comparison osx/include/kj/mutex.h @ 49:3ab5a40c4e3b

Add Capnp and KJ builds for OSX

+// Copyright (c) 2013-2014 Sandstorm Development Group, Inc. and contributors
+// Licensed under the MIT License:
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+
+#ifndef KJ_MUTEX_H_
+#define KJ_MUTEX_H_
+
+#if defined(__GNUC__) && !KJ_HEADER_WARNINGS
+#pragma GCC system_header
+#endif
+
+#include "memory.h"
+#include <inttypes.h>
+
+#if __linux__ && !defined(KJ_USE_FUTEX)
+#define KJ_USE_FUTEX 1
+#endif
+
+#if !KJ_USE_FUTEX && !_WIN32
+// On Linux we use futex.  On other platforms we wrap pthreads.
+// TODO(someday):  Write efficient low-level locking primitives for other platforms.
+#include <pthread.h>
+#endif
+
+namespace kj {
+
+// =======================================================================================
+// Private details -- public interfaces follow below.
+
+namespace _ {  // private
+
+class Mutex {
+  // Internal implementation details.  See `MutexGuarded<T>`.
+
+public:
+  Mutex();
+  ~Mutex();
+  KJ_DISALLOW_COPY(Mutex);
+
+  enum Exclusivity {
+    EXCLUSIVE,
+    SHARED
+  };
+
+  void lock(Exclusivity exclusivity);
+  void unlock(Exclusivity exclusivity);
+
+  void assertLockedByCaller(Exclusivity exclusivity);
+  // In debug mode, assert that the mutex is locked by the calling thread, or if that is
+  // non-trivial, assert that the mutex is locked (which should be good enough to catch problems
+  // in unit tests).  In non-debug builds, do nothing.
+
+private:
+#if KJ_USE_FUTEX
+  uint futex;
+  // bit 31 (msb) = set if exclusive lock held
+  // bit 30 (msb) = set if threads are waiting for exclusive lock
+  // bits 0-29 = count of readers; If an exclusive lock is held, this is the count of threads
+  //   waiting for a read lock, otherwise it is the count of threads that currently hold a read
+  //   lock.
+
+  static constexpr uint EXCLUSIVE_HELD = 1u << 31;
+  static constexpr uint EXCLUSIVE_REQUESTED = 1u << 30;
+  static constexpr uint SHARED_COUNT_MASK = EXCLUSIVE_REQUESTED - 1;
+
+#elif _WIN32
+  uintptr_t srwLock;  // Actually an SRWLOCK, but don't want to #include <windows.h> in header.
+
+#else
+  mutable pthread_rwlock_t mutex;
+#endif
+};
+
+class Once {
+  // Internal implementation details.  See `Lazy<T>`.
+
+public:
+#if KJ_USE_FUTEX
+  inline Once(bool startInitialized = false)
+      : futex(startInitialized ? INITIALIZED : UNINITIALIZED) {}
+#else
+  Once(bool startInitialized = false);
+  ~Once();
+#endif
+  KJ_DISALLOW_COPY(Once);
+
+  class Initializer {
+  public:
+    virtual void run() = 0;
+  };
+
+  void runOnce(Initializer& init);
+
+#if _WIN32  // TODO(perf): Can we make this inline on win32 somehow?
+  bool isInitialized() noexcept;
+
+#else
+  inline bool isInitialized() noexcept {
+    // Fast path check to see if runOnce() would simply return immediately.
+#if KJ_USE_FUTEX
+    return __atomic_load_n(&futex, __ATOMIC_ACQUIRE) == INITIALIZED;
+#else
+    return __atomic_load_n(&state, __ATOMIC_ACQUIRE) == INITIALIZED;
+#endif
+  }
+#endif
+
+  void reset();
+  // Returns the state from initialized to uninitialized.  It is an error to call this when
+  // not already initialized, or when runOnce() or isInitialized() might be called concurrently in
+  // another thread.
+
+private:
+#if KJ_USE_FUTEX
+  uint futex;
+
+  enum State {
+    UNINITIALIZED,
+    INITIALIZING,
+    INITIALIZING_WITH_WAITERS,
+    INITIALIZED
+  };
+
+#elif _WIN32
+  uintptr_t initOnce;  // Actually an INIT_ONCE, but don't want to #include <windows.h> in header.
