Source/WebKitLegacy/Storage/StorageAreaSync.cpp - WebKit - Git at Google

 /*
  * Copyright (C) 2008, 2009, 2010 Apple Inc. All Rights Reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
  *    documentation and/or other materials provided with the distribution.
  *
  * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
  * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
  * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL APPLE INC. OR
  * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
  * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
  * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
  * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
  * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */

 #include "StorageAreaSync.h"

 #include "StorageAreaImpl.h"
 #include "StorageSyncManager.h"
 #include "StorageTracker.h"
 #include <WebCore/SQLiteDatabaseTracker.h>
 #include <WebCore/SQLiteStatement.h>
 #include <WebCore/SQLiteTransaction.h>
 #include <WebCore/SuddenTermination.h>
 #include <wtf/FileSystem.h>
 #include <wtf/MainThread.h>

 using namespace WebCore;

 namespace WebKit {

 // If the StorageArea undergoes rapid changes, don't sync each change to disk.
 // Instead, queue up a batch of items to sync and actually do the sync at the following interval.
 static const Seconds StorageSyncInterval { 1_s };

 // A sane limit on how many items we'll schedule to sync all at once.  This makes it
 // much harder to starve the rest of LocalStorage and the OS's IO subsystem in general.
 static const int MaxiumItemsToSync = 100;

 inline StorageAreaSync::StorageAreaSync(RefPtr<StorageSyncManager>&& storageSyncManager, Ref<StorageAreaImpl>&& storageArea, const String& databaseIdentifier)
     : m_syncTimer(*this, &StorageAreaSync::syncTimerFired)
     , m_itemsCleared(false)
     , m_finalSyncScheduled(false)
     , m_storageArea(WTFMove(storageArea))
     , m_syncManager(WTFMove(storageSyncManager))
     , m_databaseIdentifier(databaseIdentifier.isolatedCopy())
     , m_clearItemsWhileSyncing(false)
     , m_syncScheduled(false)
     , m_syncInProgress(false)
     , m_databaseOpenFailed(false)
     , m_syncCloseDatabase(false)
     , m_importComplete(false)
 {
     ASSERT(isMainThread());
     ASSERT(m_storageArea);
     ASSERT(m_syncManager);

     // FIXME: If it can't import, then the default WebKit behavior should be that of private browsing,
     // not silently ignoring it. https://bugs.webkit.org/show_bug.cgi?id=25894
     RefPtr<StorageAreaSync> protector(this);
     m_syncManager->dispatch([protector] {
         protector->performImport();
     });
 }

 Ref<StorageAreaSync> StorageAreaSync::create(RefPtr<StorageSyncManager>&& storageSyncManager, Ref<StorageAreaImpl>&& storageArea, const String& databaseIdentifier)
 {
     return adoptRef(*new StorageAreaSync(WTFMove(storageSyncManager), WTFMove(storageArea), databaseIdentifier));
 }

 StorageAreaSync::~StorageAreaSync()
 {
     ASSERT(isMainThread());
     ASSERT(!m_syncTimer.isActive());
     ASSERT(m_finalSyncScheduled);
 }

 void StorageAreaSync::scheduleFinalSync()
 {
     ASSERT(isMainThread());
     // FIXME: We do this to avoid races, but it'd be better to make things safe without blocking.
     blockUntilImportComplete();
     m_storageArea = nullptr; // This is done in blockUntilImportComplete() but this is here as a form of documentation that we must be absolutely sure the ref count cycle is broken.

     if (m_syncTimer.isActive())
         m_syncTimer.stop();
     else {
         // The following is balanced by the call to enableSuddenTermination in the
         // syncTimerFired function.
         disableSuddenTermination();
     }
     // FIXME: This is synchronous. We should do it on the background process, but
     // we should do it safely.
     m_finalSyncScheduled = true;
     syncTimerFired();

     RefPtr<StorageAreaSync> protector(this);
     m_syncManager->dispatch([protector] {
         protector->deleteEmptyDatabase();
     });
 }

 void StorageAreaSync::scheduleItemForSync(const String& key, const String& value)
 {
     ASSERT(isMainThread());
     ASSERT(!m_finalSyncScheduled);

     m_changedItems.set(key, value);
     if (!m_syncTimer.isActive()) {
         m_syncTimer.startOneShot(StorageSyncInterval);

