ppapi/utility/completion_callback_factory_thread_traits.h

   1 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #ifndef PPAPI_UTILITY_THREAD_SAFE_THREAD_TRAITS_H_
   6 #define PPAPI_UTILITY_THREAD_SAFE_THREAD_TRAITS_H_
   7
   8 #include "ppapi/cpp/logging.h"
   9 #include "ppapi/cpp/module.h"
  10 #include "ppapi/utility/threading/lock.h"
  11
  12 /// @file
  13 /// Defines the traits structures for thread-safety of a completion callback
  14 /// factory. We provide thread-safe and non-thread-safe version. The thread-safe
  15 /// version is always correct (if you follow the thread usage rules of the
  16 /// callback factory), but if you know your object will only be used on one
  17 /// thread, you can uses the non-thread-safe version.
  18 ///
  19 /// The traits defines three nested classes to perform reference counting,
  20 /// locks, and scoped locking.
  21
  22 namespace pp {
  23
  24 /// The thread-safe version of thread traits. Using this class as the "traits"
  25 /// template argument to a completion callback factory will make it "somewhat
  26 /// thread-friendly." It will allow you to create completion callbacks from
  27 /// background threads and post them to another thread to run.
  28 ///
  29 /// Care still must be taken to ensure that the completion callbacks are
  30 /// executed on the same thread that the factory is destroyed on to avoid a
  31 /// race on destruction.
  32 ///
  33 /// Implementation note: this uses a lock instead of atomic add instructions.
  34 /// The number of platforms we need to support right now makes atomic
  35 /// operations unwieldy for this case that we don't actually use that often.
  36 /// As a further optimization, we can add support for this later.
  37 class ThreadSafeThreadTraits {
  38  public:
  39   class RefCount {
  40    public:
  41     /// Default constructor. In debug mode, this checks that the object is being
  42     /// created on the main thread.
  43     RefCount() : ref_(0) {
  44     }
  45
  46     /// AddRef() increments the reference counter.
  47     ///
  48     /// @return An int32_t with the incremented reference counter.
  49     int32_t AddRef() {
  50       AutoLock lock(lock_);
  51       return ++ref_;
  52     }
  53
  54     /// Release() decrements the reference counter.
  55     ///
  56     /// @return An int32_t with the decremeneted reference counter.
  57     int32_t Release() {
  58       AutoLock lock(lock_);
  59       PP_DCHECK(ref_ > 0);
  60       return --ref_;
  61     }
  62
  63    private:
  64     Lock lock_;
  65     int32_t ref_;
  66   };
  67
  68   typedef pp::Lock Lock;
  69   typedef pp::AutoLock AutoLock;
  70 };
  71
  72 /// The non-thread-safe version of thread traits. Using this class as the
  73 /// "traits" template argument to a completion callback factory will make it
  74 /// not thread-safe but with potential extra performance.
  75 class NonThreadSafeThreadTraits {
  76  public:
  77   /// A simple reference counter that is not thread-safe.
  78   ///
  79   /// <strong>Note:</strong> in Debug mode, it checks that it is either called
  80   /// on the main thread, or always called on another thread.
  81   class RefCount {
  82    public:
  83     /// Default constructor. In debug mode, this checks that the object is being
  84     /// created on the main thread.
  85     RefCount() : ref_(0) {
  86 #ifndef NDEBUG
  87       is_main_thread_ = Module::Get()->core()->IsMainThread();
  88 #endif
  89     }
  90
  91     /// Destructor.
  92     ~RefCount() {
  93       PP_DCHECK(is_main_thread_ == Module::Get()->core()->IsMainThread());
  94     }
  95
  96     /// AddRef() increments the reference counter.
  97     ///
  98     /// @return An int32_t with the incremented reference counter.
  99     int32_t AddRef() {
 100       PP_DCHECK(is_main_thread_ == Module::Get()->core()->IsMainThread());
 101       return ++ref_;
 102     }
 103
 104     /// Release() decrements the reference counter.
 105     ///
 106     /// @return An int32_t with the decremeneted reference counter.
 107     int32_t Release() {
 108       PP_DCHECK(is_main_thread_ == Module::Get()->core()->IsMainThread());
 109       return --ref_;
 110     }
 111
 112    private:
 113     int32_t ref_;
 114 #ifndef NDEBUG
 115     bool is_main_thread_;
 116 #endif
 117   };
 118
 119   /// A simple object that acts like a lock but does nothing.
 120   ///
 121   /// <strong>Note:</strong> in Debug mode, it checks that it is either
 122   /// called on the main thread, or always called on another thread. It also
 123   /// asserts that the caller does not recursively lock.
 124   class Lock {
 125    public:
 126     Lock() {
 127 #ifndef NDEBUG
 128       is_main_thread_ = Module::Get()->core()->IsMainThread();
 129       lock_held_ = false;
 130 #endif
 131     }
 132
 133     ~Lock() {
 134       PP_DCHECK(is_main_thread_ == Module::Get()->core()->IsMainThread());
 135     }
 136
 137     /// Acquires the fake "lock". This does nothing except perform checks in
 138     /// debug mode.
 139     void Acquire() {
 140 #ifndef NDEBUG
 141       PP_DCHECK(!lock_held_);
 142       lock_held_ = true;
 143 #endif
 144     }
 145
 146     /// Releases the fake "lock". This does nothing except perform checks in
 147     /// debug mode.
 148     void Release() {
 149 #ifndef NDEBUG
 150       PP_DCHECK(lock_held_);
 151       lock_held_ = false;
 152 #endif
 153     }
 154
 155    private:
 156 #ifndef NDEBUG
 157     bool is_main_thread_;
 158     bool lock_held_;
 159 #endif
 160   };
 161
 162   class AutoLock {
 163    public:
 164     explicit AutoLock(Lock& lock) : lock_(lock) {
 165       lock_.Acquire();
 166     }
 167     ~AutoLock() {
 168       lock_.Release();
 169     }
 170
 171    private:
 172     Lock& lock_;
 173   };
 174 };
 175
 176 }  // namespace pp
 177
 178 #endif  // PPAPI_UTILITY_THREAD_SAFE_THREAD_TRAITS_H_