+
+#else
+  enum State {
+    UNINITIALIZED,
+    INITIALIZED
+  };
+  State state;
+  pthread_mutex_t mutex;
+#endif
+};
+
+}  // namespace _ (private)
+
+// =======================================================================================
+// Public interface
+
+template <typename T>
+class Locked {
+  // Return type for `MutexGuarded<T>::lock()`.  `Locked<T>` provides access to the guarded object
+  // and unlocks the mutex when it goes out of scope.
+
+public:
+  KJ_DISALLOW_COPY(Locked);
+  inline Locked(): mutex(nullptr), ptr(nullptr) {}
+  inline Locked(Locked&& other): mutex(other.mutex), ptr(other.ptr) {
+    other.mutex = nullptr;
+    other.ptr = nullptr;
+  }
+  inline ~Locked() {
+    if (mutex != nullptr) mutex->unlock(isConst<T>() ? _::Mutex::SHARED : _::Mutex::EXCLUSIVE);
+  }
+
+  inline Locked& operator=(Locked&& other) {
+    if (mutex != nullptr) mutex->unlock(isConst<T>() ? _::Mutex::SHARED : _::Mutex::EXCLUSIVE);
+    mutex = other.mutex;
+    ptr = other.ptr;
+    other.mutex = nullptr;
+    other.ptr = nullptr;
+    return *this;
+  }
+
+  inline void release() {
+    if (mutex != nullptr) mutex->unlock(isConst<T>() ? _::Mutex::SHARED : _::Mutex::EXCLUSIVE);
+    mutex = nullptr;
+    ptr = nullptr;
+  }
+
+  inline T* operator->() { return ptr; }
+  inline const T* operator->() const { return ptr; }
+  inline T& operator*() { return *ptr; }
+  inline const T& operator*() const { return *ptr; }
+  inline T* get() { return ptr; }
+  inline const T* get() const { return ptr; }
+  inline operator T*() { return ptr; }
+  inline operator const T*() const { return ptr; }
+
+private:
+  _::Mutex* mutex;
+  T* ptr;
+
+  inline Locked(_::Mutex& mutex, T& value): mutex(&mutex), ptr(&value) {}
+
+  template <typename U>
+  friend class MutexGuarded;
+};
+
+template <typename T>
+class MutexGuarded {
+  // An object of type T, guarded by a mutex.  In order to access the object, you must lock it.
+  //
+  // Write locks are not "recursive" -- trying to lock again in a thread that already holds a lock
+  // will deadlock.  Recursive write locks are usually a sign of bad design.
+  //
+  // Unfortunately, **READ LOCKS ARE NOT RECURSIVE** either.  Common sense says they should be.
+  // But on many operating systems (BSD, OSX), recursively read-locking a pthread_rwlock is
+  // actually unsafe.  The problem is that writers are "prioritized" over readers, so a read lock
+  // request will block if any write lock requests are outstanding.  So, if thread A takes a read
+  // lock, thread B requests a write lock (and starts waiting), and then thread A tries to take
+  // another read lock recursively, the result is deadlock.
+
+public:
+  template <typename... Params>
+  explicit MutexGuarded(Params&&... params);
+  // Initialize the mutex-guarded object by passing the given parameters to its constructor.
+
+  Locked<T> lockExclusive() const;
+  // Exclusively locks the object and returns it.  The returned `Locked<T>` can be passed by
+  // move, similar to `Own<T>`.
+  //
+  // This method is declared `const` in accordance with KJ style rules which say that constness
+  // should be used to indicate thread-safety.  It is safe to share a const pointer between threads,
+  // but it is not safe to share a mutable pointer.  Since the whole point of MutexGuarded is to
+  // be shared between threads, its methods should be const, even though locking it produces a
+  // non-const pointer to the contained object.
+
+  Locked<const T> lockShared() const;
+  // Lock the value for shared access.  Multiple shared locks can be taken concurrently, but cannot
+  // be held at the same time as a non-shared lock.
+
+  inline const T& getWithoutLock() const { return value; }
+  inline T& getWithoutLock() { return value; }
+  // Escape hatch for cases where some external factor guarantees that it's safe to get the
+  // value.  You should treat these like const_cast -- be highly suspicious of any use.
