leveldb_wt.cc

/*-
 * Public Domain 2008-2014 WiredTiger, Inc.
 *
 * This is free and unencumbered software released into the public domain.
 *
 * Anyone is free to copy, modify, publish, use, compile, sell, or
 * distribute this software, either in source code form or as a compiled
 * binary, for any purpose, commercial or non-commercial, and by any
 * means.
 *
 * In jurisdictions that recognize copyright laws, the author or authors
 * of this software dedicate any and all copyright interest in the
 * software to the public domain. We make this dedication for the benefit
 * of the public at large and to the detriment of our heirs and
 * successors. We intend this dedication to be an overt act of
 * relinquishment in perpetuity of all present and future rights to this
 * software under copyright law.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
 * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
 * IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR
 * OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
 * ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
 * OTHER DEALINGS IN THE SOFTWARE.
 */

#include "leveldb_wt.h"
#include <errno.h>
#include <sys/stat.h>
#include <unistd.h>
#include <sstream>

#if HAVE_BASHOLEVELDB
namespace leveldb {
Value::~Value() {}

class StringValue : public Value {
 public:
  explicit StringValue(std::string& val) : value_(val) {}
  ~StringValue() {}

  StringValue& assign(const char* data, size_t size) {
    value_.assign(data, size);
    return *this;
  }

 private:
  std::string& value_;
};
}
#endif

Status leveldb::DestroyDB(const std::string& name, const Options& options) {
  WT_CONNECTION *conn;
  int ret, t_ret;
  /* If the database doesn't exist, there is nothing to destroy. */
  if (access((name + "/WiredTiger").c_str(), F_OK) != 0)
    return Status::OK();
  if ((ret = ::wiredtiger_open(name.c_str(), NULL, NULL, &conn)) != 0)
    return WiredTigerErrorToStatus(ret, NULL);
  WT_SESSION *session;
  if ((ret = conn->open_session(conn, NULL, NULL, &session)) != 0)
    goto cleanup;
  if ((ret = session->drop(session, WT_URI, "force")) != 0)
    goto cleanup;

cleanup:
  if ((t_ret = conn->close(conn, NULL)) != 0 && ret == 0)
    ret = t_ret;
  return WiredTigerErrorToStatus(ret, NULL);
}

Status leveldb::RepairDB(const std::string& dbname, const Options& options) {
  return Status::NotSupported("sorry!");
}

/* Destructors required for interfaces. */
leveldb::DB::~DB() {}
Snapshot::~Snapshot() {}

Status WiredTigerErrorToStatus(int wiredTigerError, const char *msg) {
  if (wiredTigerError == 0)
    return Status::OK();

  if (msg == NULL)
    msg = wiredtiger_strerror(wiredTigerError);

  if (wiredTigerError != WT_NOTFOUND)
    printf("Failing status: %d -> %s\n", wiredTigerError, msg);

  if (wiredTigerError == WT_NOTFOUND)
    return Status::NotFound(Slice(msg));
  else if (wiredTigerError == WT_ERROR || wiredTigerError == WT_PANIC)
    return Status::Corruption(Slice(msg));
  else if (wiredTigerError == ENOTSUP)
    return Status::NotSupported(Slice(msg));
  else if (wiredTigerError == EINVAL)
    return Status::InvalidArgument(Slice(msg));
  else if (wiredTigerError == EPERM || wiredTigerError == ENOENT ||
      wiredTigerError == EIO || wiredTigerError == EBADF ||
      wiredTigerError == EEXIST || wiredTigerError == ENOSPC)
    return Status::IOError(Slice(msg));
  else if (wiredTigerError == WT_ROLLBACK)
    return Status::IOError("ROLLBACK"); // TODO: Is this the best translation?
  else
    return Status::Corruption(Slice(msg));
}

/* Iterators, from leveldb/table/iterator.cc */
Iterator::Iterator() {
  cleanup_.function = NULL;
  cleanup_.next = NULL;
}

