fix deadlock folly::RequestContext::try_get()

Summary:
`folly::RequestContext::try_get()` might deadlock if accessor to all threads is
held alive.

Reviewed By: amlannayak

Differential Revision: D31496646

fbshipit-source-id: 6edfd0a54f8bfaf72218d78ba1a6c09dc92a8e63

folly/io/async/Request.cpp

@@ -608,13 +608,18 @@ void RequestContext::clearContextData(const RequestToken& val) {
   return prevCtx;
 }
 
+namespace {
+thread_local bool getStaticContextCalled = false;
+}
+
 /* static */ RequestContext::StaticContext& RequestContext::getStaticContext() {
+  getStaticContextCalled = true;
   return StaticContextThreadLocal::get();
 }
 
 /* static */ RequestContext::StaticContext*
 RequestContext::tryGetStaticContext() {
-  return StaticContextThreadLocal::try_get();
+  return getStaticContextCalled ? &StaticContextThreadLocal::get() : nullptr;
 }
 
 /* static */ RequestContext::StaticContextAccessor

folly/io/async/test/RequestContextTest.cpp

@@ -584,4 +584,19 @@ TEST(RequestContextTryGetTest, TryGetTest) {
   EXPECT_TRUE(rc->hasContextData("test"));
   auto* dataPtr = dynamic_cast<TestData*>(rc->getContextData("test"));
   EXPECT_EQ(dataPtr->data_, 10);
+
+  auto thread = std::thread([&] {
+    auto accessor = RequestContext::accessAllThreads();
+    // test there is no deadlock with try_get()
+    RequestContext::try_get();
+  });
+  thread.join();
+
+  thread = std::thread([&] {
+    RequestContext::get();
+    auto accessor = RequestContext::accessAllThreads();
+    // test there is no deadlock with get()
+    RequestContext::get();
+  });
+  thread.join();
 }