Fixing bug in http_listener that can cause hangs on single core machines on Windows.

Author: stgates

Release/src/http/listener/http_server_httpsys.cpp

@@ -425,7 +425,17 @@ void http_windows_server::receive_requests()
 {
     HTTP_REQUEST p_request;
     ULONG bytes_received;
-    for(;;)
+
+    // Oversubscribe since this is a blocking call and we don't want to count
+    // towards the concurrency runtime's thread count. A more proper fix
+    // would be to use Overlapped I/O and asynchronously call HttpReceiveHttpRequest.
+    // This requires additional work to be careful sychronizing with the listener
+    // shutdown. This is much easier especially given the http_listener is 'experimental'
+    // and with VS2015 PPL tasks run on the threadpool.
+#if _MSC_VER < 1900
+    concurrency::Context::Oversubscribe(true);
+#endif
+    for (;;)
     {
         unsigned long error_code = HttpReceiveHttpRequest(
             m_hRequestQueue,
@@ -436,7 +446,7 @@ void http_windows_server::receive_requests()
             &bytes_received,
             0);
 
-        if(error_code != NO_ERROR && error_code != ERROR_MORE_DATA)
+        if (error_code != NO_ERROR && error_code != ERROR_MORE_DATA)
         {
             break;
         }
@@ -447,7 +457,9 @@ void http_windows_server::receive_requests()
         http_request msg = http_request::_create_request(std::move(pRequestContext));
         pContext->async_process_request(p_request.RequestId, msg, bytes_received);
     }
-}
+#if _MSC_VER < 1900
+    concurrency::Context::Oversubscribe(false);
+#endif}
 
 pplx::task<void> http_windows_server::respond(http::http_response response)
 {

Release/tests/functional/http/listener/connections_and_errors.cpp

@@ -143,6 +143,41 @@ TEST_FIXTURE(uri_address, save_request_reply)
     listener.close().wait();
 }
 
+TEST_FIXTURE(uri_address, single_core_request)
+{
+#ifdef _MSC_VER
+	// Fake having a scheduler with only 1 core.
+	concurrency::CurrentScheduler::Create(concurrency::SchedulerPolicy(2, 1, Concurrency::MinConcurrency, 1, Concurrency::MaxConcurrency));
+#endif
+
+	http_listener listener(m_uri);
+	listener.open().wait();
+	test_http_client::scoped_client client(m_uri);
+	test_http_client * p_client = client.client();
+
+	listener.support([](http_request request)
+	{
+		request.reply(status_codes::OK).get();
+	});
+	VERIFY_ARE_EQUAL(0, p_client->request(methods::POST, U("")));
+
+	// Don't wait on the task otherwise it could inline allowing other tasks to run on the scheduler.
+	std::atomic_flag responseEvent = ATOMIC_FLAG_INIT;
+	responseEvent.test_and_set();
+	p_client->next_response().then([&](test_response *p_response)
+	{
+		http_asserts::assert_test_response_equals(p_response, status_codes::OK);
+		responseEvent.clear();
+	});
+	while (responseEvent.test_and_set()) {}
+
+	listener.close().wait();
+
+#ifdef _MSC_VER
+	concurrency::CurrentScheduler::Detach();
+#endif
+}
+
 TEST_FIXTURE(uri_address, save_request_response)
 {
     http_listener listener(m_uri);