content/browser/dom_storage/session_storage

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This source file includes following definitions.
DBOperation
DBOperation
operation_count_
ReadAreaValues
CommitAreaChanges
CloneNamespace
DeleteArea
DeleteNamespace
ReadNamespacesAndOrigins
LazyOpen
TryToOpen
IsOpen
CallerErrorCheck
ConsistencyCheck
DatabaseErrorCheck
CreateNamespace
GetAreasInNamespace
AddAreaToNamespace
DeleteAreaHelper
GetMapForArea
CreateMapForArea
ReadMap
WriteValuesToMap
GetMapRefCount
IncreaseMapRefCount
DecreaseMapRefCount
ClearMap
DeepCopyArea
NamespaceStartKey
NamespaceKey
NamespacePrefix
MapRefCountKey
MapKey
NextMapIdKey
// Copyright 2013 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include "content/browser/dom_storage/session_storage_database.h"

#include "base/file_util.h"
#include "base/logging.h"
#include "base/metrics/histogram.h"
#include "base/strings/string_number_conversions.h"
#include "base/strings/stringprintf.h"
#include "base/strings/utf_string_conversions.h"
#include "third_party/leveldatabase/src/include/leveldb/db.h"
#include "third_party/leveldatabase/src/include/leveldb/iterator.h"
#include "third_party/leveldatabase/src/include/leveldb/options.h"
#include "third_party/leveldatabase/src/include/leveldb/status.h"
#include "third_party/leveldatabase/src/include/leveldb/write_batch.h"
#include "url/gurl.h"


namespace {

const char session_storage_uma_name[] = "SessionStorageDatabase.Open";

enum SessionStorageUMA {
  SESSION_STORAGE_UMA_SUCCESS,
  SESSION_STORAGE_UMA_RECREATED,
  SESSION_STORAGE_UMA_FAIL,
  SESSION_STORAGE_UMA_MAX
};

}  // namespace

// Layout of the database:
// | key                            | value                              |
// -----------------------------------------------------------------------
// | map-1-                         | 2 (refcount, start of map-1-* keys)|
// | map-1-a                        | b (a = b in map 1)                 |
// | ...                            |                                    |
// | namespace-                     | dummy (start of namespace-* keys)  |
// | namespace-1- (1 = namespace id)| dummy (start of namespace-1-* keys)|
// | namespace-1-origin1            | 1 (mapid)                          |
// | namespace-1-origin2            | 2                                  |
// | namespace-2-                   | dummy                              |
// | namespace-2-origin1            | 1 (shallow copy)                   |
// | namespace-2-origin2            | 2 (shallow copy)                   |
// | namespace-3-                   | dummy                              |
// | namespace-3-origin1            | 3 (deep copy)                      |
// | namespace-3-origin2            | 2 (shallow copy)                   |
// | next-map-id                    | 4                                  |

namespace content {

// This class keeps track of ongoing operations across different threads. When
// DB inconsistency is detected, we need to 1) make sure no new operations start
// 2) wait until all current operations finish, and let the last one of them
// close the DB and delete the data. The DB will remain empty for the rest of
// the run, and will be recreated during the next run. We cannot hope to recover
// during this run, since the upper layer will have a different idea about what
// should be in the database.
class SessionStorageDatabase::DBOperation {
 public:
  DBOperation(SessionStorageDatabase* session_storage_database)
      : session_storage_database_(session_storage_database) {
    base::AutoLock auto_lock(session_storage_database_->db_lock_);
    ++session_storage_database_->operation_count_;
  }

  ~DBOperation() {
    base::AutoLock auto_lock(session_storage_database_->db_lock_);
    --session_storage_database_->operation_count_;
    if ((session_storage_database_->is_inconsistent_ ||
         session_storage_database_->db_error_) &&
        session_storage_database_->operation_count_ == 0 &&
        !session_storage_database_->invalid_db_deleted_) {
      // No other operations are ongoing and the data is bad -> delete it now.
      session_storage_database_->db_.reset();
#if defined(OS_WIN)
      leveldb::DestroyDB(
          base::WideToUTF8(session_storage_database_->file_path_.value()),
          leveldb::Options());
#else
      leveldb::DestroyDB(session_storage_database_->file_path_.value(),
                         leveldb::Options());
#endif
      session_storage_database_->invalid_db_deleted_ = true;
    }
  }

 private:
  SessionStorageDatabase* session_storage_database_;
};


