+/*
+ * Copyright 2015-present Facebook, Inc.
+ *
+ * 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 <folly/Function.h>
+#include <condition_variable>
+#include <thread>
+#include <vector>
+
+namespace folly {
+
+/**
+ * For each function `fn` you add to this object, `fn` will be run in a loop
+ * in its own thread, with the thread sleeping between invocations of `fn`
+ * for the duration returned by `fn`'s previous run.
+ *
+ * To clean up these threads, invoke `stop()`, which will interrupt sleeping
+ * threads. `stop()` will wait for already-running functions to return.
+ *
+ * == Alternatives ==
+ *
+ * If you want to multiplex multiple functions on the same thread, you can
+ * either use EventBase with AsyncTimeout objects, or FunctionScheduler for
+ * a slightly simpler API.
+ *
+ * == Thread-safety ==
+ *
+ * This type follows the common rule that:
+ * (1) const member functions are safe to call concurrently with const
+ * member functions, but
+ * (2) non-const member functions are not safe to call concurrently with
+ * any member functions.
+ *
+ * == Pitfalls ==
+ *
+ * Threads and classes don't mix well in C++, so you have to be very careful
+ * if you want to have ThreadedRepeatingFunctionRunner as a member of your
+ * class. A reasonable pattern looks like this:
+ *
+ * struct MyClass {
+ * // Note that threads are NOT added in the constructor, for two reasons:
+ * //
+ * // (1) If you added some, and had any subsequent initialization (e.g.
+ * // derived class constructors), 'this' would not be fully
+ * // constructed when the worker threads came up, causing
+ * // heisenbugs.
+ * //
+ * // (2) Also, if your constructor threw after thread creation, the
+ * // class destructor would not be invoked, potentially leaving the
+ * // threads running too long.
+ * //
+ * // It's better to have explicit two-step initialization, or to lazily
+ * // add threads the first time they are needed.
+ * MyClass() : count_(0) {}
+ *
+ * // You must stop the threads as early as possible in the destruction
+ * // process (or even before). In the case of a class hierarchy, the
+ * // final class MUST always call stop() as the first thing in its
+ * // destructor -- otherwise, the worker threads may access already-
+ * // destroyed state.
+ * ~MyClass() {
+ * // if MyClass is abstract:
+ * threads_.stopAndWarn("MyClass");
+ * // Otherwise:
+ * threads_.stop();
+ * }
+ *
+ * // See the constructor for why two-stage initialization is preferred.
+ * void init() {
+ * threads_.add(bind(&MyClass::incrementCount, this));
+ * }
+ *
+ * std::chrono::milliseconds incrementCount() {
+ * ++count_;
+ * return 10;
+ * }
+ *
+ * private:
+ * std::atomic<int> count_;
+ * // Declared last because the threads' functions access other members.
+ * ThreadedRepeatingFunctionRunner threads_;
+ * };
+ */
+class ThreadedRepeatingFunctionRunner final {
+ public:
+ // Returns how long to wait before the next repetition. Must not throw.
+ using RepeatingFn = folly::Function<std::chrono::milliseconds() noexcept>;
+
+ ThreadedRepeatingFunctionRunner();
+ ~ThreadedRepeatingFunctionRunner();
+
+ /**
+ * Ideally, you will call this before initiating the destruction of the
+ * host object. Otherwise, this should be the first thing in the
+ * destruction sequence. If it comes any later, worker threads may access
+ * class state that had already been destroyed.
+ */
+ void stop();
+
+ /**
+ * Must be called at the TOP of the destructor of any abstract class that
+ * contains ThreadedRepeatingFunctionRunner (directly or through a
+ * parent). Any non-abstract class destructor must instead stop() at the
+ * top.
+ */
+ void stopAndWarn(const std::string& class_of_destructor);
+
+ /**
+ * Run your noexcept function `f` in a background loop, sleeping between
+ * calls for a duration returned by `f`. Optionally waits for
+ * `initialSleep` before calling `f` for the first time.
+ *
+ * DANGER: If a non-final class has a ThreadedRepeatingFunctionRunner
+ * member (which, by the way, must be declared last in the class), then
+ * you must not call add() in your constructor. Otherwise, your thread
+ * risks accessing uninitialized data belonging to a child class. To
+ * avoid this design bug, prefer to use two-stage initialization to start
+ * your threads.
+ */
+ void add(
+ RepeatingFn f,
+ std::chrono::milliseconds initialSleep = std::chrono::milliseconds(0));
+
+ size_t size() const { return threads_.size(); }
+
+ private:
+ // Returns true if this is the first stop().
+ bool stopImpl();
+
+ // Sleep for a duration, or until stop() is called.
+ bool waitFor(std::chrono::milliseconds duration) noexcept;
+
+ // Noexcept allows us to get a good backtrace on crashes -- otherwise,
+ // std::terminate would get called **outside** of the thread function.
+ void executeInLoop(
+ RepeatingFn,
+ std::chrono::milliseconds initialSleep) noexcept;
+
+ std::mutex stopMutex_;
+ bool stopping_{false}; // protected by stopMutex_
+ std::condition_variable stopCv_;
+
+ std::vector<std::thread> threads_;
+};
+
+} // namespace folly
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