SQLite

        ** to be the right size but is not actually valid. Avoid this
        ** possibility by unmapping the db here. */
        if( USEFETCH(pPager) ){
          sqlite3OsUnfetch(pPager->fd, 0, 0);
        }
      }
    }

    rc = pagerServerConnect(pPager);

    /* If there is a WAL file in the file-system, open this database in WAL
    ** mode. Otherwise, the following function call is a no-op.
    */
    if( rc==SQLITE_OK ){
      rc = pagerOpenWalIfPresent(pPager);
    }
#ifndef SQLITE_OMIT_WAL
    assert( pPager->pWal==0 || rc==SQLITE_OK );
#endif




  }

#ifdef SQLITE_SERVER_EDITION
  if( pagerIsServer(pPager) ){
    assert( rc==SQLITE_OK );
    pager_reset(pPager);
    rc = sqlite3ServerBegin(pPager->pServer);

  }
#endif
  if( pagerUseWal(pPager) ){
    assert( rc==SQLITE_OK );
    rc = pagerBeginReadTransaction(pPager);
  }

  if( pPager->tempFile==0 && pPager->eState==PAGER_OPEN && rc==SQLITE_OK ){

/*
** Return true if the underlying VFS for the given pager supports the
** primitives necessary for write-ahead logging.
*/
int sqlite3PagerWalSupported(Pager *pPager){
  const sqlite3_io_methods *pMethods = pPager->fd->pMethods;
  if( pPager->noLock && !pagerIsServer(pPager) ) return 0;
  return pPager->exclusiveMode || (pMethods->iVersion>=2 && pMethods->xShmMap);
}

/*
** Attempt to take an exclusive lock on the database file. If a PENDING lock
** is obtained instead, immediately release it.
*/

    /* Close any rollback journal previously open */
    sqlite3OsClose(pPager->jfd);

    rc = pagerOpenWal(pPager);
    if( rc==SQLITE_OK ){
      pPager->journalMode = PAGER_JOURNALMODE_WAL;
      pPager->eState = PAGER_OPEN;
      sqlite3WalServer(pPager->pWal, pPager->pServer);
    }
  }else{
    *pbOpen = 1;
  }

  return rc;
}

  u32 nCkpt;                 /* Checkpoint sequence counter in the wal-header */
#ifdef SQLITE_DEBUG
  u8 lockError;              /* True if a locking error has occurred */
#endif
#ifdef SQLITE_ENABLE_SNAPSHOT
  WalIndexHdr *pSnapshot;    /* Start transaction here if not NULL */
#endif
#ifdef SQLITE_SERVER_EDITION
  Server *pServer;
#endif
};

#ifdef SQLITE_SERVER_EDITION
# define walIsServer(p) ((p)->pServer!=0)
#else
# define walIsServer(p) 0
#endif

/*
** Candidate values for Wal.exclusiveMode.
*/
#define WAL_NORMAL_MODE     0
#define WAL_EXCLUSIVE_MODE  1     
#define WAL_HEAPMEMORY_MODE 2

      sqlite3_free((void *)pWal->apWiData[i]);
      pWal->apWiData[i] = 0;
    }
  }else{
    sqlite3OsShmUnmap(pWal->pDbFd, isDelete);
  }
}

#ifdef SQLITE_SERVER_EDITION
int sqlite3WalServer(Wal *pWal, Server *pServer){
  assert( pWal->pServer==0 );
  pWal->pServer = pServer;
  return SQLITE_OK;
}
#endif

/* 
** Open a connection to the WAL file zWalName. The database file must 
** already be opened on connection pDbFd. The buffer that zWalName points
** to must remain valid for the lifetime of the returned Wal* handle.
**
** A SHARED lock should be held on the database file when this function

      }
    }
    if( rc!=SQLITE_OK ){
      return rc;
    }
  }

  assert( rc==SQLITE_OK );
  if( walIsServer(pWal) ) return SQLITE_OK;

  pInfo = walCkptInfo(pWal);
  if( !useWal && pInfo->nBackfill==pWal->hdr.mxFrame 
#ifdef SQLITE_ENABLE_SNAPSHOT
   && (pWal->pSnapshot==0 || pWal->hdr.mxFrame==0
     || 0==memcmp(&pWal->hdr, pWal->pSnapshot, sizeof(WalIndexHdr)))
#endif
  ){

){
  u32 iRead = 0;                  /* If !=0, WAL frame to return data from */
  u32 iLast = pWal->hdr.mxFrame;  /* Last page in WAL for this reader */
  int iHash;                      /* Used to loop through N hash tables */
  int iMinHash;

  /* This routine is only be called from within a read transaction. */
  assert( walIsServer(pWal) || pWal->readLock>=0 || pWal->lockError );

  if( walIsServer(pWal) ){
    /* A server mode connection must read from the most recent snapshot. */
    iLast = walIndexHdr(pWal)->mxFrame;
  }

