SQLite

struct SuperlockBusy {
  int (*xBusy)(void*,int);        /* Pointer to busy-handler function */
  void *pBusyArg;                 /* First arg to pass to xBusy */
  int nBusy;                      /* Number of times xBusy has been invoked */
};
typedef struct SuperlockBusy SuperlockBusy;

/*
** 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.
*/
static int superlockBusyHandler(void *pCtx, int UNUSED){
  SuperlockBusy *pBusy = (SuperlockBusy *)pCtx;
  if( pBusy->xBusy==0 ) return 0;
  return pBusy->xBusy(pBusy->pBusyArg, pBusy->nBusy++);
}

/*
** This function is used to determine if the main database file for 
** connection db is open in WAL mode or not. If no error occurs and the
** database file is in WAL mode, set *pbWal to true and return SQLITE_OK.
** If it is not in WAL mode, set *pbWal to false.
**
** If an error occurs, return an SQLite error code. The value of *pbWal
** is undefined in this case.
*/
static int superlockIsWal(Superlock *pLock){
  int rc;                         /* Return Code */
  sqlite3_stmt *pStmt;            /* Compiled PRAGMA journal_mode statement */

  rc = sqlite3_prepare(pLock->db, "PRAGMA main.journal_mode", -1, &pStmt, 0);
  if( rc!=SQLITE_OK ) return rc;

  pLock->bWal = 0;
  if( SQLITE_ROW==sqlite3_step(pStmt) ){
    const char *zMode = (const char *)sqlite3_column_text(pStmt, 0);
    if( zMode && strlen(zMode)==3 && sqlite3_strnicmp("wal", zMode, 3)==0 ){
      pLock->bWal = 1;
    }
  }

  return sqlite3_finalize(pStmt);
}

/*

  /* 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().
*/
void sqlite3demo_superunlock(void *pLock){
  Superlock *p = (Superlock *)pLock;
  if( p->bWal ){
    int flags = SQLITE_SHM_UNLOCK | SQLITE_SHM_EXCLUSIVE;
    sqlite3_file *fd = 0;
    sqlite3_file_control(p->db, "main", SQLITE_FCNTL_FILE_POINTER, (void *)&fd);
    fd->pMethods->xShmLock(fd, 2, SQLITE_SHM_NLOCK-2, flags);
  }
  sqlite3_close(p->db);
  sqlite3_free(p);
}

/*
** Obtain a superlock on the database file identified by zPath, using the
** locking primitives provided by VFS zVfs. If successful, SQLITE_OK is
** returned and output variable *ppLock is populated with an opaque handle
** that may be used with sqlite3demo_superunlock() to release the lock.
**

int sqlite3demo_superlock(
  const char *zPath,              /* Path to database file to lock */
  const char *zVfs,               /* VFS to use to access database file */
  int (*xBusy)(void*,int),        /* Busy handler callback */
  void *pBusyArg,                 /* Context arg for busy handler */
  void **ppLock                   /* OUT: Context to pass to superunlock() */
){

  SuperlockBusy busy = {0, 0, 0}; /* Busy handler wrapper object */
  int rc;                         /* Return code */
  Superlock *pLock;

  pLock = sqlite3_malloc(sizeof(Superlock));
  if( !pLock ) return SQLITE_NOMEM;
  memset(pLock, 0, sizeof(Superlock));

  /* Open a database handle on the file to superlock. */
  rc = sqlite3_open_v2(
      zPath, &pLock->db, SQLITE_OPEN_READWRITE|SQLITE_OPEN_CREATE, zVfs
  );

  /* Install a busy-handler and execute a BEGIN EXCLUSIVE. If this is not
  ** a WAL database, this is all we need to do.  
  **
  ** A wrapper function is used to invoke the busy-handler instead of
  ** registering the busy-handler function supplied by the user directly
  ** with SQLite. This is because the same busy-handler function may be
  ** invoked directly later on when attempting to obtain the extra locks
  ** required in WAL mode. By using the wrapper, we are able to guarantee
  ** that the "nBusy" integer parameter passed to the users busy-handler
  ** represents the total number of busy-handler invocations made within
  ** this call to sqlite3demo_superlock(), including any made during the
  ** "BEGIN EXCLUSIVE".
  */
  if( rc==SQLITE_OK ){
    busy.xBusy = xBusy;
    busy.pBusyArg = pBusyArg;
    sqlite3_busy_handler(pLock->db, superlockBusyHandler, (void *)&busy);
    rc = sqlite3_exec(pLock->db, "BEGIN EXCLUSIVE", 0, 0, 0);
  }

  /* If the BEGIN EXCLUSIVE was executed successfully and this is a WAL
  ** database, call superlockWalLock() to obtain the extra locks required
  ** to prevent readers, writers and/or checkpointers from accessing the
  ** db while this process is holding the superlock.
  **
  ** Before attempting any WAL locks, commit the transaction started above
  ** 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;
  }else{
    *ppLock = pLock;
  }

  return rc;
}










/*
** End of example code. Everything below here is the test harness.
**************************************************************************
**************************************************************************
*************************************************************************/

    list [catch {sqlite3demo_superlock unlock test.db} msg] $msg
  } {1 {database is locked}}
  do_test 5.$tn.12 { 
    sql3 COMMIT
    list [catch {sqlite3demo_superlock unlock test.db} msg] $msg
  } {0 unlock}
  unlock


  do_test 5.$tn.13 { sql1 { SELECT * FROM t1 } } {1 2 3 4 5 6}
  do_test 5.$tn.14 { sql2 { SELECT * FROM t1 } } {1 2 3 4 5 6}
  do_test 5.$tn.15 { sqlite3demo_superlock unlock test.db } {unlock}
  do_test 5.$tn.16 { unlock } {}
  do_test 5.$tn.17 { sql2 { SELECT * FROM t1 } } {1 2 3 4 5 6}
  do_test 5.$tn.18 { sql1 { SELECT * FROM t1 } } {1 2 3 4 5 6}
  do_test 5.$tn.19 { sql2 { SELECT * FROM t1 } } {1 2 3 4 5 6}
}

proc read_content {file} {
  if {[file exists $file]==0} {return ""}
  set fd [open $file]
  fconfigure $fd -encoding binary -translation binary
  set content [read $fd]