SessionStorageDatabase::SessionStorageDatabase(const base::FilePath& file_path)
    : file_path_(file_path),
      db_error_(false),
      is_inconsistent_(false),
      invalid_db_deleted_(false),
      operation_count_(0) {
}

SessionStorageDatabase::~SessionStorageDatabase() {
}

void SessionStorageDatabase::ReadAreaValues(const std::string& namespace_id,
                                            const GURL& origin,
                                            DOMStorageValuesMap* result) {
  // We don't create a database if it doesn't exist. In that case, there is
  // nothing to be added to the result.
  if (!LazyOpen(false))
    return;
  DBOperation operation(this);

  // While ReadAreaValues is in progress, another thread can call
  // CommitAreaChanges. CommitAreaChanges might update map ref count key while
  // this thread is iterating over the map ref count key. To protect the reading
  // operation, create a snapshot and read from it.
  leveldb::ReadOptions options;
  options.snapshot = db_->GetSnapshot();

  std::string map_id;
  bool exists;
  if (GetMapForArea(namespace_id, origin.spec(), options, &exists, &map_id) &&
      exists)
    ReadMap(map_id, options, result, false);
  db_->ReleaseSnapshot(options.snapshot);
}

bool SessionStorageDatabase::CommitAreaChanges(
    const std::string& namespace_id,
    const GURL& origin,
    bool clear_all_first,
    const DOMStorageValuesMap& changes) {
  // Even if |changes| is empty, we need to write the appropriate placeholders
  // in the database, so that it can be later shallow-copied succssfully.
  if (!LazyOpen(true))
    return false;
  DBOperation operation(this);

  leveldb::WriteBatch batch;
  // Ensure that the keys "namespace-" "namespace-N" (see the schema above)
  // exist.
  const bool kOkIfExists = true;
  if (!CreateNamespace(namespace_id, kOkIfExists, &batch))
    return false;

  std::string map_id;
  bool exists;
  if (!GetMapForArea(namespace_id, origin.spec(), leveldb::ReadOptions(),
                     &exists, &map_id))
    return false;
  if (exists) {
    int64 ref_count;
    if (!GetMapRefCount(map_id, &ref_count))
      return false;
    if (ref_count > 1) {
      if (!DeepCopyArea(namespace_id, origin, !clear_all_first,
                        &map_id, &batch))
        return false;
    }
    else if (clear_all_first) {
      if (!ClearMap(map_id, &batch))
        return false;
    }
  } else {
    // Map doesn't exist, create it now if needed.
    if (!changes.empty()) {
      if (!CreateMapForArea(namespace_id, origin, &map_id, &batch))
        return false;
    }
  }

  WriteValuesToMap(map_id, changes, &batch);

  leveldb::Status s = db_->Write(leveldb::WriteOptions(), &batch);
  return DatabaseErrorCheck(s.ok());
}

bool SessionStorageDatabase::CloneNamespace(
    const std::string& namespace_id, const std::string& new_namespace_id) {
  // Go through all origins in the namespace |namespace_id|, create placeholders
  // for them in |new_namespace_id|, and associate them with the existing maps.

  // Example, data before shallow copy:
  // | map-1-                         | 1 (refcount)        |
  // | map-1-a                        | b                   |
  // | namespace-1- (1 = namespace id)| dummy               |
  // | namespace-1-origin1            | 1 (mapid)           |

