**      plus implementations of sqlite3_os_init() and sqlite3_os_end().
*/
#include "sqliteInt.h"
#if SQLITE_OS_UNIX              /* This file is used on unix only */

/* Turn this feature on in all builds for now */
#define SQLITE_MUTEXFREE_SHMLOCK 1



/*
** There are various methods for file locking used for concurrency
** control:
**
**   1. POSIX locking (the default),
**   2. No locking,
................................................................................
  ** 0, and so on.
  **
  ** If the 8-bits corresponding to a shm-locking locking slot are set to
  ** 0xFF, then a write-lock is held on the slot. Or, if they are set to
  ** a non-zero value smaller than 0xFF, then they represent the total 
  ** number of read-locks held on the slot. There is no way to distinguish
  ** between a write-lock and 255 read-locks.  */
  u64 lockmask;



#endif
};

/*
** Atomic CAS primitive used in multi-process mode. Equivalent to:
** 
**   int unixCompareAndSwap(u32 *ptr, u32 oldval, u32 newval){
................................................................................
struct unixShm {
  unixShmNode *pShmNode;     /* The underlying unixShmNode object */
  unixShm *pNext;            /* Next unixShm with the same unixShmNode */
  u8 hasMutex;               /* True if holding the unixShmNode->pShmMutex */
  u8 id;                     /* Id of this connection within its unixShmNode */
  u16 sharedMask;            /* Mask of shared locks held */
  u16 exclMask;              /* Mask of exclusive locks held */



};

/*
** Constants used for locking
*/
#define UNIX_SHM_BASE   ((22+SQLITE_SHM_NLOCK)*4)         /* first lock byte */
#define UNIX_SHM_DMS    (UNIX_SHM_BASE+SQLITE_SHM_NLOCK)  /* deadman switch */
................................................................................
    *pp = 0;
  }
  if( pShmNode->isReadonly && rc==SQLITE_OK ) rc = SQLITE_READONLY;
  sqlite3_mutex_leave(pShmNode->pShmMutex);
  return rc;
}














































































/*
** Change the lock state for a shared-memory segment.
**
** Note that the relationship between SHAREd and EXCLUSIVE locks is a little
** different here than in posix.  In xShmLock(), one can go from unlocked
** to shared and back or from unlocked to exclusive and back.  But one may
** not go from shared to exclusive or from exclusive to shared.
................................................................................
  assert( flags==(SQLITE_SHM_LOCK | SQLITE_SHM_SHARED)
       || flags==(SQLITE_SHM_LOCK | SQLITE_SHM_EXCLUSIVE)
       || flags==(SQLITE_SHM_UNLOCK | SQLITE_SHM_SHARED)
       || flags==(SQLITE_SHM_UNLOCK | SQLITE_SHM_EXCLUSIVE) );
  assert( n==1 || (flags & SQLITE_SHM_EXCLUSIVE)!=0 );
  assert( pShmNode->hShm>=0 || pDbFd->pInode->bProcessLock==1 );
  assert( pShmNode->hShm<0 || pDbFd->pInode->bProcessLock==0 );

  mask = (1<<(ofst+n)) - (1<<ofst);
  assert( n>1 || mask==(1<<ofst) );