Iterator::~Iterator() {
  if (cleanup_.function != NULL) {
    (*cleanup_.function)(cleanup_.arg1, cleanup_.arg2);
    for (Cleanup* c = cleanup_.next; c != NULL; ) {
      (*c->function)(c->arg1, c->arg2);
      Cleanup* next = c->next;
      delete c;
      c = next;
    }
  }
}

void Iterator::RegisterCleanup(CleanupFunction func, void* arg1, void* arg2) {
  assert(func != NULL);
  Cleanup* c;
  if (cleanup_.function == NULL) {
    c = &cleanup_;
  } else {
    c = new Cleanup;
    c->next = cleanup_.next;
    cleanup_.next = c;
  }
  c->function = func;
  c->arg1 = arg1;
  c->arg2 = arg2;
}

namespace {
class EmptyIterator : public Iterator {
 public:
  EmptyIterator(const Status& s) : status_(s) { }
  virtual bool Valid() const { return false; }
  virtual void Seek(const Slice& target) { }
  virtual void SeekToFirst() { }
  virtual void SeekToLast() { }
  virtual void Next() { assert(false); }
  virtual void Prev() { assert(false); }
  Slice key() const { assert(false); return Slice(); }
  Slice value() const { assert(false); return Slice(); }
  virtual Status status() const { return status_; }
 private:
  Status status_;
};
}  // namespace

Iterator* NewEmptyIterator() {
  return new EmptyIterator(Status::OK());
}

Iterator* NewErrorIterator(const Status& status) {
  return new EmptyIterator(status);
}

namespace {
class FilterPolicyImpl : public FilterPolicy {
public:
  FilterPolicyImpl(int bits_per_key) : bits_per_key_(bits_per_key) {}
  ~FilterPolicyImpl() {}
  virtual const char *Name() const { return "FilterPolicyImpl"; }
  virtual void CreateFilter(const Slice *keys, int n, std::string *dst) const {}
  virtual bool KeyMayMatch(const Slice &key, const Slice &filter) const { return true; }

  int bits_per_key_;
};
};

namespace leveldb {
FilterPolicy::~FilterPolicy() {}

const FilterPolicy *NewBloomFilterPolicy(int bits_per_key) {
  return new FilterPolicyImpl(bits_per_key);
}
#if HAVE_BASHOLEVELDB
const FilterPolicy *NewBloomFilterPolicy2(int bits_per_key) {
  return NewBloomFilterPolicy(bits_per_key);
}
#endif

Cache::~Cache() {}
Cache *NewLRUCache(size_t capacity) {
  return new CacheImpl(capacity);
}
}

int
wtleveldb_create(
    WT_CONNECTION *conn, const Options &options, std::string const &uri)
{
  int ret;
  std::stringstream s_table;
  s_table << WT_TABLE_CONFIG;
  s_table << "internal_page_max=" << options.block_size << ",";
  s_table << "leaf_page_max=" << options.block_size << ",";
  s_table << "leaf_item_max=" << options.block_size / 4 << ",";
  if (options.compression == leveldb::kSnappyCompression)
    s_table << "block_compressor=snappy,";
#ifdef HAVE_ROCKSDB
  if (options.compression == leveldb::kZlibCompression)
    s_table << "block_compressor=zlib,";
#endif
  s_table << "lsm=(";
  s_table << "chunk_size=" << options.write_buffer_size << ",";
  if (options.filter_policy) {
    int bits = ((FilterPolicyImpl *)options.filter_policy)->bits_per_key_;
    s_table << "bloom_bit_count=" << bits << ",";
    // Approximate the optimal number of hashes
    s_table << "bloom_hash_count=" << (int)(0.6 * bits) << ",";
  }
  s_table << "),";
  WT_SESSION *session;
  std::string table_config = s_table.str();
  if ((ret = conn->open_session(conn, NULL, NULL, &session)) != 0)
    return (ret);
  if ((ret = session->create(session, uri.c_str(), table_config.c_str())) != 0)
    return (ret);
  if ((ret = session->close(session, NULL)) != 0)
    return (ret);