         // The following is balanced by the call to enableSuddenTermination in the
         // syncTimerFired function.
         disableSuddenTermination();
     }
 }

 void StorageAreaSync::scheduleClear()
 {
     ASSERT(isMainThread());
     ASSERT(!m_finalSyncScheduled);

     m_changedItems.clear();
     m_itemsCleared = true;
     if (!m_syncTimer.isActive()) {
         m_syncTimer.startOneShot(StorageSyncInterval);

         // The following is balanced by the call to enableSuddenTermination in the
         // syncTimerFired function.
         disableSuddenTermination();
     }
 }

 void StorageAreaSync::scheduleCloseDatabase()
 {
     ASSERT(isMainThread());
     ASSERT(!m_finalSyncScheduled);

     if (!m_database.isOpen())
         return;

     m_syncCloseDatabase = true;

     if (!m_syncTimer.isActive()) {
         m_syncTimer.startOneShot(StorageSyncInterval);

         // The following is balanced by the call to enableSuddenTermination in the
         // syncTimerFired function.
         disableSuddenTermination();
     }
 }

 void StorageAreaSync::syncTimerFired()
 {
     ASSERT(isMainThread());

     bool partialSync = false;
     {
         Locker locker { m_syncLock };

         // Do not schedule another sync if we're still trying to complete the
         // previous one. But, if we're shutting down, schedule it anyway.
         if (m_syncInProgress && !m_finalSyncScheduled) {
             ASSERT(!m_syncTimer.isActive());
             m_syncTimer.startOneShot(StorageSyncInterval);
             return;
         }

         if (m_itemsCleared) {
             m_itemsPendingSync.clear();
             m_clearItemsWhileSyncing = true;
             m_itemsCleared = false;
         }

         HashMap<String, String>::iterator changed_it = m_changedItems.begin();
         HashMap<String, String>::iterator changed_end = m_changedItems.end();
         for (int count = 0; changed_it != changed_end; ++count, ++changed_it) {
             if (count >= MaxiumItemsToSync && !m_finalSyncScheduled) {
                 partialSync = true;
                 break;
             }
             m_itemsPendingSync.set(changed_it->key.isolatedCopy(), changed_it->value.isolatedCopy());
         }

         if (partialSync) {
             // We can't do the fast path of simply clearing all items, so we'll need to manually
             // remove them one by one. Done under lock since m_itemsPendingSync is modified by
             // the background thread.
             HashMap<String, String>::iterator pending_it = m_itemsPendingSync.begin();
             HashMap<String, String>::iterator pending_end = m_itemsPendingSync.end();
             for (; pending_it != pending_end; ++pending_it)
                 m_changedItems.remove(pending_it->key);
         }

         if (!m_syncScheduled) {
             m_syncScheduled = true;

             // The following is balanced by the call to enableSuddenTermination in the
             // performSync function.
             disableSuddenTermination();

             RefPtr<StorageAreaSync> protector(this);
             m_syncManager->dispatch([protector] {
                 protector->performSync();
             });
         }
     }

     if (partialSync) {
         // If we didn't finish syncing, then we need to finish the job later.
         ASSERT(!m_syncTimer.isActive());
         m_syncTimer.startOneShot(StorageSyncInterval);
     } else {
         // The following is balanced by the calls to disableSuddenTermination in the
         // scheduleItemForSync, scheduleClear, and scheduleFinalSync functions.
         enableSuddenTermination();

         m_changedItems.clear();
     }
 }

 void StorageAreaSync::openDatabase(OpenDatabaseParamType openingStrategy)
 {
     ASSERT(!isMainThread());
     ASSERT(!m_database.isOpen());
     ASSERT(!m_databaseOpenFailed);

     SQLiteTransactionInProgressAutoCounter transactionCounter;

     String databaseFilename = m_syncManager->fullDatabaseFilename(m_databaseIdentifier);

     if (!FileSystem::fileExists(databaseFilename) && openingStrategy == SkipIfNonExistent)
         return;

     if (databaseFilename.isEmpty()) {
         LOG_ERROR("Filename for local storage database is empty - cannot open for persistent storage");
         markImported();
         m_databaseOpenFailed = true;
         return;
     }

     // A StorageTracker thread may have been scheduled to delete the db we're
     // reopening, so cancel possible deletion.
     StorageTracker::tracker().cancelDeletingOrigin(m_databaseIdentifier);

     if (!m_database.open(databaseFilename)) {
         LOG_ERROR("Failed to open database file %s for local storage", databaseFilename.utf8().data());
         markImported();
         m_databaseOpenFailed = true;
         return;
     }

     migrateItemTableIfNeeded();

     if (!m_database.executeCommand("CREATE TABLE IF NOT EXISTS ItemTable (key TEXT UNIQUE ON CONFLICT REPLACE, value BLOB NOT NULL ON CONFLICT FAIL)"_s)) {
         LOG_ERROR("Failed to create table ItemTable for local storage");
         markImported();
         m_databaseOpenFailed = true;
         return;
     }