  // Example, data after shallow copy:
  // | map-1-                         | 2 (inc. refcount)   |
  // | map-1-a                        | b                   |
  // | namespace-1-(1 = namespace id) | dummy               |
  // | namespace-1-origin1            | 1 (mapid)           |
  // | namespace-2-                   | dummy               |
  // | namespace-2-origin1            | 1 (mapid) << references the same map

  if (!LazyOpen(true))
    return false;
  DBOperation operation(this);

  leveldb::WriteBatch batch;
  const bool kOkIfExists = false;
  if (!CreateNamespace(new_namespace_id, kOkIfExists, &batch))
    return false;

  std::map<std::string, std::string> areas;
  if (!GetAreasInNamespace(namespace_id, &areas))
    return false;

  for (std::map<std::string, std::string>::const_iterator it = areas.begin();
       it != areas.end(); ++it) {
    const std::string& origin = it->first;
    const std::string& map_id = it->second;
    if (!IncreaseMapRefCount(map_id, &batch))
      return false;
    AddAreaToNamespace(new_namespace_id, origin, map_id, &batch);
  }
  leveldb::Status s = db_->Write(leveldb::WriteOptions(), &batch);
  return DatabaseErrorCheck(s.ok());
}

bool SessionStorageDatabase::DeleteArea(const std::string& namespace_id,
                                        const GURL& origin) {
  if (!LazyOpen(false)) {
    // No need to create the database if it doesn't exist.
    return true;
  }
  DBOperation operation(this);
  leveldb::WriteBatch batch;
  if (!DeleteAreaHelper(namespace_id, origin.spec(), &batch))
    return false;
  leveldb::Status s = db_->Write(leveldb::WriteOptions(), &batch);
  return DatabaseErrorCheck(s.ok());
}

bool SessionStorageDatabase::DeleteNamespace(const std::string& namespace_id) {
  if (!LazyOpen(false)) {
    // No need to create the database if it doesn't exist.
    return true;
  }
  DBOperation operation(this);
  // Itereate through the areas in the namespace.
  leveldb::WriteBatch batch;
  std::map<std::string, std::string> areas;
  if (!GetAreasInNamespace(namespace_id, &areas))
    return false;
  for (std::map<std::string, std::string>::const_iterator it = areas.begin();
       it != areas.end(); ++it) {
    const std::string& origin = it->first;
    if (!DeleteAreaHelper(namespace_id, origin, &batch))
      return false;
  }
  batch.Delete(NamespaceStartKey(namespace_id));
  leveldb::Status s = db_->Write(leveldb::WriteOptions(), &batch);
  return DatabaseErrorCheck(s.ok());
}

bool SessionStorageDatabase::ReadNamespacesAndOrigins(
    std::map<std::string, std::vector<GURL> >* namespaces_and_origins) {
  if (!LazyOpen(true))
    return false;
  DBOperation operation(this);

  // While ReadNamespacesAndOrigins is in progress, another thread can call
  // CommitAreaChanges. To protect the reading operation, create a snapshot and
  // read from it.
  leveldb::ReadOptions options;
  options.snapshot = db_->GetSnapshot();

  std::string namespace_prefix = NamespacePrefix();
  scoped_ptr<leveldb::Iterator> it(db_->NewIterator(options));
  it->Seek(namespace_prefix);
  // If the key is not found, the status of the iterator won't be IsNotFound(),
  // but the iterator will be invalid.
  if (!it->Valid()) {
    db_->ReleaseSnapshot(options.snapshot);
    return true;
  }

  if (!DatabaseErrorCheck(it->status().ok())) {
    db_->ReleaseSnapshot(options.snapshot);
    return false;
  }

  // Skip the dummy entry "namespace-" and iterate the namespaces.
  std::string current_namespace_start_key;
  std::string current_namespace_id;
  for (it->Next(); it->Valid(); it->Next()) {
    std::string key = it->key().ToString();
    if (key.find(namespace_prefix) != 0) {
      // Iterated past the "namespace-" keys.
      break;
    }
    // For each namespace, the first key is "namespace-<namespaceid>-", and the
    // subsequent keys are "namespace-<namespaceid>-<origin>". Read the unique
    // "<namespaceid>" parts from the keys.
    if (current_namespace_start_key.empty() ||
        key.substr(0, current_namespace_start_key.length()) !=
        current_namespace_start_key) {
      // The key is of the form "namespace-<namespaceid>-" for a new
      // <namespaceid>.
      current_namespace_start_key = key;
      current_namespace_id =
          key.substr(namespace_prefix.length(),
                     key.length() - namespace_prefix.length() - 1);
      // Ensure that we keep track of the namespace even if it doesn't contain
      // any origins.
      namespaces_and_origins->insert(
          std::make_pair(current_namespace_id, std::vector<GURL>()));
    } else {
      // The key is of the form "namespace-<namespaceid>-<origin>".
      std::string origin = key.substr(current_namespace_start_key.length());
      (*namespaces_and_origins)[current_namespace_id].push_back(GURL(origin));
    }
  }
  db_->ReleaseSnapshot(options.snapshot);
  return true;
}

