[automerge] FTL: Add thread safety helpers 2p: bd006786a7 (81a91aef) · Commits · e / os / android_frameworks_native

include/ftl/fake_guard.h

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		/*
		* Copyright 2022 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

		#define FTL_ATTRIBUTE(a) __attribute__((a))

		namespace android::ftl {

		// Granular alternative to [[clang::no_thread_safety_analysis]]. Given a std::mutex-like object,
		// FakeGuard suppresses enforcement of thread-safe access to guarded variables within its scope.
		// While FakeGuard is scoped to a block, there are macro shorthands for a single expression, as
		// well as function/lambda scope (though calls must be indirect, e.g. virtual or std::function):
		//
		// struct {
		// std::mutex mutex;
		// int x FTL_ATTRIBUTE(guarded_by(mutex)) = -1;
		//
		// int f() {
		// {
		// ftl::FakeGuard guard(mutex);
		// x = 0;
		// }
		//
		// return FTL_FAKE_GUARD(mutex, x + 1);
		// }
		//
		// std::function<int()> g() const {
		// return [this]() FTL_FAKE_GUARD(mutex) { return x; };
		// }
		// } s;
		//
		// assert(s.f() == 1);
		// assert(s.g()() == 0);
		//
		// An example of a situation where FakeGuard helps is a mutex that guards writes on Thread 1, and
		// reads on Thread 2. Reads on Thread 1, which is the only writer, need not be under lock, so can
		// use FakeGuard to appease the thread safety analyzer. Another example is enforcing and documenting
		// exclusive access by a single thread. This is done by defining a global constant that represents a
		// thread context, and annotating guarded variables as if it were a mutex (though without any effect
		// at run time):
		//
		// constexpr class [[clang::capability("mutex")]] {
		// } kMainThreadContext;
		//
		template <typename Mutex>
		struct [[clang::scoped_lockable]] FakeGuard final {
		explicit FakeGuard(const Mutex& mutex) FTL_ATTRIBUTE(acquire_capability(mutex)) {}
		[[clang::release_capability()]] ~FakeGuard() {}

		FakeGuard(const FakeGuard&) = delete;
		FakeGuard& operator=(const FakeGuard&) = delete;
		};

		} // namespace android::ftl

		// TODO: Enable in C++23 once standard attributes can be used on lambdas.
		#if 0
		#define FTL_FAKE_GUARD1(mutex) [[using clang: acquire_capability(mutex), release_capability(mutex)]]
		#else
		#define FTL_FAKE_GUARD1(mutex) \
		FTL_ATTRIBUTE(acquire_capability(mutex)) \
		FTL_ATTRIBUTE(release_capability(mutex))
		#endif

		// The parentheses around `expr` are needed to deduce an lvalue or rvalue reference.
		#define FTL_FAKE_GUARD2(mutex, expr) \
		[&]() -> decltype(auto) { \
		const android::ftl::FakeGuard guard(mutex); \
		return (expr); \
		}()

		#define FTL_MAKE_FAKE_GUARD(arg1, arg2, guard, ...) guard

		// The void argument suppresses a warning about zero variadic macro arguments.
		#define FTL_FAKE_GUARD(...) \
		FTL_MAKE_FAKE_GUARD(__VA_ARGS__, FTL_FAKE_GUARD2, FTL_FAKE_GUARD1, void)(__VA_ARGS__)

libs/ftl/Android.bp

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		@@ -18,6 +18,7 @@ cc_test {
		"cast_test.cpp",
		"concat_test.cpp",
		"enum_test.cpp",
		"fake_guard_test.cpp",
		"future_test.cpp",
		"small_map_test.cpp",
		"small_vector_test.cpp",
		@@ -29,6 +30,7 @@ cc_test {
		"-Werror",
		"-Wextra",
		"-Wpedantic",
		"-Wthread-safety",
		],

		header_libs: [

libs/ftl/fake_guard_test.cpp

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		/*
		* Copyright 2022 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.
		*/

		#include <ftl/fake_guard.h>
		#include <gtest/gtest.h>

		#include <functional>
		#include <mutex>

		namespace android::test {

		// Keep in sync with example usage in header file.
		TEST(FakeGuard, Example) {
		struct {
		std::mutex mutex;
		int x FTL_ATTRIBUTE(guarded_by(mutex)) = -1;

		int f() {
		{
		ftl::FakeGuard guard(mutex);
		x = 0;
		}

		return FTL_FAKE_GUARD(mutex, x + 1);
		}

		std::function<int()> g() const {
		return [this]() FTL_FAKE_GUARD(mutex) { return x; };
		}
		} s;

		EXPECT_EQ(s.f(), 1);
		EXPECT_EQ(s.g()(), 0);
		}

		} // namespace android::test