InputTracer: Separate the threading logic into a wrapper backend
InputTracer writes to the tracing backend interface. However, since
perfetto expects events to be written from a single thread, InputTracer
would write the events from a separate thread.
To simplify the logic inside InputTracer, remove the threading logic.
Instead, we introduce the ThreadingBackend class, which wraps around an
abitrary backend and forwards events to the inner backend from a new
thread.
Another benefit of this is that we no longer have to use the threaded
backend for tests, which makes the tracing part of InputDispatcher_test
deterministic.
Bug: 210460522
Test: atest inputflinger_tests
Change-Id: If7bbc912c05bc975ec9585f0a0ebce68683925bb
diff --git a/services/inputflinger/dispatcher/trace/ThreadedBackend.h b/services/inputflinger/dispatcher/trace/ThreadedBackend.h
new file mode 100644
index 0000000..c42f896
--- /dev/null
+++ b/services/inputflinger/dispatcher/trace/ThreadedBackend.h
@@ -0,0 +1,59 @@
+/*
+ * Copyright 2024 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include "InputTracingPerfettoBackend.h"
+
+#include <android-base/thread_annotations.h>
+#include <mutex>
+#include <thread>
+#include <variant>
+#include <vector>
+
+namespace android::inputdispatcher::trace::impl {
+
+/**
+ * A wrapper around an InputTracingBackend implementation that writes to the inner tracing backend
+ * from a single new thread that it creates. The new tracing thread is started when the
+ * ThreadedBackend is created, and is stopped when it is destroyed. The ThreadedBackend is
+ * thread-safe.
+ */
+template <typename Backend>
+class ThreadedBackend : public InputTracingBackendInterface {
+public:
+ ThreadedBackend(Backend&& innerBackend);
+ ~ThreadedBackend() override;
+
+ void traceKeyEvent(const TracedKeyEvent&) override;
+ void traceMotionEvent(const TracedMotionEvent&) override;
+ void traceWindowDispatch(const WindowDispatchArgs&) override;
+
+private:
+ std::mutex mLock;
+ std::thread mTracerThread;
+ bool mThreadExit GUARDED_BY(mLock){false};
+ std::condition_variable mThreadWakeCondition;
+ Backend mBackend;
+ std::vector<std::variant<TracedKeyEvent, TracedMotionEvent, WindowDispatchArgs>> mQueue
+ GUARDED_BY(mLock);
+
+ using WindowDispatchArgs = InputTracingBackendInterface::WindowDispatchArgs;
+
+ void threadLoop();
+};
+
+} // namespace android::inputdispatcher::trace::impl