[Pyobjc-dev] Bug Fixed!!! Dealing with alloc / init oddities

[Bill B. <bb...@co...>]

(Another "debugging out loud" session -- but this one ended happily.   
I'll put the 'solution' first and those interested can read down for 
the details that led to it...)

#if 1
	self_obj = nil;
	if (*[methinfo methodReturnType] == _C_ID) {
		[inv setReturnValue:&self_obj];
	}
	[inv setTarget:self_obj];
	[inv setArgument:&self_obj atIndex:0];
#endif

	[inv release];
	[methinfo release];
	inv = nil;

The above is found in objc_support.m around line 1423.   By changing 
the "#if 1" to an "#if 0", the bug goes away and all unit tests pass 
(except invocation of class methods when an instance method of the same 
name also exists).

The code #if'd in/out looks to be there to force the NSInvocation to 
clean up after itself.  In theory, that should be handled by the [inv 
-release] on the line immediately after that block of code.    In 
practice, this still shouldn't cause a crash assuming that the 
NSInvocation only retains things that it releases.

Why the crash was happening isn't clear to me.   The [lack of the 
above] code may also potentially be causing a memory leak?

Change committed.

---

I believe I have tracked the crasher w/the collection classes down to 
being related to the following behavior of NSArray:

int main (int argc, const char * argv[]) {
   NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

   NSArray *a;

   a = [NSMutableArray alloc];
   NSLog(@"0x%x", a);
   a = [a init];
   NSLog(@"0x%x", a);

   a = [NSMutableArray alloc];
   NSLog(@"0x%x", a);
   a = [a init];
   NSLog(@"0x%x", a);

   [pool release];
   return 0;
}

The above outputs -- of note, If I change the second [NSMutableArray 
alloc] to [NSArray alloc], the output remains *unchanged*.

2002-10-24 09:35:37.398 barfoo[19606] 0x4b580
2002-10-24 09:35:37.424 barfoo[19606] 0x57c90
2002-10-24 09:35:37.439 barfoo[19606] 0x4b580
2002-10-24 09:35:37.452 barfoo[19606] 0x57cb0

Clearly, the Foundation/Core are playing games with the 
NSArray/NSMutableArray class clusters.   The +alloc methods return 
placeholders that are turned into the appropriate instance upon 
initialization.  This makes sense given that the -init* method used 
determines the role and, hence, the appropriate private subclass, of 
the instance of the class cluster.

Now, consider the following:

[bumbox:~/bbum-developer/sourceforge/pyobjc] bbum% python
 >>> from Foundation import *
 >>> NSArray.alloc().init()
()
 >>> NSMutableArray.alloc().init()
()
 >>> NSArray.alloc().init()
2002-10-24 09:37:46.189 python[19612] Did you forget to nest alloc and 
init?
2002-10-24 09:37:46.191 python[19612] *** Uncaught exception: 
<NSInvalidArgumentException> *** -length only defined for abstract 
class.  Define -[NSPlaceholderMutableString length]!

It appears that the bridge somehow caches the placeholder instance and 
ends up releasing or corrupting it upon the second call to the alloc() 
method *for that particular class*?

Given that last bug, it seems like the shared placeholder array 
instance may be being released one time to many??

It is! (see above)