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

[Kuba Brecka]

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.

http://reviews.llvm.org/D6747

Files:
  lib/asan/asan_mac.cc

Index: lib/asan/asan_mac.cc
===================================================================
--- lib/asan/asan_mac.cc
+++ lib/asan/asan_mac.cc
@@ -403,6 +403,10 @@
 
 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);

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