     StorageTracker::tracker().setOriginDetails(m_databaseIdentifier, databaseFilename);
 }

 void StorageAreaSync::migrateItemTableIfNeeded()
 {
     if (!m_database.tableExists("ItemTable"))
         return;

     {
         auto query = m_database.prepareStatement("SELECT value FROM ItemTable LIMIT 1"_s);
         // this query isn't ever executed.
         if (query && query->isColumnDeclaredAsBlob(0))
             return;
     }

     // alter table for backward compliance, change the value type from TEXT to BLOB.
     static const ASCIILiteral commands[] = {
         "DROP TABLE IF EXISTS ItemTable2"_s,
         "CREATE TABLE ItemTable2 (key TEXT UNIQUE ON CONFLICT REPLACE, value BLOB NOT NULL ON CONFLICT FAIL)"_s,
         "INSERT INTO ItemTable2 SELECT * from ItemTable"_s,
         "DROP TABLE ItemTable"_s,
         "ALTER TABLE ItemTable2 RENAME TO ItemTable"_s,
         ASCIILiteral::null(),
     };

     SQLiteTransaction transaction(m_database, false);
     transaction.begin();
     for (size_t i = 0; commands[i]; ++i) {
         if (!m_database.executeCommand(commands[i])) {
             LOG_ERROR("Failed to migrate table ItemTable for local storage when executing: %s", commands[i].characters());
             transaction.rollback();

             // finally it will try to keep a backup of ItemTable for the future restoration.
             // NOTICE: this will essentially DELETE the current database, but that's better
             // than continually hitting this case and never being able to use the local storage.
             // if this is ever hit, it's definitely a bug.
             ASSERT_NOT_REACHED();
             if (!m_database.executeCommand("ALTER TABLE ItemTable RENAME TO Backup_ItemTable"_s))
                 LOG_ERROR("Failed to save ItemTable after migration job failed.");

             return;
         }
     }
     transaction.commit();
 }

 void StorageAreaSync::performImport()
 {
     ASSERT(!isMainThread());
     ASSERT(!m_database.isOpen());

     openDatabase(SkipIfNonExistent);
     if (!m_database.isOpen()) {
         markImported();
         return;
     }

     auto query = m_database.prepareStatement("SELECT key, value FROM ItemTable"_s);
     if (!query) {
         LOG_ERROR("Unable to select items from ItemTable for local storage");
         markImported();
         return;
     }

     HashMap<String, String> itemMap;

     int result = query->step();
     while (result == SQLITE_ROW) {
         itemMap.set(query->columnText(0), query->columnBlobAsString(1));
         result = query->step();
     }

     if (result != SQLITE_DONE) {
         LOG_ERROR("Error reading items from ItemTable for local storage");
         markImported();
         return;
     }

     m_storageArea->importItems(WTFMove(itemMap));

     markImported();
 }

 void StorageAreaSync::markImported()
 {
     Locker locker { m_importLock };
     m_importComplete = true;
     m_importCondition.notifyOne();
 }

 // FIXME: In the future, we should allow use of StorageAreas while it's importing (when safe to do so).
 // Blocking everything until the import is complete is by far the simplest and safest thing to do, but
 // there is certainly room for safe optimization: Key/length will never be able to make use of such an
 // optimization (since the order of iteration can change as items are being added). Get can return any
 // item currently in the map. Get/remove can work whether or not it's in the map, but we'll need a list
 // of items the import should not overwrite. Clear can also work, but it'll need to kill the import
 // job first.
 void StorageAreaSync::blockUntilImportComplete()
 {
     ASSERT(isMainThread());

     // Fast path. We set m_storageArea to 0 only after m_importComplete being true.
     if (!m_storageArea)
         return;

     Locker locker { m_importLock };
     while (!m_importComplete)
         m_importCondition.wait(m_importLock);
     m_storageArea = nullptr;
 }

 void StorageAreaSync::sync(bool clearItems, const HashMap<String, String>& items)
 {
     ASSERT(!isMainThread());

     if (items.isEmpty() && !clearItems && !m_syncCloseDatabase)
         return;
     if (m_databaseOpenFailed)
         return;

     if (!m_database.isOpen() && m_syncCloseDatabase) {
         m_syncCloseDatabase = false;
         return;
     }

     if (!m_database.isOpen())
         openDatabase(CreateIfNonExistent);
     if (!m_database.isOpen())
         return;

