SQLite

  sqlite3OsEnterMutex();

  /* If some thread using this PID has a lock via a different OsFile*
  ** handle that precludes the requested lock, return BUSY.
  */
  if( (id->locktype!=pLock->locktype && 
          (pLock->locktype>=PENDING_LOCK || locktype>SHARED_LOCK))

  ){
    rc = SQLITE_BUSY;
    goto end_lock;
  }

  /* If a SHARED lock is requested, and some thread using this PID already
  ** has a SHARED or RESERVED lock, then increment reference counts and

    id->pOpen->nLock++;
    goto end_lock;
  }

  lock.l_len = 1L;
  lock.l_whence = SEEK_SET;

  /* A PENDING lock is needed before acquiring a SHARED lock and before
  ** acquiring an EXCLUSIVE lock.  For the SHARED lock, the PENDING will
  ** be released.
  */
  if( locktype==SHARED_LOCK 

      || (locktype==EXCLUSIVE_LOCK && id->locktype<PENDING_LOCK)
  ){



    lock.l_type = F_RDLCK;
    lock.l_start = PENDING_BYTE;
    s = fcntl(id->h, F_SETLK, &lock);
    if( s ){
      rc = (errno==EINVAL) ? SQLITE_NOLFS : SQLITE_BUSY;
      goto end_lock;
    }
  }


  /* If control gets to this point, then actually go ahead and make
  ** operating system calls for the specified lock.
  */
  if( locktype==SHARED_LOCK ){
    assert( pLock->cnt==0 );
    assert( pLock->locktype==0 );

    /* Now get the read-lock */
    lock.l_start = SHARED_FIRST;
    lock.l_len = SHARED_SIZE;
    s = fcntl(id->h, F_SETLK, &lock);

    /* Drop the temporary PENDING lock */
    lock.l_start = PENDING_BYTE;
    lock.l_len = 1L;
    lock.l_type = F_UNLCK;
    fcntl(id->h, F_SETLK, &lock);
    if( s ){
      rc = (errno==EINVAL) ? SQLITE_NOLFS : SQLITE_BUSY;
    }else{
      id->locktype = SHARED_LOCK;
      id->pOpen->nLock++;
      pLock->cnt = 1;
    }
  }else if( locktype==EXCLUSIVE_LOCK && pLock->cnt>1 ){
    /* We are trying for an exclusive lock but another thread in this
    ** same process is still holding a shared lock. */
    rc = SQLITE_BUSY;
  }else{
    /* The request was for a RESERVED or EXCLUSIVE lock.  It is
    ** assumed that there is a SHARED or greater lock on the file
    ** already.
    */
    assert( 0!=id->locktype );
    lock.l_type = F_WRLCK;
    switch( locktype ){
      case RESERVED_LOCK:
        lock.l_start = RESERVED_BYTE;
        break;



      case EXCLUSIVE_LOCK:
        lock.l_start = SHARED_FIRST;
        lock.l_len = SHARED_SIZE;
        break;
      default:
        assert(0);
    }
    s = fcntl(id->h, F_SETLK, &lock);
    if( s ){
      rc = (errno==EINVAL) ? SQLITE_NOLFS : SQLITE_BUSY;
    }
  }
  
  if( rc==SQLITE_OK ){
    id->locktype = locktype;
    pLock->locktype = locktype;
  }else if( locktype==EXCLUSIVE_LOCK ){
    id->locktype = PENDING_LOCK;
    pLock->locktype = PENDING_LOCK;
  }

end_lock:
  sqlite3OsLeaveMutex();
  TRACE4("LOCK %d %d %s\n", id->h, locktype, rc==SQLITE_OK ? "ok" : "failed");
  return rc;
}

  assert( locktype!=PENDING_LOCK );
  assert( locktype!=RESERVED_LOCK || id->locktype==SHARED_LOCK );

  /* Lock the PENDING_LOCK byte if we need to acquire a PENDING lock or
  ** a SHARED lock.  If we are acquiring a SHARED lock, the acquisition of
  ** the PENDING_LOCK byte is temporary.
  */
  newLocktype = id->locktype;
  if( id->locktype==NO_LOCK
   || (locktype==EXCLUSIVE_LOCK && id->locktype==RESERVED_LOCK)
  ){
    int cnt = 3;
    while( cnt-->0 && (res = LockFile(id->h, PENDING_BYTE, 0, 1, 0))==0 ){
      /* Try 3 times to get the pending lock.  The pending lock might be
      ** held by another reader process who will release it momentarily.

** the requested locking level, this routine is a no-op.
**
** It is not possible for this routine to fail.
*/
int sqlite3OsUnlock(OsFile *id, int locktype){
  int rc, type;
  assert( locktype<=SHARED_LOCK );
  TRACE5("UNLOCK %d to %d was %d(%d)\n", id->h, locktype,
          id->locktype, id->sharedLockByte);
  type = id->locktype;
  if( type>=EXCLUSIVE_LOCK ){
    UnlockFile(id->h, SHARED_FIRST, 0, SHARED_SIZE, 0);
  }
  if( type>=RESERVED_LOCK ){
    UnlockFile(id->h, RESERVED_BYTE, 0, 1, 0);

#    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 ATTACH and DETACH commands
# and related functionality.
#
# $Id: attach2.test,v 1.16 2004/06/12 02:17:15 drh Exp $
#

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


# Ticket #354

    INSERT INTO t1 VALUES(1, 2)
  }
} {1 {database is locked}}

lock_status 4.9.1 db {main shared temp reserved file2 shared}
lock_status 4.9.2 db2 {main reserved temp reserved file2 reserved}


btree_breakpoint
do_test attach2-4.10 {
  # We cannot commit db2 while db is holding a read-lock
  catchsql {COMMIT} db2
} {1 {database is locked}}

lock_status 4.10.1 db {main shared temp reserved file2 shared}
lock_status 4.10.2 db2 {main pending temp reserved file2 reserved}

set sqlite_os_trace 0
btree_breakpoint
do_test attach2-4.11 {
  # db is able to commit.
  catchsql {COMMIT}
} {0 {}}

lock_status 4.11.1 db {main unlocked temp unlocked file2 unlocked}
lock_status 4.11.2 db2 {main pending temp reserved file2 reserved}

do_test attach2-4.12 {
  # Now we can commit db2
  catchsql {COMMIT} db2
} {0 {}}

lock_status 4.12.1 db {main unlocked temp unlocked file2 unlocked}