Possible deadlock with InlineTask and Mutex

Summary: This demonstrates a deadlock that's possible when using InlineTask.

Reviewed By: lbrandy

Differential Revision: D9883432

fbshipit-source-id: 45bb14181f6cfec41851315c0369232a30ba13d9

folly/experimental/coro/test/MutexTest.cpp

@@ -27,6 +27,7 @@
 #include <folly/experimental/coro/Mutex.h>
 #include <folly/experimental/coro/Promise.h>
 #include <folly/experimental/coro/Task.h>
+#include <folly/experimental/coro/detail/InlineTask.h>
 #include <folly/portability/GTest.h>
 
 #include <mutex>
@@ -157,4 +158,37 @@ TEST(Mutex, ThreadSafety) {
   CHECK_EQ(30'000, value);
 }
 
+TEST(Mutex, InlineTaskDeadlock) {
+  coro::Mutex coroMutex;
+  std::timed_mutex stdMutex;
+
+  std::thread thread1([&] {
+    coro::blockingWait(
+        [](auto& coroMutex, auto& stdMutex) -> coro::detail::InlineTask<void> {
+          co_await coroMutex.co_lock();
+          std::this_thread::sleep_for(std::chrono::milliseconds{200});
+          stdMutex.lock();
+          // At this point the other coroutine is suspended waiting on
+          // coroMutex.co_lock(). coroMutex.unlock() will unlock the mutex and
+          // run the other coroutine *inline*. That coroutine will
+          // try to acquire stdMutex resulting in a deadlock.
+          coroMutex.unlock();
+          stdMutex.unlock();
+        }(coroMutex, stdMutex));
+  });
+
+  std::thread thread2([&] {
+    coro::blockingWait(
+        [](auto& coroMutex, auto& stdMutex) -> coro::detail::InlineTask<void> {
+          std::this_thread::sleep_for(std::chrono::milliseconds{100});
+          co_await coroMutex.co_lock();
+          EXPECT_FALSE(stdMutex.try_lock_for(std::chrono::milliseconds{500}));
+          coroMutex.unlock();
+        }(coroMutex, stdMutex));
+  });
+
+  thread1.join();
+  thread2.join();
+}
+
 #endif