Am using an NSMutableSet that is working great, up until this weird SIGABRT error popped up:

2011-07-05 17:01:00.249 Dama[1497:cd57] * Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSCFSet removeObject:]: attempt to remove nil'

ok, I can't remove nil, but here is my code:

    id item = nil;
if ([theSet count] == 0) { // This equals 1 !!
    item = [[[self getClass] alloc] init];
} else {
    item = [[theSet anyObject] retain]; //this returns nil !!
    [theSet removeObject:item]; // ERROR !!
}

Using gdb console, I found theSet looks like this:

(gdb) po theSet

{(

 (null)

)}

(gdb) print (int) [theSet count]

$1 = 1

How is this even possible?

P.S.: Am running in a multi-threaded environment .. I can't promise that the isn't being accessed by two threads at the same time, but it shouldn't be 开发者_如何转开发the case..

EDIT

Please, I really want to know how can the NSSet end up with (null), I dont care how it got there .. No matter how dirty my multi-threaded program is, This shouldn't happen, I think. [And I double checked, this set is only accessed by the background worker thread].

EDIT

2011-07-05 17:39:55.884 App[1608:c59f] Set count = 2 Contains: {(

< Step: 0xc43f3e0>,
< Step: 0x62f1800> )} 

2011-07-05 17:39:55.886 App[1608:c59f] Set count = 9 Contains: {(

>     < Step: 0x62eece0>,
>     < Step: 0xc490660>,
>     < Step: 0xcb597d0>,
>     < Step: 0x65a4f60>,
>     < Step: 0xc43f4b0>,
>     < Step: 0xc43f3e0>,
>     < Step: 0x65c8f60>,
>     < Step: 0xc499230>,
>     (null) )} //null appeared?!

HINT!!

Check out the Step dealloc method (xD):

-(void)dealloc{
    if (![[PoolStep sharedPool] purgeFlag]) {
        [[PoolStep sharedPool] doneWithItem:self]; //put it in the object pool for reuse..
    } else {
        NSLog(@"=== STEP ====> [ %d ]", --_counter);
        [children release];
        [super dealloc];
    }
}

Another HINT!!

PoolStep is a singleton without @synchronize block in the sharedPool method, since it's accessed by one thread only. (and the performance is needed)

Have you tried wrapping it and any other code that uses theSet in a synchronized(theSet) block, a threading issue could explain your issue. Maybe the null is caused by some intermediate state.