bool SessionStorageDatabase::LazyOpen(bool create_if_needed) {
  base::AutoLock auto_lock(db_lock_);
  if (db_error_ || is_inconsistent_) {
    // Don't try to open a database that we know has failed already.
    return false;
  }
  if (IsOpen())
    return true;

  if (!create_if_needed &&
      (!base::PathExists(file_path_) || base::IsDirectoryEmpty(file_path_))) {
    // If the directory doesn't exist already and we haven't been asked to
    // create a file on disk, then we don't bother opening the database. This
    // means we wait until we absolutely need to put something onto disk before
    // we do so.
    return false;
  }

  leveldb::DB* db;
  leveldb::Status s = TryToOpen(&db);
  if (!s.ok()) {
    LOG(WARNING) << "Failed to open leveldb in " << file_path_.value()
                 << ", error: " << s.ToString();
    DCHECK(db == NULL);

    // Clear the directory and try again.
    base::DeleteFile(file_path_, true);
    s = TryToOpen(&db);
    if (!s.ok()) {
      LOG(WARNING) << "Failed to open leveldb in " << file_path_.value()
                   << ", error: " << s.ToString();
      UMA_HISTOGRAM_ENUMERATION(session_storage_uma_name,
                                SESSION_STORAGE_UMA_FAIL,
                                SESSION_STORAGE_UMA_MAX);
      DCHECK(db == NULL);
      db_error_ = true;
      return false;
    }
    UMA_HISTOGRAM_ENUMERATION(session_storage_uma_name,
                              SESSION_STORAGE_UMA_RECREATED,
                              SESSION_STORAGE_UMA_MAX);
  } else {
    UMA_HISTOGRAM_ENUMERATION(session_storage_uma_name,
                              SESSION_STORAGE_UMA_SUCCESS,
                              SESSION_STORAGE_UMA_MAX);
  }
  db_.reset(db);
  return true;
}

leveldb::Status SessionStorageDatabase::TryToOpen(leveldb::DB** db) {
  leveldb::Options options;
  // The directory exists but a valid leveldb database might not exist inside it
  // (e.g., a subset of the needed files might be missing). Handle this
  // situation gracefully by creating the database now.
  options.max_open_files = 0;  // Use minimum.
  options.create_if_missing = true;
#if defined(OS_WIN)
  return leveldb::DB::Open(options, base::WideToUTF8(file_path_.value()), db);
#elif defined(OS_POSIX)
  return leveldb::DB::Open(options, file_path_.value(), db);
#endif
}

bool SessionStorageDatabase::IsOpen() const {
  return db_.get() != NULL;
}

bool SessionStorageDatabase::CallerErrorCheck(bool ok) const {
  DCHECK(ok);
  return ok;
}

bool SessionStorageDatabase::ConsistencyCheck(bool ok) {
  if (ok)
    return true;
  base::AutoLock auto_lock(db_lock_);
  // We cannot recover the database during this run, e.g., the upper layer can
  // have a different understanding of the database state (shallow and deep
  // copies). Make further operations fail. The next operation that finishes
  // will delete the data, and next run will recerate the database.
  is_inconsistent_ = true;
  return false;
}

bool SessionStorageDatabase::DatabaseErrorCheck(bool ok) {
  if (ok)
    return true;
  base::AutoLock auto_lock(db_lock_);
  // Make further operations fail. The next operation that finishes
  // will delete the data, and next run will recerate the database.
  db_error_ = true;
  return false;
}