  return (0);
}

Status
leveldb::DB::Open(const Options &options, const std::string &name, leveldb::DB **dbptr)
{
  // Build the wiredtiger_open config.
  std::stringstream s_conn;
  s_conn << WT_CONN_CONFIG;
  if (options.create_if_missing) {
    (void)mkdir(name.c_str(), 0777);
    s_conn << "create,";
  }
  if (options.error_if_exists)
    s_conn << "exclusive,";
#ifndef HAVE_BUILTIN_EXTENSION_SNAPPY
  if (options.compression == kSnappyCompression)
    s_conn << "extensions=[libwiredtiger_snappy.so],";
#endif
#ifdef HAVE_ROCKSDB
#ifndef HAVE_BUILTIN_ZLIB
  if (options.compression == kZlibCompression)
    s_conn << "extensions=[libwiredtiger_zlib.so],";
#endif
#endif
  size_t cache_size = 2 * options.write_buffer_size;
  cache_size += (size_t)options.max_open_files * (4 << 20);
  if (options.block_cache)
    cache_size += ((CacheImpl *)options.block_cache)->capacity_;
  else
    cache_size += 100 << 20;
  s_conn << "cache_size=" << cache_size << ",";
  std::string conn_config = s_conn.str();

  WT_CONNECTION *conn;
  printf("Open: home %s config %s\r\n",name.c_str(),conn_config.c_str());
  int ret = ::wiredtiger_open(name.c_str(), NULL, conn_config.c_str(), &conn);
  if (ret == ENOENT)
    return Status::NotFound(Slice("Database does not exist."));
  else if (ret == EEXIST)
    return Status::NotFound(Slice("Database already exists."));
  else if (ret != 0)
    return WiredTigerErrorToStatus(ret, NULL);

  if (options.create_if_missing)
    ret = wtleveldb_create(conn, options, WT_URI);

  if (ret != 0) {
    conn->close(conn, NULL);
    return WiredTigerErrorToStatus(ret, NULL);
  }
  *dbptr = new DbImpl(conn);
  return Status::OK();
}

// Set the database entry for "key" to "value".  Returns OK on success,
// and a non-OK status on error.
// Note: consider setting options.sync = true.
Status
DbImpl::Put(const WriteOptions& options,
       const Slice& key, const Slice& value)
{
  WT_CURSOR *cursor = GetContext()->GetCursor();
  WT_ITEM item;

  item.data = key.data();
  item.size = key.size();
  cursor->set_key(cursor, &item);
  item.data = value.data();
  item.size = value.size();
  cursor->set_value(cursor, &item);
  int ret = cursor->insert(cursor);
  return WiredTigerErrorToStatus(ret, NULL);
}

// Remove the database entry (if any) for "key".  Returns OK on
// success, and a non-OK status on error.  It is not an error if "key"
// did not exist in the database.
// Note: consider setting options.sync = true.
Status
DbImpl::Delete(const WriteOptions& options, const Slice& key)
{
  WT_CURSOR *cursor = GetContext()->GetCursor();
  WT_ITEM item;

  item.data = key.data();
  item.size = key.size();
  cursor->set_key(cursor, &item);
  int ret = cursor->remove(cursor);
  // Reset the WiredTiger cursor so it doesn't keep any pages pinned. Track
  // failures in debug builds since we don't expect failure, but don't pass
  // failures on - it's not necessary for correct operation.
  if (ret == 0) {
    int t_ret = cursor->reset(cursor);
    assert(t_ret == 0);
  } else if (ret == WT_NOTFOUND)
    ret = 0;
  return WiredTigerErrorToStatus(ret, NULL);
}

void
WriteBatchHandler::Put(const Slice& key, const Slice& value) {
  WT_CURSOR *cursor = context_->GetCursor();
  WT_ITEM item;

  item.data = key.data();
  item.size = key.size();
  cursor->set_key(cursor, &item);
  item.data = value.data();
  item.size = value.size();
  cursor->set_value(cursor, &item);
  int ret = cursor->insert(cursor);
  if (ret != 0 && status_ == 0)
    status_ = ret;
}

void WriteBatchHandler::Delete(const Slice& key) {
  WT_CURSOR *cursor = context_->GetCursor();
  WT_ITEM item;

  item.data = key.data();
  item.size = key.size();
  cursor->set_key(cursor, &item);
  int ret = cursor->remove(cursor);
  if (ret != 0 && ret != WT_NOTFOUND && status_ == 0)
    status_ = ret;
}

// Apply the specified updates to the database.
// Returns OK on success, non-OK on failure.
// Note: consider setting options.sync = true.
Status
DbImpl::Write(const WriteOptions& options, WriteBatch* updates)
{
  const char *errmsg = NULL;
  Status status = Status::OK();
  OperationContext *context = GetContext();
  WT_SESSION *session = context->GetSession();
  int ret = 0, t_ret;

