SQLite Unlock-Notify API

/* This example uses the pthreads API */
#include <pthread.h>

/*
** A pointer to an instance of this structure is passed as the user-context
** pointer when registering for an unlock-notify callback.
*/
typedef struct UnlockNotification UnlockNotification;
struct UnlockNotification {
  int fired;                         /* True after unlock event has occurred */
  pthread_cond_t cond;               /* Condition variable to wait on */
  pthread_mutex_t mutex;             /* Mutex to protect structure */
};

/*
** This function is an unlock-notify callback registered with SQLite.
*/
static void unlock_notify_cb(void **apArg, int nArg){
  int i;
  for(i=0; i<nArg; i++){
    UnlockNotification *p = (UnlockNotification *)apArg[i];
    pthread_mutex_lock(&p->mutex);
    p->fired = 1;
    pthread_cond_signal(&p->cond);
    pthread_mutex_unlock(&p->mutex);
  }
}

/*
** This function assumes that an SQLite API call (either sqlite3_prepare_v2() 
** or sqlite3_step()) has just returned SQLITE_LOCKED. The argument is the
** associated database connection.
**
** This function calls sqlite3_unlock_notify() to register for an 
** unlock-notify callback, then blocks until that callback is delivered 
** and returns SQLITE_OK. The caller should then retry the failed operation.
**
** Or, if sqlite3_unlock_notify() indicates that to block would deadlock 
** the system, then this function returns SQLITE_LOCKED immediately. In 
** this case the caller should not retry the operation and should roll 
** back the current transaction (if any).
*/
static int wait_for_unlock_notify(sqlite3 *db){
  int rc;
  UnlockNotification un;

  /* Initialize the UnlockNotification structure. */
  un.fired = 0;
  pthread_mutex_init(&un.mutex, 0);
  pthread_cond_init(&un.cond, 0);

  /* Register for an unlock-notify callback. */
  rc = sqlite3_unlock_notify(db, unlock_notify_cb, (void *)&un);
  assert( rc==SQLITE_LOCKED || rc==SQLITE_OK );

  /* The call to sqlite3_unlock_notify() always returns either SQLITE_LOCKED 
  ** or SQLITE_OK. 
  **
  ** If SQLITE_LOCKED was returned, then the system is deadlocked. In this
  ** case this function needs to return SQLITE_LOCKED to the caller so 
  ** that the current transaction can be rolled back. Otherwise, block
  ** until the unlock-notify callback is invoked, then return SQLITE_OK.
  */
  if( rc==SQLITE_OK ){
    pthread_mutex_lock(&un.mutex);
    if( !un.fired ){
      pthread_cond_wait(&un.cond, &un.mutex);
    }
    pthread_mutex_unlock(&un.mutex);
  }

  /* Destroy the mutex and condition variables. */
  pthread_cond_destroy(&un.cond);
  pthread_mutex_destroy(&un.mutex);

  return rc;
}

/*
** This function is a wrapper around the SQLite function sqlite3_step().
** It functions in the same way as step(), except that if a required
** shared-cache lock cannot be obtained, this function may block waiting for
** the lock to become available. In this scenario the normal API step()
** function always returns SQLITE_LOCKED.
**
** If this function returns SQLITE_LOCKED, the caller should rollback
** the current transaction (if any) and try again later. Otherwise, the
** system may become deadlocked.
*/
int sqlite3_blocking_step(sqlite3_stmt *pStmt){
  int rc;
  while( SQLITE_LOCKED==(rc = sqlite3_step(pStmt)) ){
    rc = wait_for_unlock_notify(sqlite3_db_handle(pStmt));
    if( rc!=SQLITE_OK ) break;
    sqlite3_reset(pStmt);
  }
  return rc;
}

/*
** This function is a wrapper around the SQLite function sqlite3_prepare_v2().
** It functions in the same way as prepare_v2(), except that if a required
** shared-cache lock cannot be obtained, this function may block waiting for
** the lock to become available. In this scenario the normal API prepare_v2()
** function always returns SQLITE_LOCKED.
**
** If this function returns SQLITE_LOCKED, the caller should rollback
** the current transaction (if any) and try again later. Otherwise, the
** system may become deadlocked.
*/
int sqlite3_blocking_prepare_v2(
  sqlite3 *db,              /* Database handle. */
  const char *zSql,         /* UTF-8 encoded SQL statement. */
  int nSql,                 /* Length of zSql in bytes. */
  sqlite3_stmt **ppStmt,    /* OUT: A pointer to the prepared statement */
  const char **pz           /* OUT: End of parsed string */
){
  int rc;
  while( SQLITE_LOCKED==(rc = sqlite3_prepare_v2(db, zSql, nSql, ppStmt, pz)) ){
    rc = wait_for_unlock_notify(db);
    if( rc!=SQLITE_OK ) break;
  }
  return rc;
}

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