Fix Uninitialized Try exception when co_awaiting on fulfilled futures.

Summary:
FutureAwaitable currently does not properly handle the case where an
already fulfilled future is awaited on. In particular, await_ready() is based
on the value of future_.isReady(), while await_resume() pulls the value from
result_, regardless of whether or not result_ has been correctly initialized by
the callback registered in await_suspend. When an already fulfilled
future is awaited on, await_ready() will return true immediately, and await_suspend() will
be bypassed and await_resume() called immediately, throwing an exception since result_ hasn't been initialized.

Reviewed By: lewissbaker

Differential Revision: D12816274

fbshipit-source-id: f5f697e433c1621f829af6f5509d08bac2229ddb

folly/experimental/coro/test/CoroTest.cpp

@@ -321,4 +321,15 @@ TEST(Coro, Baton) {
   EXPECT_EQ(42, std::move(future).get());
 }
 
+coro::Task<int> taskFuture(int value) {
+  co_return co_await folly::makeFuture<int>(std::move(value));
+}
+
+TEST(Coro, FulfilledFuture) {
+  ManualExecutor executor;
+  auto value =
+      taskFuture(42).scheduleOn(&executor).start().via(&executor).getVia(
+          &executor);
+  EXPECT_EQ(42, value);
+}
 #endif

folly/futures/Future.h

@@ -2053,18 +2053,17 @@ class FutureAwaitable {
   }
 
   T await_resume() {
-    return std::move(result_).value();
+    return std::move(future_).value();
   }
 
   void await_suspend(std::experimental::coroutine_handle<> h) {
-    future_.setCallback_([this, h](Try<T>&& result) mutable {
-      result_ = std::move(result);
+    future_.setCallback_([h](Try<T>&&) mutable {
+      // Don't std::move() so the try is left in the future for await_resume()
       h.resume();
     });
   }
 
  private:
-  folly::Try<T> result_;
   folly::Future<T> future_;
 };