SQLite切换日志模式优化

 private void setMaxConnectionPoolSizeLocked() { if (!mConfiguration.isInMemoryDb() && (mConfiguration.openFlags & SQLiteDatabase.ENABLE_WRITE_AHEAD_LOGGING) != 0) { mMaxConnectionPoolSize = SQLiteGlobal.getWALConnectionPoolSize();<<=====关键步骤=== } else { // We don't actually need to always restrict the connection pool size to 1 // for non-WAL databases. There might be reasons to use connection pooling // with other journal modes. However, we should always keep pool size of 1 for in-memory // databases since every :memory: db is separate from another. // For now, enabling connection pooling and using WAL are the same thing in the API. mMaxConnectionPoolSize = 1; } }
 /** * Gets the connection pool size when in WAL mode. */ public static int getWALConnectionPoolSize() { int value = SystemProperties.getInt("debug.sqlite.wal.poolsize", Resources.getSystem().getInteger( com.android.internal.R.integer.db_connection_pool_size)); return Math.max(2, value); } 

 <!-- Maximum number of database connections opened and managed by framework layer to handle queries on each database when using Write-Ahead Logging. --> <integer name="db_connection_pool_size">4</integer>

int sqlite3PagerSharedLock(Pager *pPager){ int rc = SQLITE_OK; /* Return code */
 /* This routine is only called from b-tree and only when there are no ** outstanding pages. This implies that the pager state should either ** be OPEN or READER. READER is only possible if the pager is or was in  ** exclusive access mode. */ assert( sqlite3PcacheRefCount(pPager->pPCache)==0 ); assert( assert_pager_state(pPager) ); assert( pPager->eState==PAGER_OPEN || pPager->eState==PAGER_READER ); assert( pPager->errCode==SQLITE_OK );
 if( !pagerUseWal(pPager) && pPager->eState==PAGER_OPEN ){ int bHotJournal = 1; /* True if there exists a hot journal-file */
 assert( !MEMDB ); assert( pPager->tempFile==0 || pPager->eLock==EXCLUSIVE_LOCK );
 rc = pager_wait_on_lock(pPager, SHARED_LOCK); if( rc!=SQLITE_OK ){ assert( pPager->eLock==NO_LOCK || pPager->eLock==UNKNOWN_LOCK ); goto failed; }
 /* If a journal file exists, and there is no RESERVED lock on the ** database file, then it either needs to be played back or deleted. */ if( pPager->eLock<=SHARED_LOCK ){ rc = hasHotJournal(pPager, &bHotJournal);<<=========关键步骤============= } if( rc!=SQLITE_OK ){ goto failed; } if( bHotJournal ){ if( pPager->readOnly ){ rc = SQLITE_READONLY_ROLLBACK; goto failed; }
 /* Get an EXCLUSIVE lock on the database file. At this point it is ** important that a RESERVED lock is not obtained on the way to the ** EXCLUSIVE lock. If it were, another process might open the ** database file, detect the RESERVED lock, and conclude that the ** database is safe to read while this process is still rolling the  ** hot-journal back.*/
 ...
 if( isOpen(pPager->jfd) ){ assert( rc==SQLITE_OK ); rc = pagerSyncHotJournal(pPager); <<============关键步骤============== if( rc==SQLITE_OK ){ rc = pager_playback(pPager, !pPager->tempFile); pPager->eState = PAGER_OPEN; } }else if( !pPager->exclusiveMode ){

static int hasHotJournal(Pager *pPager, int *pExists){ sqlite3_vfs * const pVfs = pPager->pVfs; int rc = SQLITE_OK; /* Return code */ int exists = 1; /* True if a journal file is present */ int jrnlOpen = !!isOpen(pPager->jfd);
 assert( pPager->useJournal ); assert( isOpen(pPager->fd) ); assert( pPager->eState==PAGER_OPEN );
 assert( jrnlOpen==0 || ( sqlite3OsDeviceCharacteristics(pPager->jfd) & SQLITE_IOCAP_UNDELETABLE_WHEN_OPEN ));
 *pExists = 0; if( !jrnlOpen ){ rc = sqlite3OsAccess(pVfs, pPager->zJournal, SQLITE_ACCESS_EXISTS, &exists); } if( rc==SQLITE_OK && exists ){ int locked = 0; /* True if some process holds a RESERVED lock */
 /* Race condition here: Another process might have been holding the ** the RESERVED lock and have a journal open at the sqlite3OsAccess()  ** call above, but then delete the journal and drop the lock before ** we get to the following sqlite3OsCheckReservedLock() call. If that ** is the case, this routine might think there is a hot journal when ** in fact there is none. This results in a false-positive which will ** be dealt with by the playback routine. Ticket #3883. */  rc = sqlite3OsCheckReservedLock(pPager->fd, &locked); if( rc==SQLITE_OK && !locked ){ Pgno nPage; /* Number of pages in database file */
 assert( pPager->tempFile==0 ); rc = pagerPagecount(pPager, &nPage); if( rc==SQLITE_OK ){ /* If the database is zero pages in size, that means that either (1) the ** journal is a remnant from a prior database with the same name where ** the database file but not the journal was deleted, or (2) the initial ** transaction that populates a new database is being rolled back. ** In either case, the journal file can be deleted. However, take care ** not to delete the journal file if it is already open due to ** journal_mode=PERSIST. */ if( nPage==0 && !jrnlOpen ){ sqlite3BeginBenignMalloc(); if( pagerLockDb(pPager, RESERVED_LOCK)==SQLITE_OK ){ sqlite3OsDelete(pVfs, pPager->zJournal, 0); if( !pPager->exclusiveMode ) pagerUnlockDb(pPager, SHARED_LOCK); } sqlite3EndBenignMalloc(); }else{ /* The journal file exists and no other connection has a reserved ** or greater lock on the database file. Now check that there is ** at least one non-zero bytes at the start of the journal file. ** If there is, then we consider this journal to be hot. If not,  ** it can be ignored. */ if( !jrnlOpen ){ int f = SQLITE_OPEN_READONLY|SQLITE_OPEN_MAIN_JOURNAL; rc = sqlite3OsOpen(pVfs, pPager->zJournal, pPager->jfd, f, &f); } if( rc==SQLITE_OK ){ u8 first = 0; rc = sqlite3OsRead(pPager->jfd, (void *)&first, 1, 0); if( rc==SQLITE_IOERR_SHORT_READ ){ rc = SQLITE_OK; } if( !jrnlOpen ){ sqlite3OsClose(pPager->jfd); } *pExists = (first!=0); }else if( rc==SQLITE_CANTOPEN ){ /* If we cannot open the rollback journal file in order to see if ** it has a zero header, that might be due to an I/O error, or ** it might be due to the race condition described above and in ** ticket #3883. Either way, assume that the journal is hot. ** This might be a false positive. But if it is, then the ** automatic journal playback and recovery mechanism will deal ** with it under an EXCLUSIVE lock where we do not need to ** worry so much with race conditions. */ *pExists = 1; rc = SQLITE_OK; } } } } }
 return rc;}