bool SessionStorageDatabase::CreateNamespace(const std::string& namespace_id,
                                             bool ok_if_exists,
                                             leveldb::WriteBatch* batch) {
  leveldb::Slice namespace_prefix = NamespacePrefix();
  std::string dummy;
  leveldb::Status s = db_->Get(leveldb::ReadOptions(), namespace_prefix,
                               &dummy);
  if (!DatabaseErrorCheck(s.ok() || s.IsNotFound()))
    return false;
  if (s.IsNotFound())
    batch->Put(namespace_prefix, "");

  std::string namespace_start_key = NamespaceStartKey(namespace_id);
  s = db_->Get(leveldb::ReadOptions(), namespace_start_key, &dummy);
  if (!DatabaseErrorCheck(s.ok() || s.IsNotFound()))
    return false;
  if (s.IsNotFound()) {
    batch->Put(namespace_start_key, "");
    return true;
  }
  return CallerErrorCheck(ok_if_exists);
}

bool SessionStorageDatabase::GetAreasInNamespace(
    const std::string& namespace_id,
    std::map<std::string, std::string>* areas) {
  std::string namespace_start_key = NamespaceStartKey(namespace_id);
  scoped_ptr<leveldb::Iterator> it(db_->NewIterator(leveldb::ReadOptions()));
  it->Seek(namespace_start_key);
  // If the key is not found, the status of the iterator won't be IsNotFound(),
  // but the iterator will be invalid.
  if (!it->Valid()) {
    // The namespace_start_key is not found when the namespace doesn't contain
    // any areas. We don't need to do anything.
    return true;
  }
  if (!DatabaseErrorCheck(it->status().ok()))
    return false;

  // Skip the dummy entry "namespace-<namespaceid>-" and iterate the origins.
  for (it->Next(); it->Valid(); it->Next()) {
    std::string key = it->key().ToString();
    if (key.find(namespace_start_key) != 0) {
      // Iterated past the origins for this namespace.
      break;
    }
    std::string origin = key.substr(namespace_start_key.length());
    std::string map_id = it->value().ToString();
    (*areas)[origin] = map_id;
  }
  return true;
}

void SessionStorageDatabase::AddAreaToNamespace(const std::string& namespace_id,
                                                const std::string& origin,
                                                const std::string& map_id,
                                                leveldb::WriteBatch* batch) {
  std::string namespace_key = NamespaceKey(namespace_id, origin);
  batch->Put(namespace_key, map_id);
}

bool SessionStorageDatabase::DeleteAreaHelper(
    const std::string& namespace_id,
    const std::string& origin,
    leveldb::WriteBatch* batch) {
  std::string map_id;
  bool exists;
  if (!GetMapForArea(namespace_id, origin, leveldb::ReadOptions(), &exists,
                     &map_id))
    return false;
  if (!exists)
    return true;  // Nothing to delete.
  if (!DecreaseMapRefCount(map_id, 1, batch))
    return false;
  std::string namespace_key = NamespaceKey(namespace_id, origin);
  batch->Delete(namespace_key);

  // If this was the only area in the namespace, delete the namespace start key,
  // too.
  std::string namespace_start_key = NamespaceStartKey(namespace_id);
  scoped_ptr<leveldb::Iterator> it(db_->NewIterator(leveldb::ReadOptions()));
  it->Seek(namespace_start_key);
  if (!ConsistencyCheck(it->Valid()))
    return false;
  // Advance the iterator 2 times (we still haven't really deleted
  // namespace_key).
  it->Next();
  if (!ConsistencyCheck(it->Valid()))
    return false;
  it->Next();
  if (!it->Valid())
    return true;
  std::string key = it->key().ToString();
  if (key.find(namespace_start_key) != 0)
    batch->Delete(namespace_start_key);
  return true;
}

bool SessionStorageDatabase::GetMapForArea(const std::string& namespace_id,
                                           const std::string& origin,
                                           const leveldb::ReadOptions& options,
                                           bool* exists, std::string* map_id) {
  std::string namespace_key = NamespaceKey(namespace_id, origin);
  leveldb::Status s = db_->Get(options, namespace_key, map_id);
  if (s.IsNotFound()) {
    *exists = false;
    return true;
  }
  *exists = true;
  return DatabaseErrorCheck(s.ok());
}

