fix(blocks runtime): Implement _Block_isDeallocating()

This allows us to check whether a block is currently undergoing deallocation in a portable fashion.

arc.mm

@@ -8,12 +8,15 @@
 #include <assert.h>
 #include <vector>
 #include <tsl/robin_map.h>
-#ifdef HAVE_BLOCK_USE_RR2
-#include <Block_private.h>
-#endif
 #import "lock.h"
 #import "objc/runtime.h"
+#ifdef EMBEDDED_BLOCKS_RUNTIME
+#import "objc/blocks_private.h"
 #import "objc/blocks_runtime.h"
+#else
+#include <Block.h>
+#include <Block_private.h>
+#endif
 #import "nsobject.h"
 #import "class.h"
 #import "selector.h"
@@ -867,9 +870,20 @@ static BOOL setObjectHasWeakRefs(id obj)
 		*addr = obj;
 		return obj;
 	}
-	// If the object is being deallocated return nil.
-	if (object_getRetainCount_np(obj) == 0)
+	Class cls = classForObject(obj);
+	if (cls == static_cast<void*>(&_NSConcreteMallocBlock) ||
+		cls == static_cast<void*>(&_NSConcreteAutoBlock) ||
+		cls == static_cast<void*>(&_NSConcreteFinalizingBlock))
+	{
+		// Check whether the block is being deallocated and return nil if so
+		if (_Block_isDeallocating(obj)) {
+			*addr = nil;
+			return nil;
+		}
+	}
+	else if (object_getRetainCount_np(obj) == 0)
 	{
+		// If the object is being deallocated return nil.
 		*addr = nil;
 		return nil;
 	}

blocks_runtime.m

@@ -291,3 +291,13 @@ OBJC_PUBLIC void _Block_release(const void *src)
 		}
 	}
 }
+
+#ifdef EMBEDDED_BLOCKS_RUNTIME
+OBJC_PUBLIC bool _Block_isDeallocating(const void* arg)
+{
+	struct Block_layout *block = (struct Block_layout*)arg;
+	int *refCountPtr = &((struct Block_layout*)arg)->reserved;
+	int refCount = __sync_fetch_and_add(refCountPtr, 0);
+	return refCount == 0;
+}
+#endif

objc/blocks_private.h

@@ -3,7 +3,14 @@
 #if defined(__clang__) && !defined(__OBJC_RUNTIME_INTERNAL__)
 #pragma clang system_header
 #endif
+#ifdef __cplusplus
+#define BLOCKS_EXPORT extern "C"
+#else
+#define BLOCKS_EXPORT extern
+#endif
 
+#include <stdbool.h>
+#include "Availability.h"
 
 /*
  * This header file exposes some implementation details of the blocks runtime
@@ -38,6 +45,8 @@ struct Block_descriptor
 	const char *encoding;
 };
 
+OBJC_PUBLIC BLOCKS_EXPORT bool _Block_isDeallocating(const void *aBlock);
+
 // Helper structure
 struct Block_layout
 {