title: db executeUpdate... in FMDB block and doesn't go over, without error
body: <p>i'm using the amazing FMDB project in my app in development, i have a NSOperation like this:</p>
  
  <pre><code>- (void)main
  {
   @autoreleasepool {
  
  FMDatabaseQueue *queue = [FMDatabaseQueue databaseQueueWithPath:[[NSUserDefaults standardUserDefaults] valueForKey:@"pathDB"]];
  
  [queue inDatabase:^(FMDatabase *db) {
  
      FMResultSet *toQuery;
  
      if (self._id == nil) {
          toQuery = [db executeQuery:@"SELECT id,language,update_time FROM task"];
  
      while ([toQuery next]) {
  
          [myarray addObject:[toQuery resultDictionary]];
  
      }
  }];
  
  for (int i = 0; i&lt;[myarray count]; i++){
  
  ...Do Something
  
  [queue inDatabase:^(FMDatabase *db) {
  
  FMResultSet *checkImgQuery = [db executeQuery:@"SELECT img_url,img_path FROM task WHERE id = ? AND language = ?",myTask.id ,myTask.lang];
  
  while ([checkImgQuery next]) {
  
   if (![[checkImgQuery stringForColumn:@"img_url"] isEqualToString:myTask.img]) {
                                      NSData *my_img = [NSData dataWithContentsOfURL:[NSURL URLWithString:myTask.img]];
  
  if (my_img != nil) {
  
  NSError *er;
  
  [my_img writeToFile:[checkImgQuery stringForColumn:@"img_path"] options:NSDataWritingAtomic error:&amp;er];
  
  //In the line under here the code block, the app still running, but this operation doesn't
  //go over this task
  
  [db executeUpdate:@"UPDATE task SET img_url = ? WHERE id = ? AND language = ?",myTask.img,[NSNumber numberWithInt:myTask.id],[NSNumber numberWithInt:myTask.language];
  
  NSLog(@"%@",[db lastErrorMessage]);
  }
  ...Do Something
  }
  }
  }
  }];
  }
  }
  </code></pre>
  
  <p>The problem is in <code>[db executeUpdate:...]</code> that sometime works with no problem and sometime freeze and doesn't go over that line, the <code>NSLog</code> i have put there doesn't print anything, the app doesn't crash and continue working, but the thread is stuck there, if i shutdown the run of the app, and i restart it again the thread doesn't stop on the same task, but random on another, with no criteria, some time one works, and some time doesn't...anyone can help?</p>