them, ovs-vsctl and similar tools tolerate initialization races, where more than one client at a time tries to initialize a singleton table. The upshot is that if you create a database and then run multiple ovs-vsctl (etc.) commands against it in parallel (without first initializing it serially), then without this patch sometimes you will sometimes get failures but this patch avoids them. Signed-off-by: Ben Pfaff <[email protected]> Acked-by: Alin Gabriel Serdean <[email protected]>
marbindrakon · Dec 11, 2017 · 25540a7 · 25540a7
1 parent 7a36082
commit 25540a7
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Showing 3 changed files with 27 additions and 6 deletions.
diff --git a/lib/ovsdb-idl-provider.h b/lib/ovsdb-idl-provider.h
@@ -101,6 +101,7 @@ struct ovsdb_idl_column {
 struct ovsdb_idl_table_class {
     char *name;
     bool is_root;
+    bool is_singleton;
     const struct ovsdb_idl_column *columns;
     size_t n_columns;
     size_t allocation_size;

diff --git a/lib/ovsdb-idl.c b/lib/ovsdb-idl.c
@@ -3537,12 +3537,28 @@ ovsdb_idl_txn_commit(struct ovsdb_idl_txn *txn)
             }
         } else if (row->old_datum != row->new_datum) {
             struct json *row_json;
-            struct json *op;
             size_t idx;
 
-            op = json_object_create();
-            json_object_put_string(op, "op", row->old_datum ? "update"
-                                                            : "insert");
+            if (!row->old_datum && class->is_singleton) {
+                /* We're inserting a row into a table that allows only a
+                 * single row.  (This is a fairly common OVSDB pattern for
+                 * storing global data.)  Verify that the table is empty
+                 * before inserting the row, so that we get a clear
+                 * verification-related failure if there was an insertion
+                 * race with another client. */
+                struct json *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, "where", json_array_create_empty());
+                json_object_put(op, "timeout", json_integer_create(0));
+                json_object_put_string(op, "until", "==");
+                json_object_put(op, "rows", json_array_create_empty());
+            }
+
+            struct json *op = json_object_create();
+            json_object_put_string(op, "op",
+                                   row->old_datum ? "update" : "insert");
             json_object_put_string(op, "table", class->name);
             if (row->old_datum) {
                 json_object_put(op, "where", where_uuid_equals(&row->uuid));

diff --git a/ovsdb/ovsdb-idlc.in b/ovsdb/ovsdb-idlc.in
@@ -1298,9 +1298,13 @@ void
             is_root = "true"
         else:
             is_root = "false"
-        print("    {\"%s\", %s," % (tableName, is_root))
+        if table.max_rows == 1:
+            is_singleton = "true"
+        else:
+            is_singleton = "false"
+        print("    {\"%s\", %s, %s," % (tableName, is_root, is_singleton))
         print("     %s_columns, ARRAY_SIZE(%s_columns)," % (
-            structName, structName))
+              structName, structName))
         print("     sizeof(struct %s), %s_init__}," % (structName, structName))
     print("};")