Add utility class Defer (#6382)
Summary: Add a utility class `Defer` to defer the execution of a function until the Defer object goes out of scope. Used in VersionSet:: ProcessManifestWrites as an example. The inline comments for class `Defer` have more details. Pull Request resolved: https://github.com/facebook/rocksdb/pull/6382 Test Plan: `make defer_test version_set_test && ./defer_test && ./version_set_test` Differential Revision: D19797538 Pulled By: cheng-chang fbshipit-source-id: b1a9b7306e4fd4f48ec2ab55783caa561a315f0fmain
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// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
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// This source code is licensed under both the GPLv2 (found in the
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// COPYING file in the root directory) and Apache 2.0 License
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// (found in the LICENSE.Apache file in the root directory).
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#pragma once |
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#include <functional> |
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namespace rocksdb { |
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// Defers the execution of the provided function until the Defer
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// object goes out of scope.
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//
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// Usage example:
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//
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// Status DeferTest() {
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// Status s;
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// Defer defer([&s]() {
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// if (!s.ok()) {
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// // do cleanups ...
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// }
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// });
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// // do something ...
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// if (!s.ok()) return;
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// // do some other things ...
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// return s;
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// }
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//
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// The above code ensures that cleanups will always happen on returning.
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//
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// Without the help of Defer, you can
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// 1. every time when !s.ok(), do the cleanup;
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// 2. instead of returning when !s.ok(), continue the work only when s.ok(),
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// but sometimes, this might lead to nested blocks of "if (s.ok()) {...}".
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//
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// With the help of Defer, you can centralize the cleanup logic inside the
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// lambda passed to Defer, and you can return immediately on failure when necessary.
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class Defer final { |
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public: |
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Defer(std::function<void()>&& fn) : fn_(std::move(fn)) {} |
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~Defer() { fn_(); } |
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// Disallow copy.
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Defer(const Defer&) = delete; |
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Defer& operator=(const Defer&) = delete; |
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private: |
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std::function<void()> fn_; |
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}; |
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} // namespace rocksdb
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// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
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// This source code is licensed under both the GPLv2 (found in the
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// COPYING file in the root directory) and Apache 2.0 License
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// (found in the LICENSE.Apache file in the root directory).
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#include "port/port.h" |
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#include "port/stack_trace.h" |
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#include "test_util/testharness.h" |
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#include "util/defer.h" |
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namespace rocksdb { |
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class DeferTest {}; |
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TEST(DeferTest, BlockScope) { |
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int v = 1; |
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{ |
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Defer defer([&v]() { v *= 2; }); |
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} |
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ASSERT_EQ(2, v); |
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} |
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TEST(DeferTest, FunctionScope) { |
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int v = 1; |
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auto f = [&v]() { |
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Defer defer([&v]() { v *= 2; }); |
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v = 2; |
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}; |
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f(); |
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ASSERT_EQ(4, v); |
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} |
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} // namespace rocksdb
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int main(int argc, char** argv) { |
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rocksdb::port::InstallStackTraceHandler(); |
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::testing::InitGoogleTest(&argc, argv); |
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return RUN_ALL_TESTS(); |
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} |
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