#ifdef SQLITE_MUTEXFREE_SHMLOCK
  if( pDbFd->pInode->bProcessLock ){


    while( 1 ){
      u64 lockmask = pShmNode->lockmask;
      u64 newmask = lockmask;
      int i;
      for(i=ofst; i<n+ofst; i++){
        int ix8 = i*8;
        u8 v = (lockmask >> (ix8)) & 0xFF;
        if( flags & SQLITE_SHM_UNLOCK ){
          if( flags & SQLITE_SHM_EXCLUSIVE ){
            if( p->exclMask & (1 << i) ){
              newmask = newmask & ~((u64)0xFF<<ix8);
            }
          }else{
            if( p->sharedMask & (1 << i) ){
              newmask = newmask & ~((u64)0xFF<<ix8) | ((u64)(v-1)<<ix8);
            }
          }
        }else{
          if( flags & SQLITE_SHM_EXCLUSIVE ){
            if( v ) return SQLITE_BUSY;
            if( (p->exclMask & (1 << i))==0 ){
              newmask = newmask | ((u64)0xFF<<ix8);
            }
          }else{
            if( v==0xFF ) return SQLITE_BUSY;
            if( (p->sharedMask & (1 << i))==0 ){
              newmask = newmask & ~((u64)0xFF<<ix8) | ((u64)(v+1)<<ix8);
            }
          }
        }
      }

      if( unixCompareAndSwap(&pShmNode->lockmask, lockmask, newmask) ) break;
    }



    if( flags & SQLITE_SHM_UNLOCK ){
      p->sharedMask &= ~mask;
      p->exclMask &= ~mask;

    }else if( flags & SQLITE_SHM_EXCLUSIVE ){
      p->exclMask |= mask;
    }else{
      p->sharedMask |= mask;


    }

    return SQLITE_OK;
  }
#endif

  sqlite3_mutex_enter(pShmNode->pShmMutex);
  if( flags & SQLITE_SHM_UNLOCK ){
    u16 allMask = 0; /* Mask of locks held by siblings */

    /* See if any siblings hold this same lock */
    for(pX=pShmNode->pFirst; pX; pX=pX->pNext){

**      plus implementations of sqlite3_os_init() and sqlite3_os_end().
*/
#include "sqliteInt.h"
#if SQLITE_OS_UNIX              /* This file is used on unix only */

/* Turn this feature on in all builds for now */
#define SQLITE_MUTEXFREE_SHMLOCK 1
#define SQLITE_MFS_NSHARD        5
#define SQLITE_MFS_EXCLUSIVE     255

/*
** There are various methods for file locking used for concurrency
** control:
**
**   1. POSIX locking (the default),
**   2. No locking,
................................................................................
  ** 0, and so on.
  **
  ** If the 8-bits corresponding to a shm-locking locking slot are set to
  ** 0xFF, then a write-lock is held on the slot. Or, if they are set to
  ** a non-zero value smaller than 0xFF, then they represent the total 
  ** number of read-locks held on the slot. There is no way to distinguish
  ** between a write-lock and 255 read-locks.  */
  struct LockingSlot {
    u32 nLock;
    u64 aPadding[7];
  } aMFSlot[3 + SQLITE_MFS_NSHARD*5];
#endif
};

/*
** Atomic CAS primitive used in multi-process mode. Equivalent to:
** 
**   int unixCompareAndSwap(u32 *ptr, u32 oldval, u32 newval){
................................................................................
struct unixShm {
  unixShmNode *pShmNode;     /* The underlying unixShmNode object */
  unixShm *pNext;            /* Next unixShm with the same unixShmNode */
  u8 hasMutex;               /* True if holding the unixShmNode->pShmMutex */
  u8 id;                     /* Id of this connection within its unixShmNode */
  u16 sharedMask;            /* Mask of shared locks held */
  u16 exclMask;              /* Mask of exclusive locks held */
#ifdef SQLITE_MUTEXFREE_SHMLOCK
  u8 aMFCurrent[8];          /* Current slot used for each shared lock */
#endif
};

/*
** Constants used for locking
*/
#define UNIX_SHM_BASE   ((22+SQLITE_SHM_NLOCK)*4)         /* first lock byte */
#define UNIX_SHM_DMS    (UNIX_SHM_BASE+SQLITE_SHM_NLOCK)  /* deadman switch */
................................................................................
    *pp = 0;
  }
  if( pShmNode->isReadonly && rc==SQLITE_OK ) rc = SQLITE_READONLY;
  sqlite3_mutex_leave(pShmNode->pShmMutex);
  return rc;
}