#ifdef HAVE_ROCKSDB
    int need_txn = (updates->Count() > 1);
#else
    int need_txn = 1;
#endif

  for (;;) {
    if (need_txn && (ret = session->begin_transaction(session, NULL)) != 0) {
      errmsg = "Begin transaction failed in Write batch";
      goto err;
    }

    WriteBatchHandler handler(this, context);
#if 0
    status = updates->Iterate(&handler);
#else
    try {
      status = updates->Iterate(&handler);
    } catch(...) {
      if (need_txn)
        (void)session->rollback_transaction(session, NULL);
      throw;
    }
#endif
    if (!status.ok() || (ret = handler.GetWiredTigerStatus()) != WT_ROLLBACK)
      break;
    // Roll back the transaction on deadlock so we can try again
    if (need_txn && (ret = session->rollback_transaction(session, NULL)) != 0) {
      errmsg = "Rollback transaction failed in Write batch";
      goto err;
    }
  }

  if (need_txn && status.ok() && ret == 0) {
    ret = session->commit_transaction(session, NULL);
  } else if (need_txn) {
    t_ret = session->rollback_transaction(session, NULL);
    if (ret == 0)
      ret = t_ret;
  }

err:
  if (status.ok() && ret != 0)
    status = WiredTigerErrorToStatus(ret, errmsg);
  return status;
}

// If the database contains an entry for "key" store the
// corresponding value in *value and return OK.
//
// If there is no entry for "key" leave *value unchanged and return
// a status for which Status::IsNotFound() returns true.
//
// May return some other Status on an error.
Status
DbImpl::Get(const ReadOptions& options,
       const Slice& key, std::string* value)
{
  WT_CURSOR *cursor = GetContext(options)->GetCursor();
  const char *errmsg = NULL;

  WT_ITEM item;
  item.data = key.data();
  item.size = key.size();
  cursor->set_key(cursor, &item);
  int ret = cursor->search(cursor);
  if (ret == 0) {
    ret = cursor->get_value(cursor, &item);
    if (ret == 0) {
      // Make a copy of the value to return, then the cursor can be reset
      *value = std::string((const char *)item.data, item.size);
      ret = cursor->reset(cursor);
    }
  } else if (ret == WT_NOTFOUND)
    errmsg = "DB::Get key not found";
  return WiredTigerErrorToStatus(ret, errmsg);
}

#if HAVE_BASHOLEVELDB
// If the database contains an entry for "key" store the
// corresponding value in *value and return OK.
//
// If there is no entry for "key" leave *value unchanged and return
// a status for which Status::IsNotFound() returns true.
//
// May return some other Status on an error.
Status
DbImpl::Get(const ReadOptions& options,
       const Slice& key, Value* value)
{
  const char *errmsg = NULL;

  WT_CURSOR *cursor = GetContext(options)->GetCursor();
  WT_ITEM item;
  item.data = key.data();
  item.size = key.size();
  cursor->set_key(cursor, &item);
  int ret = cursor->search(cursor);
  if (ret == 0) {
    ret = cursor->get_value(cursor, &item);
    if (ret == 0) {
      // This call makes a copy, reset the cursor afterwards.
      value->assign((const char *)item.data, item.size);
      ret = cursor->reset(cursor);
    }
  } else if (ret == WT_NOTFOUND)
    errmsg = "DB::Get key not found";
err:
  return WiredTigerErrorToStatus(ret, errmsg);
}
#endif

// Return a heap-allocated iterator over the contents of the database.
// The result of NewIterator() is initially invalid (caller must
// call one of the Seek methods on the iterator before using it).
//
// Caller should delete the iterator when it is no longer needed.
// The returned iterator should be deleted before this db is deleted.
Iterator *
DbImpl::NewIterator(const ReadOptions& options)
{
  /* Iterators own the cursor until they are closed. */
  OperationContext *context = GetContext(options);
  WT_CURSOR *c = context->GetCursor();
  context->SetCursor(NULL);
  return new IteratorImpl(this, c);
}

SnapshotImpl::SnapshotImpl(DbImpl *db) :
    Snapshot(), db_(db), context_(db->NewContext()), status_(Status::OK())
{
}