+
+  inline const T& getAlreadyLockedShared() const;
+  inline T& getAlreadyLockedShared();
+  inline T& getAlreadyLockedExclusive() const;
+  // Like `getWithoutLock()`, but asserts that the lock is already held by the calling thread.
+
+private:
+  mutable _::Mutex mutex;
+  mutable T value;
+};
+
+template <typename T>
+class MutexGuarded<const T> {
+  // MutexGuarded cannot guard a const type.  This would be pointless anyway, and would complicate
+  // the implementation of Locked<T>, which uses constness to decide what kind of lock it holds.
+  static_assert(sizeof(T) < 0, "MutexGuarded's type cannot be const.");
+};
+
+template <typename T>
+class Lazy {
+  // A lazily-initialized value.
+
+public:
+  template <typename Func>
+  T& get(Func&& init);
+  template <typename Func>
+  const T& get(Func&& init) const;
+  // The first thread to call get() will invoke the given init function to construct the value.
+  // Other threads will block until construction completes, then return the same value.
+  //
+  // `init` is a functor(typically a lambda) which takes `SpaceFor<T>&` as its parameter and returns
+  // `Own<T>`.  If `init` throws an exception, the exception is propagated out of that thread's
+  // call to `get()`, and subsequent calls behave as if `get()` hadn't been called at all yet --
+  // in other words, subsequent calls retry initialization until it succeeds.
+
+private:
+  mutable _::Once once;
+  mutable SpaceFor<T> space;
+  mutable Own<T> value;
+
+  template <typename Func>
+  class InitImpl;
+};
+
+// =======================================================================================
+// Inline implementation details
+
+template <typename T>
+template <typename... Params>
+inline MutexGuarded<T>::MutexGuarded(Params&&... params)
+    : value(kj::fwd<Params>(params)...) {}
+
+template <typename T>
+inline Locked<T> MutexGuarded<T>::lockExclusive() const {
+  mutex.lock(_::Mutex::EXCLUSIVE);
+  return Locked<T>(mutex, value);
+}
+
+template <typename T>
+inline Locked<const T> MutexGuarded<T>::lockShared() const {
+  mutex.lock(_::Mutex::SHARED);
+  return Locked<const T>(mutex, value);
+}
+
+template <typename T>
+inline const T& MutexGuarded<T>::getAlreadyLockedShared() const {
+#ifdef KJ_DEBUG
+  mutex.assertLockedByCaller(_::Mutex::SHARED);
+#endif
+  return value;
+}
+template <typename T>
+inline T& MutexGuarded<T>::getAlreadyLockedShared() {
+#ifdef KJ_DEBUG
+  mutex.assertLockedByCaller(_::Mutex::SHARED);
+#endif
+  return value;
+}
+template <typename T>
+inline T& MutexGuarded<T>::getAlreadyLockedExclusive() const {
+#ifdef KJ_DEBUG
+  mutex.assertLockedByCaller(_::Mutex::EXCLUSIVE);
+#endif
+  return const_cast<T&>(value);
+}
+
+template <typename T>
+template <typename Func>
+class Lazy<T>::InitImpl: public _::Once::Initializer {
+public:
+  inline InitImpl(const Lazy<T>& lazy, Func&& func): lazy(lazy), func(kj::fwd<Func>(func)) {}
+
+  void run() override {
+    lazy.value = func(lazy.space);
+  }
+
+private:
+  const Lazy<T>& lazy;
+  Func func;
+};
+
+template <typename T>
+template <typename Func>
+inline T& Lazy<T>::get(Func&& init) {
+  if (!once.isInitialized()) {
+    InitImpl<Func> initImpl(*this, kj::fwd<Func>(init));
+    once.runOnce(initImpl);
+  }
+  return *value;
+}
+
+template <typename T>
+template <typename Func>
+inline const T& Lazy<T>::get(Func&& init) const {
+  if (!once.isInitialized()) {
+    InitImpl<Func> initImpl(*this, kj::fwd<Func>(init));
+    once.runOnce(initImpl);
+  }
+  return *value;
+}
+
+}  // namespace kj
+
+#endif  // KJ_MUTEX_H_