     // Closing this db because it is about to be deleted by StorageTracker.
     // The delete will be cancelled if StorageAreaSync needs to reopen the db
     // to write new items created after the request to delete the db.
     if (m_syncCloseDatabase) {
         m_syncCloseDatabase = false;
         m_database.close();
         return;
     }

     SQLiteTransactionInProgressAutoCounter transactionCounter;

     // If the clear flag is set, then we clear all items out before we write any new ones in.
     if (clearItems) {
         auto clear = m_database.prepareStatement("DELETE FROM ItemTable"_s);
         if (!clear) {
             LOG_ERROR("Failed to prepare clear statement - cannot write to local storage database");
             return;
         }

         int result = clear->step();
         if (result != SQLITE_DONE) {
             LOG_ERROR("Failed to clear all items in the local storage database - %i", result);
             return;
         }
     }

     auto insert = m_database.prepareStatement("INSERT INTO ItemTable VALUES (?, ?)"_s);
     if (!insert) {
         LOG_ERROR("Failed to prepare insert statement - cannot write to local storage database");
         return;
     }

     auto remove = m_database.prepareStatement("DELETE FROM ItemTable WHERE key=?"_s);
     if (!remove) {
         LOG_ERROR("Failed to prepare delete statement - cannot write to local storage database");
         return;
     }

     HashMap<String, String>::const_iterator end = items.end();

     SQLiteTransaction transaction(m_database);
     transaction.begin();
     for (HashMap<String, String>::const_iterator it = items.begin(); it != end; ++it) {
         // Based on the null-ness of the second argument, decide whether this is an insert or a delete.
         auto& query = it->value.isNull() ? remove : insert;

         query->bindText(1, it->key);

         // If the second argument is non-null, we're doing an insert, so bind it as the value.
         if (!it->value.isNull())
             query->bindBlob(2, it->value);

         int result = query->step();
         if (result != SQLITE_DONE) {
             LOG_ERROR("Failed to update item in the local storage database - %i", result);
             break;
         }

         query->reset();
     }
     transaction.commit();
 }

 void StorageAreaSync::performSync()
 {
     ASSERT(!isMainThread());

     bool clearItems;
     HashMap<String, String> items;
     {
         Locker locker { m_syncLock };

         ASSERT(m_syncScheduled);

         clearItems = m_clearItemsWhileSyncing;
         m_itemsPendingSync.swap(items);

         m_clearItemsWhileSyncing = false;
         m_syncScheduled = false;
         m_syncInProgress = true;
     }

     sync(clearItems, items);

     {
         Locker locker { m_syncLock };
         m_syncInProgress = false;
     }

     // The following is balanced by the call to disableSuddenTermination in the
     // syncTimerFired function.
     enableSuddenTermination();
 }

 void StorageAreaSync::deleteEmptyDatabase()
 {
     ASSERT(!isMainThread());
     if (!m_database.isOpen())
         return;

     auto count = [&] {
         auto query = m_database.prepareStatement("SELECT COUNT(*) FROM ItemTable"_s);
         if (!query) {
             LOG_ERROR("Unable to count number of rows in ItemTable for local storage");
             return -1;
         }

         int result = query->step();
         if (result != SQLITE_ROW) {
             LOG_ERROR("No results when counting number of rows in ItemTable for local storage");
             return -1;
         }

         return query->columnInt(0);
     }();
     if (count)
         return;

     m_database.close();
     if (StorageTracker::tracker().isActive()) {
         callOnMainThread([databaseIdentifier = m_databaseIdentifier.isolatedCopy()] {
             StorageTracker::tracker().deleteOriginWithIdentifier(databaseIdentifier);
         });
     } else {
         String databaseFilename = m_syncManager->fullDatabaseFilename(m_databaseIdentifier);
         if (!FileSystem::deleteFile(databaseFilename))
             LOG_ERROR("Failed to delete database file %s\n", databaseFilename.utf8().data());
     }
 }

 void StorageAreaSync::scheduleSync()
 {
     syncTimerFired();
 }

 } // namespace WebCore
	/*
	* Copyright (C) 2008, 2009, 2010 Apple Inc. All Rights Reserved.
	*
	* Redistribution and use in source and binary forms, with or without
	* modification, are permitted provided that the following conditions
	* are met:
	* 1. Redistributions of source code must retain the above copyright
	* notice, this list of conditions and the following disclaimer.
	* 2. Redistributions in binary form must reproduce the above copyright
	* notice, this list of conditions and the following disclaimer in the
	* documentation and/or other materials provided with the distribution.
	*
	* THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
	* EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
	* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
	* PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR
	* CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
	* EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
	* PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
	* PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
	* OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
	* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
	* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
	*/

