title: Scribble uncovering bug in block use I don't understand
body: <p>I have a bug which I have uncovered by enabling Scribble in Xcode, fixing the bug is not an issue, it isn't implemented in the best way, I can just remove the whole block, but I don't understand why I am getting the issue in the first place, which tells me I don't understand something</p>
  
  <p>If I have Scribble enabled, when it tries to execute the release line in the code below it crashes without fail,</p>
  
  <pre><code>HDClipPlaybackController    * theController =  nil;
  if( theSource != nil ) {
      theController = [[HDClipPlaybackController alloc] initWithClipProxyList:theSource];
  }
  else {
      theController = [[HDClipPlaybackController alloc] initWithClips:clips handles:[handles intValue]];
  }
  theController.startIndex = [startIndex intValue];
  theController.completionHandler = ^(BOOL success){
      theController.completionHandler = nil;
      [theController release];       // &lt;-- CRASH
  };
  [theController performSelectorInBackground:@selector(startDownloadingClips:) withObject:theController.clipProxyList.everyClipProxy];
  </code></pre>
  
  <p>Thread 1: EXC_BAD_ACCESS (code=1, address=0x55555555)</p>
  
  <p>adding a break point on the line before the release line and looking at the value of <em>theController</em> it is a valid object addres but stepping onto the next line I can see the value has changed to 0x55555555 (Scribble has freed it), which I would take to mean that the memory for the block has been freed, since my understanding is that the local variable is copied into the blocks scope, but that would mean the blocks memory is being freed before it has finished executing? If I just move the release to outside the block, the crash goes away, also if I enable Zombies instead of Scribble I don't have any issue, so it doesn't look like an over release issue to me. The variable <em>theController</em> isn't declared __block, so it should just be a simpler pointer inside the block scope if I understand things correctly.</p>
  
  <p>This is a Mac OS X app running as 32bit, with Xcode 6.1.1 and Mac OS 10.9.5.</p>