Re: NSConnection bugs

[Richard Frith-Macdonald]

On 29 Mar 2011, at 01:01, David Chisnall wrote:

> Hello the list,
> 
> I spent some time with valgrind and found out why NSConnection is so flakey 
> on 64-bit platforms.  It seems that we use the same incorrect pattern in a 
> lot of places throughout the class.  We probably use it in other places, so 
> it would be worth doing a brief audit for this bug before the next release.  
> I had a quick look, and I think that NSConnection is the only place where we 
> use GSIMap with something other than pointers.
> 
> The problem is in statements like this:
> 
>      node = GSIMapNodeForKey(IreplyMap, (GSIMapKey)seq);
> 
> Looks fine?  The problem is that (GSIMapKey) is a union (something concealed 
> by the fact that it is a macro - unions are difficult to use safely, and 
> anything that hides the fact that you are using a union is generally a bad 
> thing).  Specifically, it's a problem that GSIMapKey is a union of void*, id, 
> unsigned, and int.  On most current 64-bit platforms, unsigned and int are 
> 32-bit quantities, while void* and id are 64-bit quantities. 
> 
> The problem, therefore, is that the we are defining 4 bytes of an 8-byte 
> quantity and then (in the callee) accessing all 8 bytes.  This results in 
> some random behaviour.  The connection.m test was failing because sometimes 
> the uninitialised bytes were 0 (so the map was finding the value it was 
> looking for) and sometimes they were some other value from the stack.

Well spotted.  I've implemented a fix for that throughout base.