  /* If the "last page" field of the wal-index header snapshot is 0, then
  ** no data will be read from the wal under any circumstances. Return early
  ** in this case as an optimization.  Likewise, if pWal->readLock==0, 
  ** then the WAL is ignored by the reader so return early, as if the 
  ** WAL were empty.
  */

  return sqlite3OsRead(pWal->pWalFd, pOut, (nOut>sz ? sz : nOut), iOffset);
}

/* 
** Return the size of the database in pages (or zero, if unknown).
*/
Pgno sqlite3WalDbsize(Wal *pWal){
  if( pWal && (walIsServer(pWal) || ALWAYS(pWal->readLock>=0)) ){
    return pWal->hdr.nPage;
  }
  return 0;
}


/* 

** There can only be a single writer active at a time.
*/
int sqlite3WalBeginWriteTransaction(Wal *pWal){
  int rc;

  /* Cannot start a write transaction without first holding a read
  ** transaction. */
  assert( walIsServer(pWal) || pWal->readLock>=0 );
  assert( pWal->writeLock==0 && pWal->iReCksum==0 );

  if( pWal->readOnly ){
    return SQLITE_READONLY;
  }

  /* For a server connection, do nothing at this point. */
  if( walIsServer(pWal) ){
    return SQLITE_OK;
  }

  /* Only one writer allowed at a time.  Get the write lock.  Return
  ** SQLITE_BUSY if unable.
  */
  rc = walLockExclusive(pWal, WAL_WRITE_LOCK, 1);
  if( rc ){
    return rc;

  int szFrame;                    /* The size of a single frame */
  i64 iOffset;                    /* Next byte to write in WAL file */
  WalWriter w;                    /* The writer */
  u32 iFirst = 0;                 /* First frame that may be overwritten */
  WalIndexHdr *pLive;             /* Pointer to shared header */

  assert( pList );
  assert( pWal->writeLock || walIsServer(pWal) );
  if( pWal->writeLock==0 ){
    int bDummy = 0;
    rc = walLockExclusive(pWal, WAL_WRITE_LOCK, 1);
    if( rc==SQLITE_OK ){
      pWal->writeLock = 1;
      rc = walIndexTryHdr(pWal, &bDummy);
    }
    if( rc!=SQLITE_OK ){
      return rc;
    }
  }

  /* If this frame set completes a transaction, then nTruncate>0.  If
  ** nTruncate==0 then this frame set does not complete the transaction. */
  assert( (isCommit!=0)==(nTruncate!=0) );

#if defined(SQLITE_TEST) && defined(SQLITE_DEBUG)
  { int cnt; for(cnt=0, p=pList; p; p=p->pDirty, cnt++){}

** stored in each frame (i.e. the db page-size when the WAL was created).
*/
int sqlite3WalFramesize(Wal *pWal);
#endif

/* Return the sqlite3_file object for the WAL file */
sqlite3_file *sqlite3WalFile(Wal *pWal);

#ifdef SQLITE_SERVER_EDITION
int sqlite3WalServer(Wal *pWal, Server *pServer);
#endif

#endif /* ifndef SQLITE_OMIT_WAL */
#endif /* SQLITE_WAL_H */

# 2017 April 25
#
# The author disclaims copyright to this source code.  In place of
# a legal notice, here is a blessing:
#
#    May you do good and not evil.
#    May you find forgiveness for yourself and forgive others.
#    May you share freely, never taking more than you give.
#
#***********************************************************************
# This file implements regression tests for SQLite library.  The
# focus of this script is testing the server mode of SQLite.
#


set testdir [file dirname $argv0]
source $testdir/tester.tcl
set testprefix serverwal

# Check files are created and deleted as expected.
#
do_execsql_test 1.0 {
  PRAGMA journal_mode = wal;
} {wal}
do_execsql_test 1.1 {
  CREATE TABLE t1(a, b);
}
do_execsql_test 1.2 {
  SELECT * FROM t1;
} {}
do_test 1.3 {
  lsort [glob test.db*]
} {test.db test.db-hma test.db-shm test.db-wal}
do_test 1.4 {
  db close
  glob test.db*
} {test.db}

# Two concurrent transactions.
#
do_test 2.0 {
  sqlite3 db  test.db
  sqlite3 db2 test.db
  db eval {
    CREATE TABLE t2(a, b);
  }
} {}
do_test 2.1 {
  execsql {
    BEGIN;
      INSERT INTO t1 VALUES(1, 2);
  } db
  execsql {
    BEGIN;
      INSERT INTO t2 VALUES(1, 2);
  } db2
} {}
do_test 2.2 {
  execsql COMMIT db
  execsql COMMIT db2
} {}


finish_test