#define PAGER_JOURNALMODE_QUERY     (-1)  /* Query the value of journalmode */
#define PAGER_JOURNALMODE_DELETE      0   /* Commit by deleting journal file */

#define PAGER_JOURNALMODE_PERSIST     1   /* Commit by zeroing journal header */

#define PAGER_JOURNALMODE_OFF         2   /* Journal omitted.  */

#define PAGER_JOURNALMODE_TRUNCATE    3   /* Commit by truncating journal */

#define PAGER_JOURNALMODE_MEMORY      4   /* In-memory journal file */

#define PAGER_JOURNALMODE_WAL         5   /* Use write-ahead logging */

cflags: [
    ...
        "-DSQLITE_THREADSAFE=2",
    ...

    ],

static void sqliteInitialize() {
    // Enable multi-threaded mode.  In this mode, SQLite is safe to use by multiple
    // threads as long as no two threads use the same database connection at the same
    // time (which we guarantee in the SQLite database wrappers).
    sqlite3_config(SQLITE_CONFIG_MULTITHREAD);<<====關鍵步驟======

    ...

    // Initialize SQLite.

    sqlite3_initialize();

}

private void setMaxConnectionPoolSizeLocked() {
        if (!mConfiguration.isInMemoryDb()
                && (mConfiguration.openFlags & SQLiteDatabase.ENABLE_WRITE_AHEAD_LOGGING) != 0) {
            mMaxConnectionPoolSize = SQLiteGlobal.getWALConnectionPoolSize();<<=====關鍵步驟===
        } 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>

private OpenHelper getDelegate() {
        // getDelegate() is lazy because we don't want to File I/O until the call to
        // getReadableDatabase() or getWritableDatabase(). This is better because the call to
        // a getReadableDatabase() or a getWritableDatabase() happens on a background thread unless
        // queries are allowed on the main thread.
        // We defer computing the path the database from the constructor to getDelegate()

        // because context.getNoBackupFilesDir() does File I/O :(
        synchronized (mLock) {
            if (mDelegate == null) {
                final FrameworkSQLiteDatabase[] dbRef = new FrameworkSQLiteDatabase[1];
                if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M
                        && mName != null
                        && mUseNoBackupDirectory) {
                    File file = new File(mContext.getNoBackupFilesDir(), mName);
                    mDelegate = new OpenHelper(mContext, file.getAbsolutePath(), dbRef, mCallback);
                } else {
                    mDelegate = new OpenHelper(mContext, mName, dbRef, mCallback);
                }
                if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN) {

                    <<============關鍵步驟==================>>
                    mDelegate.setWriteAheadLoggingEnabled(mWriteAheadLoggingEnabled);
                }
            }
            return mDelegate;
        }
    }

public static void setWriteAheadLoggingEnabled(@NonNull SQLiteOpenHelper sQLiteOpenHelper,
                boolean enabled) {
            sQLiteOpenHelper.setWriteAheadLoggingEnabled(enabled);
        }

#define PAGER_JOURNALMODE_DELETE      0   /* Commit by deleting journal file */

private void setJournalMode(String newValue) {
        String value = executeForString("PRAGMA journal_mode", null, null);
        if (!value.equalsIgnoreCase(newValue)) {
            try {
                <<=======關鍵步驟=========>>
                String result = executeForString("PRAGMA journal_mode=" + newValue, null, null);
                if (result.equalsIgnoreCase(newValue)) {
                    return;
                }
                // PRAGMA journal_mode silently fails and returns the original journal
                // mode in some cases if the journal mode could not be changed.
            } catch (SQLiteDatabaseLockedException ex) {
                // This error (SQLITE_BUSY) occurs if one connection has the database
                // open in WAL mode and another tries to change it to non-WAL.
            }
            ...
        }
    }

int sqlite3PagerSetJournalMode(Pager *pPager, int eMode){
  u8 eOld = pPager->journalMode;    /* Prior journalmode */

  ...

  if( eMode!=eOld ){

    /* Change the journal mode. */
    assert( pPager->eState!=PAGER_ERROR );
    pPager->journalMode = (u8)eMode;

    ...
    if( !pPager->exclusiveMode && (eOld & 5)==1 && (eMode & 1)==0 ){

      ...

      sqlite3OsClose(pPager->jfd);
      if( pPager->eLock>=RESERVED_LOCK ){
        sqlite3OsDelete(pPager->pVfs, pPager->zJournal, 0);
      }else{
        int rc = SQLITE_OK;
        int state = pPager->eState;
        assert( state==PAGER_OPEN || state==PAGER_READER );
        if( state==PAGER_OPEN ){
          rc = sqlite3PagerSharedLock(pPager);<<=====關鍵步驟==============
        }
        ...
        assert( state==pPager->eState );
      }
    }else if( eMode==PAGER_JOURNALMODE_OFF ){
      sqlite3OsClose(pPager->jfd);
    }
  }

  /* Return the new journal mode */
  return (int)pPager->journalMode;
}

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);<<=========關鍵步驟=============
    }
    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); <<============關鍵步驟==============
        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;
}