	#include "StorageAreaSync.h"

	#include "StorageAreaImpl.h"
	#include "StorageSyncManager.h"
	#include "StorageTracker.h"
	#include <WebCore/SQLiteDatabaseTracker.h>
	#include <WebCore/SQLiteStatement.h>
	#include <WebCore/SQLiteTransaction.h>
	#include <WebCore/SuddenTermination.h>
	#include <wtf/FileSystem.h>
	#include <wtf/MainThread.h>

	using namespace WebCore;

	namespace WebKit {

	// If the StorageArea undergoes rapid changes, don't sync each change to disk.
	// Instead, queue up a batch of items to sync and actually do the sync at the following interval.
	static const Seconds StorageSyncInterval { 1_s };

	// A sane limit on how many items we'll schedule to sync all at once. This makes it
	// much harder to starve the rest of LocalStorage and the OS's IO subsystem in general.
	static const int MaxiumItemsToSync = 100;

	inline StorageAreaSync::StorageAreaSync(RefPtr<StorageSyncManager>&& storageSyncManager, Ref<StorageAreaImpl>&& storageArea, const String& databaseIdentifier)
	: m_syncTimer(*this, &StorageAreaSync::syncTimerFired)
	, m_itemsCleared(false)
	, m_finalSyncScheduled(false)
	, m_storageArea(WTFMove(storageArea))
	, m_syncManager(WTFMove(storageSyncManager))
	, m_databaseIdentifier(databaseIdentifier.isolatedCopy())
	, m_clearItemsWhileSyncing(false)
	, m_syncScheduled(false)
	, m_syncInProgress(false)
	, m_databaseOpenFailed(false)
	, m_syncCloseDatabase(false)
	, m_importComplete(false)
	{
	ASSERT(isMainThread());
	ASSERT(m_storageArea);
	ASSERT(m_syncManager);

	// FIXME: If it can't import, then the default WebKit behavior should be that of private browsing,
	// not silently ignoring it. https://bugs.webkit.org/show_bug.cgi?id=25894
	RefPtr<StorageAreaSync> protector(this);
	m_syncManager->dispatch([protector] {
	protector->performImport();
	});
	}

	Ref<StorageAreaSync> StorageAreaSync::create(RefPtr<StorageSyncManager>&& storageSyncManager, Ref<StorageAreaImpl>&& storageArea, const String& databaseIdentifier)
	{
	return adoptRef(*new StorageAreaSync(WTFMove(storageSyncManager), WTFMove(storageArea), databaseIdentifier));
	}

	StorageAreaSync::~StorageAreaSync()
	{
	ASSERT(isMainThread());
	ASSERT(!m_syncTimer.isActive());
	ASSERT(m_finalSyncScheduled);
	}

	void StorageAreaSync::scheduleFinalSync()
	{
	ASSERT(isMainThread());
	// FIXME: We do this to avoid races, but it'd be better to make things safe without blocking.
	blockUntilImportComplete();
	m_storageArea = nullptr; // This is done in blockUntilImportComplete() but this is here as a form of documentation that we must be absolutely sure the ref count cycle is broken.

	if (m_syncTimer.isActive())
	m_syncTimer.stop();
	else {
	// The following is balanced by the call to enableSuddenTermination in the
	// syncTimerFired function.
	disableSuddenTermination();
	}
	// FIXME: This is synchronous. We should do it on the background process, but
	// we should do it safely.
	m_finalSyncScheduled = true;
	syncTimerFired();

	RefPtr<StorageAreaSync> protector(this);
	m_syncManager->dispatch([protector] {
	protector->deleteEmptyDatabase();
	});
	}

	void StorageAreaSync::scheduleItemForSync(const String& key, const String& value)
	{
	ASSERT(isMainThread());
	ASSERT(!m_finalSyncScheduled);

	m_changedItems.set(key, value);
	if (!m_syncTimer.isActive()) {
	m_syncTimer.startOneShot(StorageSyncInterval);

