[compiler-rt] Properly handle dispatch_source_set_cancel_handler with a NULL handler

[kubamracek]

Summary

Per https://developer.apple.com/library/mac/documentation/Performance/Reference/GCD_libdispatch_Ref/index.html, the dispatch_source_set_cancel_handler() API *can* be called with a NULL handler. In that case, the libdispatch removes an already existing cancellation handler, if there was one. ASan's interceptor always creates a new block that always tries to call the original handler. In case the original block is NULL, a segmentation fault happens. Let's fix that by not wrapping a NULL-block at all.

It looks like all the other libdispatch APIs (which we intercept) do *not* allow NULL. So it's really only the dispatch_source_set_cancel_handler one that needs this fix.

[glider]

LGTM

[kubamracek]

Landed in r224711.

lib/asan/asan_mac.cc

INTERCEPTOR(void, dispatch_after,
             dispatch_time_t when, dispatch_queue_t queue, void(^work)(void)) {
   ENABLE_FRAME_POINTER;
   GET_ASAN_BLOCK(work);
   REAL(dispatch_after)(when, queue, asan_block);
 }
 
 INTERCEPTOR(void, dispatch_source_set_cancel_handler,
             dispatch_source_t ds, void(^work)(void)) {
+  if (!work) {
+    REAL(dispatch_source_set_cancel_handler)(ds, work);
+    return;
+  }
   ENABLE_FRAME_POINTER;
   GET_ASAN_BLOCK(work);
   REAL(dispatch_source_set_cancel_handler)(ds, asan_block);
 }
 
 INTERCEPTOR(void, dispatch_source_set_event_handler,
             dispatch_source_t ds, void(^work)(void)) {
   ENABLE_FRAME_POINTER;
   GET_ASAN_BLOCK(work);
   REAL(dispatch_source_set_event_handler)(ds, asan_block);
 }
 #endif
 
 #endif  // SANITIZER_MAC