#ifdef SQLITE_MUTEXFREE_SHMLOCK
static int unixMutexFreeShmlock(
  unixFile *pFd,             /* Database file holding the shared memory */
  int ofst,                  /* First lock to acquire or release */
  int n,                     /* Number of locks to acquire or release */
  int flags                  /* What to do with the lock */
){
  struct LockMapEntry {
    int iFirst;
    int nSlot;
  } aMap[9] = {
    { 0, 1 },
    { 1, 1 },
    { 2, 1 },
    { 3+0*SQLITE_MFS_NSHARD, SQLITE_MFS_NSHARD },
    { 3+1*SQLITE_MFS_NSHARD, SQLITE_MFS_NSHARD },
    { 3+2*SQLITE_MFS_NSHARD, SQLITE_MFS_NSHARD },
    { 3+3*SQLITE_MFS_NSHARD, SQLITE_MFS_NSHARD },
    { 3+4*SQLITE_MFS_NSHARD, SQLITE_MFS_NSHARD },
    { 3+5*SQLITE_MFS_NSHARD, 0 },
  };

  unixShm *p = pFd->pShm;               /* The shared memory being locked */
  unixShm *pX;                          /* For looping over all siblings */
  unixShmNode *pShmNode = p->pShmNode;  /* The underlying file iNode */
  int rc = SQLITE_OK;
  int iIncr;
  u16 mask;                             /* Mask of locks to take or release */

  if( flags & SQLITE_SHM_SHARED ){
    /* SHARED locks */
    u32 iOld, iNew, *ptr;
    int iIncr = -1;
    if( (flags & SQLITE_SHM_UNLOCK)==0 ){
      p->aMFCurrent[ofst] = (p->aMFCurrent[ofst] + 1) % aMap[ofst].nSlot;
      iIncr = 1;
    }
    ptr = &pShmNode->aMFSlot[aMap[ofst].iFirst + p->aMFCurrent[ofst]].nLock;
    do {
      iOld = *ptr;
      iNew = iOld + iIncr;
      if( iNew>SQLITE_MFS_EXCLUSIVE ){
        return SQLITE_BUSY;
      }
    }while( 0==unixCompareAndSwap(ptr, iOld, iNew) );
  }else{
    /* EXCLUSIVE locks */
    int iFirst = aMap[ofst].iFirst;
    int iLast = aMap[ofst+n].iFirst;
    int i;
    for(i=iFirst; i<iLast; i++){
      u32 *ptr = &pShmNode->aMFSlot[i].nLock;
      if( flags & SQLITE_SHM_UNLOCK ){
        assert( (*ptr)==SQLITE_MFS_EXCLUSIVE );
        *ptr = 0;
      }else{
        u32 iOld;
        do {
          iOld = *ptr;
          if( iOld>0 ){
            while( i>iFirst ){
              i--;
              pShmNode->aMFSlot[i].nLock = 0;
            }
            return SQLITE_BUSY;
          }
        }while( 0==unixCompareAndSwap(ptr, iOld, SQLITE_MFS_EXCLUSIVE) );
      }
    }
  }

  return SQLITE_OK;
}
#else
# define unixMutexFreeShmlock(a,b,c,d) SQLITE_OK
#endif

/*
** Change the lock state for a shared-memory segment.
**
** Note that the relationship between SHAREd and EXCLUSIVE locks is a little
** different here than in posix.  In xShmLock(), one can go from unlocked
** to shared and back or from unlocked to exclusive and back.  But one may
** not go from shared to exclusive or from exclusive to shared.
................................................................................
  assert( flags==(SQLITE_SHM_LOCK | SQLITE_SHM_SHARED)
       || flags==(SQLITE_SHM_LOCK | SQLITE_SHM_EXCLUSIVE)
       || flags==(SQLITE_SHM_UNLOCK | SQLITE_SHM_SHARED)
       || flags==(SQLITE_SHM_UNLOCK | SQLITE_SHM_EXCLUSIVE) );
  assert( n==1 || (flags & SQLITE_SHM_EXCLUSIVE)!=0 );
  assert( pShmNode->hShm>=0 || pDbFd->pInode->bProcessLock==1 );
  assert( pShmNode->hShm<0 || pDbFd->pInode->bProcessLock==0 );