	// The following is balanced by the call to enableSuddenTermination in the
	// syncTimerFired function.
	disableSuddenTermination();
	}
	}

	void StorageAreaSync::scheduleClear()
	{
	ASSERT(isMainThread());
	ASSERT(!m_finalSyncScheduled);

	m_changedItems.clear();
	m_itemsCleared = true;
	if (!m_syncTimer.isActive()) {
	m_syncTimer.startOneShot(StorageSyncInterval);

	// The following is balanced by the call to enableSuddenTermination in the
	// syncTimerFired function.
	disableSuddenTermination();
	}
	}

	void StorageAreaSync::scheduleCloseDatabase()
	{
	ASSERT(isMainThread());
	ASSERT(!m_finalSyncScheduled);

	if (!m_database.isOpen())
	return;

	m_syncCloseDatabase = true;

	if (!m_syncTimer.isActive()) {
	m_syncTimer.startOneShot(StorageSyncInterval);

	// The following is balanced by the call to enableSuddenTermination in the
	// syncTimerFired function.
	disableSuddenTermination();
	}
	}

	void StorageAreaSync::syncTimerFired()
	{
	ASSERT(isMainThread());

	bool partialSync = false;
	{
	Locker locker { m_syncLock };

	// Do not schedule another sync if we're still trying to complete the
	// previous one. But, if we're shutting down, schedule it anyway.
	if (m_syncInProgress && !m_finalSyncScheduled) {
	ASSERT(!m_syncTimer.isActive());
	m_syncTimer.startOneShot(StorageSyncInterval);
	return;
	}

	if (m_itemsCleared) {
	m_itemsPendingSync.clear();
	m_clearItemsWhileSyncing = true;
	m_itemsCleared = false;
	}

	HashMap<String, String>::iterator changed_it = m_changedItems.begin();
	HashMap<String, String>::iterator changed_end = m_changedItems.end();
	for (int count = 0; changed_it != changed_end; ++count, ++changed_it) {
	if (count >= MaxiumItemsToSync && !m_finalSyncScheduled) {
	partialSync = true;
	break;
	}
	m_itemsPendingSync.set(changed_it->key.isolatedCopy(), changed_it->value.isolatedCopy());
	}

	if (partialSync) {
	// We can't do the fast path of simply clearing all items, so we'll need to manually
	// remove them one by one. Done under lock since m_itemsPendingSync is modified by
	// the background thread.
	HashMap<String, String>::iterator pending_it = m_itemsPendingSync.begin();
	HashMap<String, String>::iterator pending_end = m_itemsPendingSync.end();
	for (; pending_it != pending_end; ++pending_it)
	m_changedItems.remove(pending_it->key);
	}

	if (!m_syncScheduled) {
	m_syncScheduled = true;

	// The following is balanced by the call to enableSuddenTermination in the
	// performSync function.
	disableSuddenTermination();

	RefPtr<StorageAreaSync> protector(this);
	m_syncManager->dispatch([protector] {
	protector->performSync();
	});
	}
	}

	if (partialSync) {
	// If we didn't finish syncing, then we need to finish the job later.
	ASSERT(!m_syncTimer.isActive());
	m_syncTimer.startOneShot(StorageSyncInterval);
	} else {
	// The following is balanced by the calls to disableSuddenTermination in the
	// scheduleItemForSync, scheduleClear, and scheduleFinalSync functions.
	enableSuddenTermination();

	m_changedItems.clear();
	}
	}

	void StorageAreaSync::openDatabase(OpenDatabaseParamType openingStrategy)
	{
	ASSERT(!isMainThread());
	ASSERT(!m_database.isOpen());
	ASSERT(!m_databaseOpenFailed);

	SQLiteTransactionInProgressAutoCounter transactionCounter;

	String databaseFilename = m_syncManager->fullDatabaseFilename(m_databaseIdentifier);

	if (!FileSystem::fileExists(databaseFilename) && openingStrategy == SkipIfNonExistent)
	return;

	if (databaseFilename.isEmpty()) {
	LOG_ERROR("Filename for local storage database is empty - cannot open for persistent storage");
	markImported();
	m_databaseOpenFailed = true;
	return;
	}

