Fix potential deadlock in AsyncGenerator when used with blockingWait()

Summary:
The AsyncGenerator was not clearing its Executor::KeepAlive when it ran to completion.

This meant that when you call `blockingWait(gen.next())` and this would result in the end-of-stream (error or done) then `blockingWait()` would never return because the destructor of the local executor created in the call was blocked waiting for all KeepAlive instances to be destroyed - but this would not happen until the AsyncGenerator was destroyed.

AsyncGenerator now clears the caller context (which includes the KeepAlive) when the coroutine runs to completion.

Reviewed By: kirkshoop

Differential Revision: D19625741

fbshipit-source-id: f3254181af14828b32bfa8f7ef5d400d53c04efa

folly/experimental/coro/AsyncGenerator.h

@@ -68,6 +68,7 @@ class AsyncGeneratorPromise {
 
   YieldAwaiter final_suspend() noexcept {
     DCHECK(!hasValue_);
+    clearContext();
     return {};
   }
 

folly/experimental/coro/test/AsyncGeneratorTest.cpp

@@ -447,4 +447,30 @@ TEST_F(AsyncGeneratorTest, CancellationTokenPropagatesFromConsumer) {
   }());
 }
 
+TEST_F(AsyncGeneratorTest, BlockingWaitOnFinalNextDoesNotDeadlock) {
+  auto gen = []() -> folly::coro::AsyncGenerator<int> { co_yield 42; };
+
+  auto g = gen();
+  auto val1 = folly::coro::blockingWait(g.next());
+  CHECK_EQ(42, val1.value());
+  auto val2 = folly::coro::blockingWait(g.next());
+  CHECK(!val2.has_value());
+}
+
+TEST_F(AsyncGeneratorTest, BlockingWaitOnThrowingFinalNextDoesNotDeadlock) {
+  auto gen = []() -> folly::coro::AsyncGenerator<int> {
+    co_yield 42;
+    throw SomeError{};
+  };
+
+  auto g = gen();
+  auto val1 = folly::coro::blockingWait(g.next());
+  CHECK_EQ(42, val1.value());
+  try {
+    folly::coro::blockingWait(g.next());
+    CHECK(false);
+  } catch (const SomeError&) {
+  }
+}
+
 #endif