  if( pDbFd->pInode->bProcessLock ){
    return unixMutexFreeShmlock(pDbFd, ofst, n, flags);
  }



































  mask = (1<<(ofst+n)) - (1<<ofst);
  assert( n>1 || mask==(1<<ofst) );
  if( flags & SQLITE_SHM_LOCK ){


    assert( !(flags&SQLITE_SHM_SHARED) || (p->sharedMask&mask)==0 );
    assert( !(flags&SQLITE_SHM_EXCLUSIVE) || !(p->exclMask&mask) );

  }else{

    assert( !(flags&SQLITE_SHM_SHARED) || (p->sharedMask&mask)==mask );
    assert( !(flags&SQLITE_SHM_EXCLUSIVE) || (p->exclMask&mask)==mask );
  }





  sqlite3_mutex_enter(pShmNode->pShmMutex);
  if( flags & SQLITE_SHM_UNLOCK ){
    u16 allMask = 0; /* Mask of locks held by siblings */

    /* See if any siblings hold this same lock */
    for(pX=pShmNode->pFirst; pX; pX=pX->pNext){

** An instance of the following structure is allocated for each active
** superlock. The opaque handle returned by sqlite3demo_superlock() is
** actually a pointer to an instance of this structure.
*/
struct Superlock {
  sqlite3 *db;                    /* Database handle used to lock db */
  int bWal;                       /* True if db is a WAL database */


};
typedef struct Superlock Superlock;

/*
** The pCtx pointer passed to this function is actually a pointer to a
** SuperlockBusy structure. Invoke the busy-handler function encapsulated
** by the structure and return the result.
................................................................................
}

/*
** Obtain the extra locks on the database file required for WAL databases.
** Invoke the supplied busy-handler as required.
*/
static int superlockWalLock(
  sqlite3 *db,                    /* Database handle open on WAL database */
  SuperlockBusy *pBusy            /* Busy handler wrapper object */
){
  int rc;                         /* Return code */
  sqlite3_file *fd = 0;           /* Main database file handle */
  void volatile *p = 0;           /* Pointer to first page of shared memory */


  /* Obtain a pointer to the sqlite3_file object open on the main db file. */
  rc = sqlite3_file_control(db, "main", SQLITE_FCNTL_FILE_POINTER, (void *)&fd);
  if( rc!=SQLITE_OK ) return rc;

  /* Obtain the "recovery" lock. Normally, this lock is only obtained by
  ** clients running database recovery.  
  */

  rc = superlockShmLock(fd, 2, 1, pBusy);
  if( rc!=SQLITE_OK ) return rc;


  /* Zero the start of the first shared-memory page. This means that any
  ** clients that open read or write transactions from this point on will
  ** have to run recovery before proceeding. Since they need the "recovery"
  ** lock that this process is holding to do that, no new read or write
  ** transactions may now be opened. Nor can a checkpoint be run, for the
  ** same reason.
................................................................................
  if( rc!=SQLITE_OK ) return rc;
  memset((void *)p, 0, 32);

  /* Obtain exclusive locks on all the "read-lock" slots. Once these locks
  ** are held, it is guaranteed that there are no active reader, writer or 
  ** checkpointer clients.
  */

  rc = superlockShmLock(fd, 3, SQLITE_SHM_NLOCK-3, pBusy);

  return rc;
}

/*
** Release a superlock held on a database file. The argument passed to 
** this function must have been obtained from a successful call to
** sqlite3demo_superlock().
................................................................................
  Superlock *p = (Superlock *)pLock;
  if( p->bWal ){
    int rc;                         /* Return code */
    int flags = SQLITE_SHM_UNLOCK | SQLITE_SHM_EXCLUSIVE;
    sqlite3_file *fd = 0;
    rc = sqlite3_file_control(p->db, "main", SQLITE_FCNTL_FILE_POINTER, (void *)&fd);
    if( rc==SQLITE_OK ){

      fd->pMethods->xShmLock(fd, 2, 1, flags);



      fd->pMethods->xShmLock(fd, 3, SQLITE_SHM_NLOCK-3, flags);