	// A StorageTracker thread may have been scheduled to delete the db we're
	// reopening, so cancel possible deletion.
	StorageTracker::tracker().cancelDeletingOrigin(m_databaseIdentifier);

	if (!m_database.open(databaseFilename)) {
	LOG_ERROR("Failed to open database file %s for local storage", databaseFilename.utf8().data());
	markImported();
	m_databaseOpenFailed = true;
	return;
	}

	migrateItemTableIfNeeded();

	if (!m_database.executeCommand("CREATE TABLE IF NOT EXISTS ItemTable (key TEXT UNIQUE ON CONFLICT REPLACE, value BLOB NOT NULL ON CONFLICT FAIL)"_s)) {
	LOG_ERROR("Failed to create table ItemTable for local storage");
	markImported();
	m_databaseOpenFailed = true;
	return;
	}

	StorageTracker::tracker().setOriginDetails(m_databaseIdentifier, databaseFilename);
	}

	void StorageAreaSync::migrateItemTableIfNeeded()
	{
	if (!m_database.tableExists("ItemTable"))
	return;

	{
	auto query = m_database.prepareStatement("SELECT value FROM ItemTable LIMIT 1"_s);
	// this query isn't ever executed.
	if (query && query->isColumnDeclaredAsBlob(0))
	return;
	}

	// alter table for backward compliance, change the value type from TEXT to BLOB.
	static const ASCIILiteral commands[] = {
	"DROP TABLE IF EXISTS ItemTable2"_s,
	"CREATE TABLE ItemTable2 (key TEXT UNIQUE ON CONFLICT REPLACE, value BLOB NOT NULL ON CONFLICT FAIL)"_s,
	"INSERT INTO ItemTable2 SELECT * from ItemTable"_s,
	"DROP TABLE ItemTable"_s,
	"ALTER TABLE ItemTable2 RENAME TO ItemTable"_s,
	ASCIILiteral::null(),
	};

	SQLiteTransaction transaction(m_database, false);
	transaction.begin();
	for (size_t i = 0; commands[i]; ++i) {
	if (!m_database.executeCommand(commands[i])) {
	LOG_ERROR("Failed to migrate table ItemTable for local storage when executing: %s", commands[i].characters());
	transaction.rollback();

	// finally it will try to keep a backup of ItemTable for the future restoration.
	// NOTICE: this will essentially DELETE the current database, but that's better
	// than continually hitting this case and never being able to use the local storage.
	// if this is ever hit, it's definitely a bug.
	ASSERT_NOT_REACHED();
	if (!m_database.executeCommand("ALTER TABLE ItemTable RENAME TO Backup_ItemTable"_s))
	LOG_ERROR("Failed to save ItemTable after migration job failed.");

	return;
	}
	}
	transaction.commit();
	}

	void StorageAreaSync::performImport()
	{
	ASSERT(!isMainThread());
	ASSERT(!m_database.isOpen());

	openDatabase(SkipIfNonExistent);
	if (!m_database.isOpen()) {
	markImported();
	return;
	}

	auto query = m_database.prepareStatement("SELECT key, value FROM ItemTable"_s);
	if (!query) {
	LOG_ERROR("Unable to select items from ItemTable for local storage");
	markImported();
	return;
	}

	HashMap<String, String> itemMap;

	int result = query->step();
	while (result == SQLITE_ROW) {
	itemMap.set(query->columnText(0), query->columnBlobAsString(1));
	result = query->step();
	}

	if (result != SQLITE_DONE) {
	LOG_ERROR("Error reading items from ItemTable for local storage");
	markImported();
	return;
	}

	m_storageArea->importItems(WTFMove(itemMap));

	markImported();
	}

	void StorageAreaSync::markImported()
	{
	Locker locker { m_importLock };
	m_importComplete = true;
	m_importCondition.notifyOne();
	}

	// FIXME: In the future, we should allow use of StorageAreas while it's importing (when safe to do so).
	// Blocking everything until the import is complete is by far the simplest and safest thing to do, but
	// there is certainly room for safe optimization: Key/length will never be able to make use of such an
	// optimization (since the order of iteration can change as items are being added). Get can return any
	// item currently in the map. Get/remove can work whether or not it's in the map, but we'll need a list
	// of items the import should not overwrite. Clear can also work, but it'll need to kill the import
	// job first.
	void StorageAreaSync::blockUntilImportComplete()
	{
	ASSERT(isMainThread());

	// Fast path. We set m_storageArea to 0 only after m_importComplete being true.
	if (!m_storageArea)
	return;

