[TSan][libdispatch] Add RunLoop helper to ease porting tests

[yln]

Summary

Many existing libdispatch tests use CFRunLoop{Run,Stop} to setup the
test harness. Unfortunately, libdispatch only offers dispatch_main
which has no built-in functionality for stopping the run loop.

To emulate the Foundation behavior we schedule an "exit block" to the
main queue that exists when a flag is set. If the flag hasn't been set
yet, the block resubmits itself to to the queue.

I included one adapted example test to show how this would be used.
Let me know if you think this is a good approach and if it sufficiently retains
the original test semantics.

[kubamracek]

See the inline notes. Is there no better approach?

compiler-rt/test/tsan/Darwin/gcd-blocks.mm

27

Can we keep the print in main?

compiler-rt/test/tsan/Darwin/runloop.h

22

Can we somehow exit() from by returning from main instead? That would be much cleaner.

24

This keeps burning the CPU until we exit.

[yln]

compiler-rt/test/tsan/Darwin/gcd-blocks.mm

27

We could pass a block that expresses "the rest of the program".

RunLoopRun(^{
  fprintf(stderr, "Done.\n");
});

I looked at all usages of CFRunLoopRun and that's what the block would ever be used for. I agree that having a CHECK: Done. line that has no obvious source in the test is ugly, but in this case I would prefer it over the block alternative.
We could also make the check lines more explicit, e.g., CHECK: RunLoopShutdown.

compiler-rt/test/tsan/Darwin/runloop.h

22

I don't know how to do this. The main crux here is that dispatch_main never returns, and doesn't provide a built-in means to stop the run loop. Do you have a suggestion?

24

Yes, we could introduce a short sleep for cases where we are waiting on something (i.e., a timer).

compiler-rt/test/tsan/Darwin/gcd-blocks.mm

-// RUN: %clangxx_tsan %s -o %t -framework Foundation
-// RUN: %run %t 2>&1 | FileCheck %s
+// RUN: %clangxx_tsan %s -o %t
+// RUN: %run %t 2>&1 | FileCheck %s --implicit-check-not='ThreadSanitizer'
 
-#import <Foundation/Foundation.h>
+#include "runloop.h"
 
 int main() {
   fprintf(stderr, "start\n");
 
   dispatch_queue_t background_q = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
   dispatch_queue_t main_q = dispatch_get_main_queue();
 
   dispatch_async(background_q, ^{
     __block long block_var = 0;
 
     dispatch_sync(main_q, ^{
       block_var = 42;
     });
 
     fprintf(stderr, "block_var = %ld\n", block_var);
 
     dispatch_sync(dispatch_get_main_queue(), ^{
-      CFRunLoopStop(CFRunLoopGetCurrent());
+      RunLoopStop();
     });
   });
-  
-  CFRunLoopRun();
-  fprintf(stderr, "done\n");
+  RunLoopRun();
 }
 
 // CHECK: start
 // CHECK: block_var = 42
-// CHECK: done
-// CHECK-NOT: WARNING: ThreadSanitizer
-// CHECK-NOT: CHECK failed
+// CHECK: Done.

compiler-rt/test/tsan/Darwin/runloop.h

+// Emulation of Foundation RunLoop functionality.
+// Supports porting of libdispatch tests to platforms without Foundation.
+
+#include <dispatch/dispatch.h>
+#include <stdio.h>  // fprintf
+#include <stdlib.h> // exit
+
+static bool runloop_exit = false;
+
+static inline void RunLoopStop() {
+  runloop_exit = true;
+}
+
+// Start executing blocks submitted to the main queue.
+// Never returns (should be the last statement in a test's main function). Loops
+// indefinitely until a call to RunLoopStop is scheduled on the main queue.
+// Prints "Done." to stderr before exiting.
+static inline void RunLoopRun() {
+  __block dispatch_block_t exitBlock = ^{
+    if (runloop_exit) {
+      fprintf(stderr, "Done.\n");
+      exit(0);
+    } else {
+      dispatch_async(dispatch_get_main_queue(), exitBlock);
+    }
+  };
+
+  dispatch_async(dispatch_get_main_queue(), exitBlock);
+  dispatch_main(); // Never returns
+}