bool SessionStorageDatabase::CreateMapForArea(const std::string& namespace_id,
                                              const GURL& origin,
                                              std::string* map_id,
                                              leveldb::WriteBatch* batch) {
  leveldb::Slice next_map_id_key = NextMapIdKey();
  leveldb::Status s = db_->Get(leveldb::ReadOptions(), next_map_id_key, map_id);
  if (!DatabaseErrorCheck(s.ok() || s.IsNotFound()))
    return false;
  int64 next_map_id = 0;
  if (s.IsNotFound()) {
    *map_id = "0";
  } else {
    bool conversion_ok = base::StringToInt64(*map_id, &next_map_id);
    if (!ConsistencyCheck(conversion_ok))
      return false;
  }
  batch->Put(next_map_id_key, base::Int64ToString(++next_map_id));
  std::string namespace_key = NamespaceKey(namespace_id, origin.spec());
  batch->Put(namespace_key, *map_id);
  batch->Put(MapRefCountKey(*map_id), "1");
  return true;
}

bool SessionStorageDatabase::ReadMap(const std::string& map_id,
                                     const leveldb::ReadOptions& options,
                                     DOMStorageValuesMap* result,
                                     bool only_keys) {
  scoped_ptr<leveldb::Iterator> it(db_->NewIterator(options));
  std::string map_start_key = MapRefCountKey(map_id);
  it->Seek(map_start_key);
  // If the key is not found, the status of the iterator won't be IsNotFound(),
  // but the iterator will be invalid. The map needs to exist, otherwise we have
  // a stale map_id in the database.
  if (!ConsistencyCheck(it->Valid()))
    return false;
  if (!DatabaseErrorCheck(it->status().ok()))
    return false;
  // Skip the dummy entry "map-<mapid>-".
  for (it->Next(); it->Valid(); it->Next()) {
    std::string key = it->key().ToString();
    if (key.find(map_start_key) != 0) {
      // Iterated past the keys in this map.
      break;
    }
    // Key is of the form "map-<mapid>-<key>".
    base::string16 key16 =
        base::UTF8ToUTF16(key.substr(map_start_key.length()));
    if (only_keys) {
      (*result)[key16] = base::NullableString16();
    } else {
      // Convert the raw data stored in std::string (it->value()) to raw data
      // stored in base::string16.
      size_t len = it->value().size() / sizeof(base::char16);
      const base::char16* data_ptr =
          reinterpret_cast<const base::char16*>(it->value().data());
      (*result)[key16] =
          base::NullableString16(base::string16(data_ptr, len), false);
    }
  }
  return true;
}

void SessionStorageDatabase::WriteValuesToMap(const std::string& map_id,
                                              const DOMStorageValuesMap& values,
                                              leveldb::WriteBatch* batch) {
  for (DOMStorageValuesMap::const_iterator it = values.begin();
       it != values.end();
       ++it) {
    base::NullableString16 value = it->second;
    std::string key = MapKey(map_id, base::UTF16ToUTF8(it->first));
    if (value.is_null()) {
      batch->Delete(key);
    } else {
      // Convert the raw data stored in base::string16 to raw data stored in
      // std::string.
      const char* data = reinterpret_cast<const char*>(value.string().data());
      size_t size = value.string().size() * 2;
      batch->Put(key, leveldb::Slice(data, size));
    }
  }
}

bool SessionStorageDatabase::GetMapRefCount(const std::string& map_id,
                                            int64* ref_count) {
  std::string ref_count_string;
  leveldb::Status s = db_->Get(leveldb::ReadOptions(),
                               MapRefCountKey(map_id), &ref_count_string);
  if (!ConsistencyCheck(s.ok()))
    return false;
  bool conversion_ok = base::StringToInt64(ref_count_string, ref_count);
  return ConsistencyCheck(conversion_ok);
}

