llvm/clang-tools-extra/clangd/support/Threading.h

//===--- Threading.h - Abstractions for multithreading -----------*- C++-*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_SUPPORT_THREADING_H
#define LLVM_CLANG_TOOLS_EXTRA_CLANGD_SUPPORT_THREADING_H

#include "support/Context.h"
#include "llvm/ADT/FunctionExtras.h"
#include "llvm/ADT/Twine.h"
#include <atomic>
#include <cassert>
#include <condition_variable>
#include <future>
#include <memory>
#include <mutex>
#include <optional>
#include <thread>
#include <vector>

namespace clang {
namespace clangd {

/// Limits the number of threads that can acquire the lock at the same time.
class Semaphore { … };

/// A point in time we can wait for.
/// Can be zero (don't wait) or infinity (wait forever).
/// (Not time_point::max(), because many std::chrono implementations overflow).
class Deadline { … };

/// Makes a deadline from a timeout in seconds. std::nullopt means wait forever.
Deadline timeoutSeconds(std::optional<double> Seconds);
/// Wait once on CV for the specified duration.
void wait(std::unique_lock<std::mutex> &Lock, std::condition_variable &CV,
          Deadline D);
/// Waits on a condition variable until F() is true or D expires.
template <typename Func>
[[nodiscard]] bool wait(std::unique_lock<std::mutex> &Lock,
                        std::condition_variable &CV, Deadline D, Func F) { … }

/// A threadsafe flag that is initially clear.
class Notification { … };

/// Runs tasks on separate (detached) threads and wait for all tasks to finish.
/// Objects that need to spawn threads can own an AsyncTaskRunner to ensure they
/// all complete on destruction.
class AsyncTaskRunner { … };

/// Runs \p Action asynchronously with a new std::thread. The context will be
/// propagated.
template <typename T>
std::future<T> runAsync(llvm::unique_function<T()> Action) { … }

/// Memoize is a cache to store and reuse computation results based on a key.
///
///   Memoize<DenseMap<int, bool>> PrimeCache;
///   for (int I : RepetitiveNumbers)
///     if (PrimeCache.get(I, [&] { return expensiveIsPrime(I); }))
///       llvm::errs() << "Prime: " << I << "\n";
///
/// The computation will only be run once for each key.
/// This class is threadsafe. Concurrent calls for the same key may run the
/// computation multiple times, but each call will return the same result.
template <typename Container> class Memoize { … };

/// Used to guard an operation that should run at most every N seconds.
///
/// Usage:
///   mutable PeriodicThrottler ShouldLog(std::chrono::seconds(1));
///   void calledFrequently() {
///     if (ShouldLog())
///       log("this is not spammy");
///   }
///
/// This class is threadsafe. If multiple threads are involved, then the guarded
/// operation still needs to be threadsafe!
class PeriodicThrottler { … };

} // namespace clangd
} // namespace clang
#endif