// Return a handle to the current DB state.  Iterators created with
// this handle will all observe a stable snapshot of the current DB
// state.  The caller must call ReleaseSnapshot(result) when the
// snapshot is no longer needed.
const Snapshot *
DbImpl::GetSnapshot()
{
  SnapshotImpl *si = new SnapshotImpl(this);
  WT_SESSION *session = si->GetContext()->GetSession();
  int ret = session->begin_transaction(session, NULL);
  assert(ret == 0);
  return si;
}

// Release a previously acquired snapshot.  The caller must not
// use "snapshot" after this call.
void
DbImpl::ReleaseSnapshot(const Snapshot* snapshot)
{
  SnapshotImpl *si =
    static_cast<SnapshotImpl *>(const_cast<Snapshot *>(snapshot));
  if (si != NULL) {
    // We started a transaction: we could commit it here, but it will be rolled
    // back automatically by closing the session, which we have to do anyway.
    int ret = si->GetContext()->Close();
    assert(ret == 0);
    delete si;
  }
}

// DB implementations can export properties about their state
// via this method.  If "property" is a valid property understood by this
// DB implementation, fills "*value" with its current value and returns
// true.  Otherwise returns false.
//
//
// Valid property names include:
//
//  "leveldb.num-files-at-level<N>" - return the number of files at level <N>,
//     where <N> is an ASCII representation of a level number (e.g. "0").
//  "leveldb.stats" - returns a multi-line string that describes statistics
//     about the internal operation of the DB.
//  "leveldb.sstables" - returns a multi-line string that describes all
//     of the sstables that make up the db contents.
bool
DbImpl::GetProperty(const Slice& property, std::string* value)
{
  /* Not supported */
  return false;
}

// For each i in [0,n-1], store in "sizes[i]", the approximate
// file system space used by keys in "[range[i].start .. range[i].limit)".
//
// Note that the returned sizes measure file system space usage, so
// if the user data compresses by a factor of ten, the returned
// sizes will be one-tenth the size of the corresponding user data size.
//
// The results may not include the sizes of recently written data.
void
DbImpl::GetApproximateSizes(const Range* range, int n,
         uint64_t* sizes)
{
  int i;

  /* XXX Not supported */
  for (i = 0; i < n; i++)
    sizes[i] = 1;
}

// Compact the underlying storage for the key range [*begin,*end].
// In particular, deleted and overwritten versions are discarded,
// and the data is rearranged to reduce the cost of operations
// needed to access the data.  This operation should typically only
// be invoked by users who understand the underlying implementation.
//
// begin==NULL is treated as a key before all keys in the database.
// end==NULL is treated as a key after all keys in the database.
// Therefore the following call will compact the entire database:
//    db->CompactRange(NULL, NULL);
void
DbImpl::CompactRange(const Slice* begin, const Slice* end)
{
  // The compact doesn't need a cursor, but the context always opens a
  // cursor when opening the session - so grab that, and use the session.
  WT_CURSOR *cursor = GetContext()->GetCursor();
  WT_SESSION *session = cursor->session;
  int ret = session->compact(session, WT_URI, NULL);
  assert(ret == 0);
}

// Suspends the background compaction thread.  This methods
// returns once suspended.
void
DbImpl::SuspendCompactions()
{
  /* Not supported */
}

// Resumes a suspended background compation thread.
void
DbImpl::ResumeCompactions()
{
  /* Not supported */
}

IteratorImpl::~IteratorImpl()
{
  if (cursor_ != NULL) {
    OperationContext *context = db_->GetContext();
    /*
     * If we are in the same thread where the iterator was opened, and there is
     * no cursor stashed there, return it.
     */
    if (cursor_->session == context->GetSession()) {
#ifdef HAVE_ROCKSDB
      if (context->GetCursor(id_) == NULL) {
        context->SetCursor(id_, cursor_);
        cursor_ = NULL;
      }
#else
      if (context->GetCursor() == NULL) {
        context->SetCursor(cursor_);
        cursor_ = NULL;
      }
#endif
    }
    if (cursor_ != NULL) {
      int ret = cursor_->close(cursor_);
      assert(ret == 0);
    }
  }
}