bool SessionStorageDatabase::IncreaseMapRefCount(const std::string& map_id,
                                                 leveldb::WriteBatch* batch) {
  // Increase the ref count for the map.
  int64 old_ref_count;
  if (!GetMapRefCount(map_id, &old_ref_count))
    return false;
  batch->Put(MapRefCountKey(map_id), base::Int64ToString(++old_ref_count));
  return true;
}

bool SessionStorageDatabase::DecreaseMapRefCount(const std::string& map_id,
                                                 int decrease,
                                                 leveldb::WriteBatch* batch) {
  // Decrease the ref count for the map.
  int64 ref_count;
  if (!GetMapRefCount(map_id, &ref_count))
    return false;
  if (!ConsistencyCheck(decrease <= ref_count))
    return false;
  ref_count -= decrease;
  if (ref_count > 0) {
    batch->Put(MapRefCountKey(map_id), base::Int64ToString(ref_count));
  } else {
    // Clear all keys in the map.
    if (!ClearMap(map_id, batch))
      return false;
    batch->Delete(MapRefCountKey(map_id));
  }
  return true;
}

bool SessionStorageDatabase::ClearMap(const std::string& map_id,
                                      leveldb::WriteBatch* batch) {
  DOMStorageValuesMap values;
  if (!ReadMap(map_id, leveldb::ReadOptions(), &values, true))
    return false;
  for (DOMStorageValuesMap::const_iterator it = values.begin();
       it != values.end(); ++it)
    batch->Delete(MapKey(map_id, base::UTF16ToUTF8(it->first)));
  return true;
}

bool SessionStorageDatabase::DeepCopyArea(
    const std::string& namespace_id, const GURL& origin, bool copy_data,
    std::string* map_id, leveldb::WriteBatch* batch) {
  // Example, data before deep copy:
  // | namespace-1- (1 = namespace id)| dummy               |
  // | namespace-1-origin1            | 1 (mapid)           |
  // | namespace-2-                   | dummy               |
  // | namespace-2-origin1            | 1 (mapid) << references the same map
  // | map-1-                         | 2 (refcount)        |
  // | map-1-a                        | b                   |

  // Example, data after deep copy copy:
  // | namespace-1-(1 = namespace id) | dummy               |
  // | namespace-1-origin1            | 1 (mapid)           |
  // | namespace-2-                   | dummy               |
  // | namespace-2-origin1            | 2 (mapid) << references the new map
  // | map-1-                         | 1 (dec. refcount)   |
  // | map-1-a                        | b                   |
  // | map-2-                         | 1 (refcount)        |
  // | map-2-a                        | b                   |

  // Read the values from the old map here. If we don't need to copy the data,
  // this can stay empty.
  DOMStorageValuesMap values;
  if (copy_data && !ReadMap(*map_id, leveldb::ReadOptions(), &values, false))
    return false;
  if (!DecreaseMapRefCount(*map_id, 1, batch))
    return false;
  // Create a new map (this will also break the association to the old map) and
  // write the old data into it. This will write the id of the created map into
  // |map_id|.
  if (!CreateMapForArea(namespace_id, origin, map_id, batch))
    return false;
  WriteValuesToMap(*map_id, values, batch);
  return true;
}

std::string SessionStorageDatabase::NamespaceStartKey(
    const std::string& namespace_id) {
  return base::StringPrintf("namespace-%s-", namespace_id.c_str());
}

std::string SessionStorageDatabase::NamespaceKey(
    const std::string& namespace_id, const std::string& origin) {
  return base::StringPrintf("namespace-%s-%s", namespace_id.c_str(),
                            origin.c_str());
}

const char* SessionStorageDatabase::NamespacePrefix() {
  return "namespace-";
}

std::string SessionStorageDatabase::MapRefCountKey(const std::string& map_id) {
  return base::StringPrintf("map-%s-", map_id.c_str());
}

std::string SessionStorageDatabase::MapKey(const std::string& map_id,
                                           const std::string& key) {
  return base::StringPrintf("map-%s-%s", map_id.c_str(), key.c_str());
}

const char* SessionStorageDatabase::NextMapIdKey() {
  return "next-map-id";
}

}  // namespace content
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root/content/browser/dom_storage/session_storage_database.cc

DEFINITIONS
