Re: NSHashTable with weak objects crashing

Hi David,

While replacing the hash table, I managed to turn it into a reproduceable fault on at least one machine. The bug is quite subtle:

We have a map from objects to weak reference structures.
The weak reference structure points to the object.
When we delete the last weak reference, we delete the object from the map.
We delete the object from the map using the object as the key.
But by the time a weak reference is deallocated, its object pointer has been zeroed...
...so we always remove a random element from the map and leave a dangling pointer in the table.

Should this already be fixed on the latest libobjc2 master?

Going back to my original issue about NSHashTable with weak objects, I’m still seeing it crash with the latest libobjc2 master and also using the arc-cxx branch.

It reproduces quite easily with the following code (compiled with ARC), which crashes either directly in addObject: on the first invocation (when using weakObjectsHashTable), or in -allObjects on the second or third invocation (when using hashTableWithWeakObjects).

static __strong NSHashTable *_hashTable = nil;
static __strong NSObject *_test = nil;

- (void)testHashTable
{
    @autoreleasepool {
        if (!_hashTable) {
            _hashTable = [NSHashTable weakObjectsHashTable]; // or hashTableWithWeakObjects 
            _test = [NSObject new];
            [_hashTable addObject:_test]; // crash 1
        } else {
            _test = nil;
        }

        NSLog(@"HashTable %@ (_test: %@)", _hashTable.allObjects, _test); // crash 2
    }
}

Similarly, NSMapTable crashes as well in the second invocation of the following function, although I’m not sure if this is the same root cause as the hash table crash:

static __strong NSMapTable *_mapTable = nil;
static __strong NSObject *_test = nil;

- (void)testMapTable
{
    @autoreleasepool {
        if (!_mapTable) {
            _mapTable = [NSMapTable strongToWeakObjectsMapTable];
            _test = [NSObject new];
            [_mapTable setObject:_test forKey:@"test"];
        } else {
            NSLog(@"obj pre: %@", [_mapTable objectForKey:@"test"]); // crash!!!
            _test = nil;
            NSLog(@"obj post: %@", [_mapTable objectForKey:@"test"]);
        }

        NSLog(@"MapTable %@ (_test: %@)", _mapTable.dictionaryRepresentation, _test);
    }
}

I’m pasting the stack traces below. I’d appreciate if anyone can shed some light on this.

Thanks!

Frederik

NSHashTable crash using weakObjectsHashTable:

* frame #0: 0xeca8c1d0 libart.so`art_sigsegv_fault

frame #1: 0xeca8c774 libart.so`art::FaultManager::HandleFault(int, siginfo*, void*) + 372

frame #2: 0xeca8c49b libart.so`art::art_fault_handler(int, siginfo*, void*) (.llvm.650222801) + 43

frame #3: 0x625bd6af app_process32`___lldb_unnamed_symbol22$$app_process32 + 623

frame #4: 0xefee7c50 libc.so`___lldb_unnamed_symbol2$$libc.so + 1

frame #5: 0xd1e56762 libobjc.so`objc_msgSend at objc_msgSend.x86-32.S:120

frame #6: 0xd14f69cd libgnustep-base.so`hashObject(item=0xcf40c100, size=0x00000000) at NSConcretePointerFunctions.m:126

frame #7: 0xd1400a72 libgnustep-base.so`pointerFunctionsHash(PF=0xd0ea7900, item=0xcf40c100) at NSConcretePointerFunctions.h:180

frame #8: 0xd13fd9b6 libgnustep-base.so`GSIMapBucketForKey(map=0xd0ea78d4, key=GSIMapKey @ 0xff99f2a4) at GSIMap.h:410

frame #9: 0xd1400ce3 libgnustep-base.so`GSIMapAddNodeToMap(map=0xd0ea78d4, node=0xcf40c0f0) at GSIMap.h:453

frame #10: 0xd13fcb64 libgnustep-base.so`GSIMapAddKey(map=0xd0ea78d4, key=GSIMapKey @ 0xff99f334) at GSIMap.h:1118

frame #11: 0xd13fe82f libgnustep-base.so`-[NSConcreteHashTable addObject:](self=0xd0ea78d4, _cmd="C", anObject=0xd0efd14c) at NSConcreteHashTable.m:848

frame #12: 0xd1e2e7ff libnative-lib.so`::-[ObjectiveCTest testHashTable](self=0xcf40c0e4, _cmd="V\x1c") at ObjectiveCTest.mm:78

NSHashTable crash using hashTableWithWeakObjects:

* frame #0: 0xeca8c1d0 libart.so`art_sigsegv_fault

frame #1: 0xeca8c774 libart.so`art::FaultManager::HandleFault(int, siginfo*, void*) + 372

frame #2: 0xeca8c49b libart.so`art::art_fault_handler(int, siginfo*, void*) (.llvm.650222801) + 43

frame #3: 0x625bd6af app_process32`___lldb_unnamed_symbol22$$app_process32 + 623

frame #4: 0xefee7c50 libc.so`___lldb_unnamed_symbol2$$libc.so + 1

frame #5: 0xd1ea975d libobjc.so`objc_msgSend at objc_msgSend.x86-32.S:120

frame #6: 0xd13170bd libgnustep-base.so`-[GSInlineArray initWithObjects:count:](self=0xcf4fd184, _cmd="\xffffff81", objects=0xff99f320, count=1) at GSArray.m:420

frame #7: 0xd131a41d libgnustep-base.so`-[GSPlaceholderArray initWithObjects:count:](self=0xcf4fd184, _cmd="\xffffff81", objects=0xff99f320, count=1) at GSArray.m:1199

frame #8: 0xd13c2b80 libgnustep-base.so`-[NSConcreteHashTable allObjects](self=0xd0da9a14, _cmd="m\x02") at NSConcreteHashTable.m:875

frame #9: 0xd1df7844 libnative-lib.so`::-[ObjectiveCTest testHashTable](self=0xcf391444, _cmd="V\x1c") at ObjectiveCTest.mm:84

NSMapTable crash:

* frame #0: 0xeca8c1d1 libart.so`art_sigsegv_fault + 1

frame #1: 0xeca8c774 libart.so`art::FaultManager::HandleFault(int, siginfo*, void*) + 372

frame #2: 0xeca8c49b libart.so`art::art_fault_handler(int, siginfo*, void*) (.llvm.650222801) + 43

frame #3: 0x625bd6af app_process32`___lldb_unnamed_symbol22$$app_process32 + 623

frame #4: 0xefee7c50 libc.so`___lldb_unnamed_symbol2$$libc.so + 1

frame #5: 0xd1e29c6d libobjc.so`::objc_retainAutoreleasedReturnValue(id) [inlined] isPersistentObject(obj=0xd0bfb27c) at arc.mm:291

frame #6: 0xd1e29c66 libobjc.so`::objc_retainAutoreleasedReturnValue(id) [inlined] retain(obj=0xd0bfb27c) at arc.mm:296

frame #7: 0xd1e29c66 libobjc.so`::objc_retainAutoreleasedReturnValue(id) [inlined] objc_retain(obj=0xd0bfb27c) at arc.mm:587

frame #8: 0xd1e29c62 libobjc.so`::objc_retainAutoreleasedReturnValue(obj=0xd0bfb27c) at arc.mm:581

frame #9: 0xd1175a04 libnative-lib.so`::-[ObjectiveCTest testMapTable](self=0xcf66c064, _cmd="X\x1c") at ObjectiveCTest.mm:97

From:	Frederik Seiffert
Subject:	Re: NSHashTable with weak objects crashing
Date:	Mon, 7 Oct 2019 16:51:43 +0200