	Locker locker { m_importLock };
	while (!m_importComplete)
	m_importCondition.wait(m_importLock);
	m_storageArea = nullptr;
	}

	void StorageAreaSync::sync(bool clearItems, const HashMap<String, String>& items)
	{
	ASSERT(!isMainThread());

	if (items.isEmpty() && !clearItems && !m_syncCloseDatabase)
	return;
	if (m_databaseOpenFailed)
	return;

	if (!m_database.isOpen() && m_syncCloseDatabase) {
	m_syncCloseDatabase = false;
	return;
	}

	if (!m_database.isOpen())
	openDatabase(CreateIfNonExistent);
	if (!m_database.isOpen())
	return;

	// Closing this db because it is about to be deleted by StorageTracker.
	// The delete will be cancelled if StorageAreaSync needs to reopen the db
	// to write new items created after the request to delete the db.
	if (m_syncCloseDatabase) {
	m_syncCloseDatabase = false;
	m_database.close();
	return;
	}

	SQLiteTransactionInProgressAutoCounter transactionCounter;

	// If the clear flag is set, then we clear all items out before we write any new ones in.
	if (clearItems) {
	auto clear = m_database.prepareStatement("DELETE FROM ItemTable"_s);
	if (!clear) {
	LOG_ERROR("Failed to prepare clear statement - cannot write to local storage database");
	return;
	}

	int result = clear->step();
	if (result != SQLITE_DONE) {
	LOG_ERROR("Failed to clear all items in the local storage database - %i", result);
	return;
	}
	}

	auto insert = m_database.prepareStatement("INSERT INTO ItemTable VALUES (?, ?)"_s);
	if (!insert) {
	LOG_ERROR("Failed to prepare insert statement - cannot write to local storage database");
	return;
	}

	auto remove = m_database.prepareStatement("DELETE FROM ItemTable WHERE key=?"_s);
	if (!remove) {
	LOG_ERROR("Failed to prepare delete statement - cannot write to local storage database");
	return;
	}

	HashMap<String, String>::const_iterator end = items.end();

	SQLiteTransaction transaction(m_database);
	transaction.begin();
	for (HashMap<String, String>::const_iterator it = items.begin(); it != end; ++it) {
	// Based on the null-ness of the second argument, decide whether this is an insert or a delete.
	auto& query = it->value.isNull() ? remove : insert;

	query->bindText(1, it->key);

	// If the second argument is non-null, we're doing an insert, so bind it as the value.
	if (!it->value.isNull())
	query->bindBlob(2, it->value);

	int result = query->step();
	if (result != SQLITE_DONE) {
	LOG_ERROR("Failed to update item in the local storage database - %i", result);
	break;
	}

	query->reset();
	}
	transaction.commit();
	}

	void StorageAreaSync::performSync()
	{
	ASSERT(!isMainThread());

	bool clearItems;
	HashMap<String, String> items;
	{
	Locker locker { m_syncLock };

	ASSERT(m_syncScheduled);

	clearItems = m_clearItemsWhileSyncing;
	m_itemsPendingSync.swap(items);

	m_clearItemsWhileSyncing = false;
	m_syncScheduled = false;
	m_syncInProgress = true;
	}

	sync(clearItems, items);

	{
	Locker locker { m_syncLock };
	m_syncInProgress = false;
	}

	// The following is balanced by the call to disableSuddenTermination in the
	// syncTimerFired function.
	enableSuddenTermination();
	}

	void StorageAreaSync::deleteEmptyDatabase()
	{
	ASSERT(!isMainThread());
	if (!m_database.isOpen())
	return;

	auto count = [&] {
	auto query = m_database.prepareStatement("SELECT COUNT(*) FROM ItemTable"_s);
	if (!query) {
	LOG_ERROR("Unable to count number of rows in ItemTable for local storage");
	return -1;
	}

	int result = query->step();
	if (result != SQLITE_ROW) {
	LOG_ERROR("No results when counting number of rows in ItemTable for local storage");
	return -1;
	}

	return query->columnInt(0);
	}();
	if (count)
	return;

	m_database.close();
	if (StorageTracker::tracker().isActive()) {
	callOnMainThread([databaseIdentifier = m_databaseIdentifier.isolatedCopy()] {
	StorageTracker::tracker().deleteOriginWithIdentifier(databaseIdentifier);
	});
	} else {
	String databaseFilename = m_syncManager->fullDatabaseFilename(m_databaseIdentifier);
	if (!FileSystem::deleteFile(databaseFilename))
	LOG_ERROR("Failed to delete database file %s\n", databaseFilename.utf8().data());
	}
	}

	void StorageAreaSync::scheduleSync()
	{
	syncTimerFired();
	}

	} // namespace WebCore