    }
  }
  sqlite3_close(p->db);
  sqlite3_free(p);
}

/*
................................................................................
  ** to drop the WAL read and write locks currently held. Otherwise, the
  ** new WAL locks may conflict with the old.
  */
  if( rc==SQLITE_OK ){
    if( SQLITE_OK==(rc = superlockIsWal(pLock)) && pLock->bWal ){
      rc = sqlite3_exec(pLock->db, "COMMIT", 0, 0, 0);
      if( rc==SQLITE_OK ){
        rc = superlockWalLock(pLock->db, &busy);
      }
    }
  }

  if( rc!=SQLITE_OK ){
    sqlite3demo_superunlock(pLock);
    *ppLock = 0;

** An instance of the following structure is allocated for each active
** superlock. The opaque handle returned by sqlite3demo_superlock() is
** actually a pointer to an instance of this structure.
*/
struct Superlock {
  sqlite3 *db;                    /* Database handle used to lock db */
  int bWal;                       /* True if db is a WAL database */
  int bRecoveryLocked;            /* True if WAL RECOVERY lock is held */
  int bReaderLocked;              /* True if WAL READER locks are held */
};
typedef struct Superlock Superlock;

/*
** The pCtx pointer passed to this function is actually a pointer to a
** SuperlockBusy structure. Invoke the busy-handler function encapsulated
** by the structure and return the result.
................................................................................
}

/*
** Obtain the extra locks on the database file required for WAL databases.
** Invoke the supplied busy-handler as required.
*/
static int superlockWalLock(
  Superlock *pLock,               /* Superlock handle */
  SuperlockBusy *pBusy            /* Busy handler wrapper object */
){
  int rc;                         /* Return code */
  sqlite3_file *fd = 0;           /* Main database file handle */
  void volatile *p = 0;           /* Pointer to first page of shared memory */
  sqlite3 *db = pLock->db;

  /* Obtain a pointer to the sqlite3_file object open on the main db file. */
  rc = sqlite3_file_control(db, "main", SQLITE_FCNTL_FILE_POINTER, (void *)&fd);
  if( rc!=SQLITE_OK ) return rc;

  /* Obtain the "recovery" lock. Normally, this lock is only obtained by
  ** clients running database recovery.  
  */
  assert( pLock->bRecoveryLocked==0 );
  rc = superlockShmLock(fd, 2, 1, pBusy);
  if( rc!=SQLITE_OK ) return rc;
  pLock->bRecoveryLocked = 1;

  /* Zero the start of the first shared-memory page. This means that any
  ** clients that open read or write transactions from this point on will
  ** have to run recovery before proceeding. Since they need the "recovery"
  ** lock that this process is holding to do that, no new read or write
  ** transactions may now be opened. Nor can a checkpoint be run, for the
  ** same reason.
................................................................................
  if( rc!=SQLITE_OK ) return rc;
  memset((void *)p, 0, 32);

  /* Obtain exclusive locks on all the "read-lock" slots. Once these locks
  ** are held, it is guaranteed that there are no active reader, writer or 
  ** checkpointer clients.
  */
  assert( pLock->bReaderLocked==0 );
  rc = superlockShmLock(fd, 3, SQLITE_SHM_NLOCK-3, pBusy);
  if( rc==SQLITE_OK ) pLock->bReaderLocked = 1;
  return rc;
}

