Fix a race in EmbedderTest.CanSpecifyCustomUITaskRunner#182649
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Fix a race in EmbedderTest.CanSpecifyCustomUITaskRunner#182649auto-submit[bot] merged 5 commits intoflutter:masterfrom
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The UI task runner will be destroyed during the call to FlutterEngineShutdown. It should continue forwarding tasks to the engine until the destruction callback is invoked. After that, it should stop using the engine. The test had been using a mutex to guard the engine, but that does not work in this case because both the platform and UI threads must be able to run tasks during engine shutdown. If the test's platform thread holds the mutex during the call to engine.reset(), then the UI task runner can not acquire it to run its tasks. The test previously avoided that by calling engine.reset() without holding the mutex. That creates a race between engine shutdown and the UI task runner's check of engine.is_valid().
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This pull request addresses a race condition in the CanSpecifyCustomUITaskRunner test. The fix involves using a destruction callback on the custom UI task runner to set a flag, preventing tasks from being run on a shutdown engine. This replaces a mutex-based approach that could lead to deadlocks. The EmbedderTestTaskRunner has been updated to use std::function for its destruction callback, improving its flexibility. Additionally, a static variable in another test was made non-static to improve test isolation.
engine/src/flutter/shell/platform/embedder/tests/embedder_unittests_util.h
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…tests_util.h Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>
gaaclarke
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Feb 20, 2026
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Nice catch.
Test exempt: this is a test
| auto ui_thread = std::make_unique<fml::Thread>("test_ui_thread"); | ||
| auto ui_task_runner = ui_thread->GetTaskRunner(); | ||
| std::mutex ui_task_runner_mutex; | ||
| bool ui_task_runner_destroyed = false; |
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Please add the IPLR_GUARDED_BY macro to this variable.
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The IPLR_ macros are defined in Impeller, but this test is in the engine tree outside Impeller. The engine/shell subsystems are not using the threading annotations.
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It's a couple of lines you can inline in this file. It's basically a preprocessor check for clang and an attribute on the variable.
| }); | ||
| test_task_runner.SetDestructionCallback( | ||
| [](void* user_data) { destruction_callback_called = true; }); | ||
| [&]() { destruction_callback_called = true; }); |
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The convention at google is to use explicit load() and store() functions with std::atomics.
| task_runner_description_.destruction_callback = [](void* user_data) { | ||
| auto thiz = reinterpret_cast<EmbedderTestTaskRunner*>(user_data); | ||
| if (thiz->destruction_callback_) { | ||
| thiz->destruction_callback_(); |
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Is this executing on a different thread than SetDestructionCallback? If so there is a race condition and we'll need a mutex.
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Changed this callback to be immutable after construction
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LGTM modulo a nit about clang's guarded_by attribute. Thanks jason.
| auto ui_thread = std::make_unique<fml::Thread>("test_ui_thread"); | ||
| auto ui_task_runner = ui_thread->GetTaskRunner(); | ||
| std::mutex ui_task_runner_mutex; | ||
| bool ui_task_runner_destroyed = false; |
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It's a couple of lines you can inline in this file. It's basically a preprocessor check for clang and an attribute on the variable.
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I'm going to try to merge master to kick the bots. CI seems... unhappy. |
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The UI task runner will be destroyed during the call to FlutterEngineShutdown. It should continue forwarding tasks to the engine until the destruction callback is invoked. After that, it should stop using the engine. The test had been using a mutex to guard the engine, but that does not work in this case because both the platform and UI threads must be able to run tasks during engine shutdown. If the test's platform thread holds the mutex during the call to engine.reset(), then the UI task runner can not acquire it to run its tasks. The test previously avoided that by calling engine.reset() without holding the mutex. That creates a race between engine shutdown and the UI task runner's check of engine.is_valid(). --------- Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com> Co-authored-by: gaaclarke <[email protected]>
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The UI task runner will be destroyed during the call to FlutterEngineShutdown. It should continue forwarding tasks to the engine until the destruction callback is invoked. After that, it should stop using the engine.
The test had been using a mutex to guard the engine, but that does not work in this case because both the platform and UI threads must be able to run tasks during engine shutdown. If the test's platform thread holds the mutex during the call to engine.reset(), then the UI task runner can not acquire it to run its tasks. The test previously avoided that by calling engine.reset() without holding the mutex. That creates a race between engine shutdown and the UI task runner's check of engine.is_valid().