// Position at the first key in the source.  The iterator is Valid()
// after this call iff the source is not empty.
void
IteratorImpl::SeekToFirst()
{
  int ret;
  WT_ITEM item;

  if (!Status().ok())
    return;

  if ((ret = cursor_->reset(cursor_)) != 0) {
    SetError(ret);
    return;
  }
  ret = cursor_->next(cursor_);
  if (ret == WT_NOTFOUND) {
    valid_ = false;
    return;
  } else if (ret != 0) {
    SetError(ret);
    return;
  }
  if ((ret = cursor_->get_key(cursor_, &item)) != 0) {
    SetError(ret);
    return;
  }
  key_ = Slice((const char *)item.data, item.size);
  if ((ret = cursor_->get_value(cursor_, &item)) != 0) {
    SetError(ret);
    return;
  }
  value_ = Slice((const char *)item.data, item.size);
  valid_ = true;
}

// Position at the last key in the source.  The iterator is
// Valid() after this call iff the source is not empty.
void
IteratorImpl::SeekToLast()
{
  int ret;
  WT_ITEM item;

  if (!Status().ok())
    return;

  if ((ret = cursor_->reset(cursor_)) != 0) {
    SetError(ret);
    return;
  }
  ret = cursor_->prev(cursor_);
  if (ret == WT_NOTFOUND) {
    valid_ = false;
    return;
  } else if (ret != 0) {
    SetError(ret);
    return;
  }
  if ((ret = cursor_->get_key(cursor_, &item)) != 0) {
    SetError(ret);
    return;
  }
  key_ = Slice((const char *)item.data, item.size);
  if ((ret = cursor_->get_value(cursor_, &item)) != 0) {
    SetError(ret);
    return;
  }
  value_ = Slice((const char *)item.data, item.size);
  valid_ = true;
}

// Position at the first key in the source that at or past target
// The iterator is Valid() after this call iff the source contains
// an entry that comes at or past target.
void
IteratorImpl::Seek(const Slice& target)
{
  WT_ITEM item;

  if (!Status().ok())
    return;

  item.data = target.data();
  item.size = target.size();
  cursor_->set_key(cursor_, &item);
  int cmp, ret = cursor_->search_near(cursor_, &cmp);
  if (ret == 0 && cmp < 0)
    ret = cursor_->next(cursor_);
  if (ret != 0) {
    if (ret != WT_NOTFOUND)
      SetError(ret);
    valid_ = false;
    return;
  }
  if ((ret = cursor_->get_key(cursor_, &item)) != 0) {
    SetError(ret);
    return;
  }
  key_ = Slice((const char *)item.data, item.size);
  if ((ret = cursor_->get_value(cursor_, &item)) != 0) {
    SetError(ret);
    return;
  }
  value_ = Slice((const char *)item.data, item.size);
  valid_ = true;
}

// Moves to the next entry in the source.  After this call, Valid() is
// true iff the iterator was not positioned at the last entry in the source.
// REQUIRES: Valid()
void
IteratorImpl::Next()
{
  int ret;
  WT_ITEM item;

  if (!Status().ok() || !valid_)
    return;

  ret = cursor_->next(cursor_);
  if (ret != 0) {
    if (ret != WT_NOTFOUND)
      SetError(ret);
    valid_ = false;
    return;
  }
  if ((ret = cursor_->get_key(cursor_, &item)) != 0) {
    SetError(ret);
    return;
  }
  key_ = Slice((const char *)item.data, item.size);
  if ((ret = cursor_->get_value(cursor_, &item)) != 0) {
    SetError(ret);
    return;
  }
  value_ = Slice((const char *)item.data, item.size);
  valid_ = true;
}

// Moves to the previous entry in the source.  After this call, Valid() is
// true iff the iterator was not positioned at the first entry in source.
// REQUIRES: Valid()
void
IteratorImpl::Prev()
{
  WT_ITEM item;

  if (!Status().ok() || !valid_)
    return;

  int ret = cursor_->prev(cursor_);
  if (ret != 0) {
    if (ret != WT_NOTFOUND)
      SetError(ret);
    valid_ = false;
    return;
  }
  if ((ret = cursor_->get_key(cursor_, &item)) != 0) {
    SetError(ret);
    return;
  }
  key_ = Slice((const char *)item.data, item.size);
  if ((ret = cursor_->get_value(cursor_, &item)) != 0) {
    SetError(ret);
    return;
  }
  value_ = Slice((const char *)item.data, item.size);
  valid_ = true;
}