/*
** Release a superlock held on a database file. The argument passed to 
** this function must have been obtained from a successful call to
** sqlite3demo_superlock().
................................................................................
  Superlock *p = (Superlock *)pLock;
  if( p->bWal ){
    int rc;                         /* Return code */
    int flags = SQLITE_SHM_UNLOCK | SQLITE_SHM_EXCLUSIVE;
    sqlite3_file *fd = 0;
    rc = sqlite3_file_control(p->db, "main", SQLITE_FCNTL_FILE_POINTER, (void *)&fd);
    if( rc==SQLITE_OK ){
      if( p->bRecoveryLocked ){
        fd->pMethods->xShmLock(fd, 2, 1, flags);
        p->bRecoveryLocked = 0;
      }
      if( p->bReaderLocked ){
        fd->pMethods->xShmLock(fd, 3, SQLITE_SHM_NLOCK-3, flags);
        p->bReaderLocked = 0;
      }
    }
  }
  sqlite3_close(p->db);
  sqlite3_free(p);
}

/*
................................................................................
  ** to drop the WAL read and write locks currently held. Otherwise, the
  ** new WAL locks may conflict with the old.
  */
  if( rc==SQLITE_OK ){
    if( SQLITE_OK==(rc = superlockIsWal(pLock)) && pLock->bWal ){
      rc = sqlite3_exec(pLock->db, "COMMIT", 0, 0, 0);
      if( rc==SQLITE_OK ){
        rc = superlockWalLock(pLock, &busy);
      }
    }
  }

  if( rc!=SQLITE_OK ){
    sqlite3demo_superunlock(pLock);
    *ppLock = 0;

      **         checkpoint need not have completed for this to cause problems.
      */
      volatile WalCkptInfo *pInfo = walCkptInfo(pWal);

      assert( pWal->readLock>0 || pWal->hdr.mxFrame==0 );
      assert( pInfo->aReadMark[pWal->readLock]<=pSnapshot->mxFrame );

      /* It is possible that there is a checkpointer thread running 
      ** concurrent with this code. If this is the case, it may be that the
      ** checkpointer has already determined that it will checkpoint 
      ** snapshot X, where X is later in the wal file than pSnapshot, but 
      ** has not yet set the pInfo->nBackfillAttempted variable to indicate 
      ** its intent. To avoid the race condition this leads to, ensure that
      ** there is no checkpointer process by taking a shared CKPT lock 
      ** before checking pInfo->nBackfillAttempted.  
      **
      ** TODO: Does the aReadMark[] lock prevent a checkpointer from doing
      **       this already?
      */
      rc = walLockShared(pWal, WAL_CKPT_LOCK);

      if( rc==SQLITE_OK ){
        /* Check that the wal file has not been wrapped. Assuming that it has
        ** not, also check that no checkpointer has attempted to checkpoint any
        ** frames beyond pSnapshot->mxFrame. If either of these conditions are
        ** true, return SQLITE_ERROR_SNAPSHOT. Otherwise, overwrite pWal->hdr
        ** with *pSnapshot and set *pChanged as appropriate for opening the
................................................................................
          assert( pWal->readLock>0 );
          memcpy(&pWal->hdr, pSnapshot, sizeof(WalIndexHdr));
          *pChanged = bChanged;
        }else{
          rc = SQLITE_ERROR_SNAPSHOT;
        }

        /* Release the shared CKPT lock obtained above. */
        walUnlockShared(pWal, WAL_CKPT_LOCK);
        pWal->minFrame = 1;
      }


      if( rc!=SQLITE_OK ){
        sqlite3WalEndReadTransaction(pWal);
      }

      **         checkpoint need not have completed for this to cause problems.
      */
      volatile WalCkptInfo *pInfo = walCkptInfo(pWal);

      assert( pWal->readLock>0 || pWal->hdr.mxFrame==0 );
      assert( pInfo->aReadMark[pWal->readLock]<=pSnapshot->mxFrame );

