Don't destroy the IAsyncInfo from inside Completed handler (#671)

Author: oldnewthing

strings/base_coroutine_foundation.h

@@ -140,6 +140,7 @@ namespace winrt::impl
 
         void await_suspend(std::experimental::coroutine_handle<> handle)
         {
+            auto extend_lifetime = async;
             async.Completed([this, handler = disconnect_aware_handler{ handle }](auto&&, auto operation_status) mutable
             {
                 status = operation_status;

test/test/async_completed.cpp

@@ -0,0 +1,66 @@
+#include "pch.h"
+
+using namespace winrt;
+using namespace Windows::Foundation;
+
+namespace
+{
+    //
+    // Checks that awaiting an already-completed async operation
+    // does not destroy the operation from within the Completed handler.
+    // The Completed handler may run synchronously, and destroying the
+    // operation from within the Completed handler pulls the rug out
+    // from under the operation!
+    //
+    struct already_completed : implements<already_completed, IAsyncAction, IAsyncInfo>
+    {
+        void Completed(AsyncActionCompletedHandler const& complete)
+        {
+            auto self = get_weak();
+            complete(*this, AsyncStatus::Completed);
+            REQUIRE(self.get() != nullptr);
+        }
+
+        auto Completed() const noexcept
+        {
+            return nullptr;
+        }
+
+        uint32_t Id() const noexcept
+        {
+            return 1;
+        }
+
+        AsyncStatus Status() const noexcept
+        {
+            return AsyncStatus::Completed;
+        }
+
+        hresult ErrorCode() const noexcept
+        {
+            return 0;
+        }
+
+        void GetResults() const noexcept
+        {
+        }
+
+        void Cancel() const noexcept
+        {
+        }
+
+        void Close() const noexcept
+        {
+        }
+    };
+
+    IAsyncAction TestCompleted()
+    {
+        co_await make<already_completed>();
+    }
+}
+
+TEST_CASE("async_completed")
+{
+    TestCompleted().get();
+}

test/test/test.vcxproj

@@ -293,6 +293,7 @@
     <ClCompile Include="async_auto_cancel.cpp" />
     <ClCompile Include="async_cancel_callback.cpp" />
     <ClCompile Include="async_check_cancel.cpp" />
+    <ClCompile Include="async_completed.cpp" />
     <ClCompile Include="box_guid.cpp" />
     <ClCompile Include="capture.cpp" />
     <ClCompile Include="coro_foundation.cpp">