      /* If it were possible for a checkpointer thread to run concurrent 
      ** with this code, it would be a problem. In this case, it could be
      ** that the checkpointer has already determined that it will checkpoint 
      ** snapshot X, where X is later in the wal file than pSnapshot, but 
      ** has not yet set the pInfo->nBackfillAttempted variable to indicate 
      ** its intent. Fortunately this is not possible, as the call to
      ** sqlite3WalSnapshotOpen() that sets pWal->pSnapshot also takes a
      ** SHARED lock on the checkpointer slot.  */






      if( rc==SQLITE_OK ){
        /* Check that the wal file has not been wrapped. Assuming that it has
        ** not, also check that no checkpointer has attempted to checkpoint any
        ** frames beyond pSnapshot->mxFrame. If either of these conditions are
        ** true, return SQLITE_ERROR_SNAPSHOT. Otherwise, overwrite pWal->hdr
        ** with *pSnapshot and set *pChanged as appropriate for opening the
................................................................................
          assert( pWal->readLock>0 );
          memcpy(&pWal->hdr, pSnapshot, sizeof(WalIndexHdr));
          *pChanged = bChanged;
        }else{
          rc = SQLITE_ERROR_SNAPSHOT;
        }



        pWal->minFrame = 1;
      }


      if( rc!=SQLITE_OK ){
        sqlite3WalEndReadTransaction(pWal);
      }

      if {$rc != "SQLITE_OK"} { error $rc }
    }

    vfs_shmlock db255 main exclusive lock 4 1
  } {SQLITE_OK}

  vfs_shmlock db255 main exclusive unlock 4 1
}































































finish_test

      if {$rc != "SQLITE_OK"} { error $rc }
    }

    vfs_shmlock db255 main exclusive lock 4 1
  } {SQLITE_OK}

  vfs_shmlock db255 main exclusive unlock 4 1

  for {set i 0} {$i < 256} {incr i} {
    db$i close
  }
}

sqlite3 db0 test.db
sqlite3 db1 test.db
do_test 3.1 { execsql { SELECT * FROM t1 } db0 } {1 2}
do_test 3.2 { execsql { SELECT * FROM t1 } db1 } {1 2}

set L(0) {n n n n n n n n}
set L(1) {n n n n n n n n}
proc random_lock_test {idx} {
  global L
  set iSlot [expr int(rand()*8)]
  if {[expr int(rand()*2)]} {
    # Unlock operation
    if {[lindex $L($idx) $iSlot]!="n"} {
      vfs_shmlock db$idx main [lindex $L($idx) $iSlot] unlock $iSlot 1
      lset L($idx) $iSlot n
    }
  } else {
    # Lock operation
    if {[lindex $L($idx) $iSlot]=="n"} {
      set locktype [lindex {e s} [expr int(rand()*2)]]
      set n 1
      if {$locktype=="e"} {
        for {set l $iSlot} {$l<8 && [lindex $L($idx) $l]=="n"} {incr l} {}
        set n [expr int(rand()*($l-$iSlot))+1]
        # puts "iSlot=$iSlot l=$l L=$L($idx)"
        # puts "$iSlot $n"
      }
      set res [vfs_shmlock db$idx main $locktype lock $iSlot $n]

      set bBusy 0
      for {set i $iSlot} {$i<($iSlot+$n)} {incr i} {
        set other [lindex $L([expr ($idx+1)%2]) $i]
        if {($other!="n" && $locktype=="e")||($other=="e" && $locktype=="s")} {
          if {$res != "SQLITE_BUSY"} { error "BUSY not detected" }
          set bBusy 1
          break
        } 
      }

      if {$bBusy==0} {
        if {$res != "SQLITE_OK"} { error "BUSY false-positive" }
        for {set i $iSlot} {$i<($iSlot+$n)} {incr i} {
          lset L($idx) $i $locktype
        }
      }
    }
  }
}

set nStep 100000
for {set i 0} {$i < $nStep} {incr i} {
  random_lock_test 0
  random_lock_test 1
}

db0 close
db1 close

finish_test