gecko-dev/mfbt/WeakPtr.h
Tom Ritter 4d9bda0d08 Bug 1317642 Disable WeakPtr thread assertions on MinGW r=froydnj
MinGW has two threading models: win32 API based, which disables std::thread,
and POSIX based which enables it but requires an emulation library (winpthreads).

Rather than attempting to switch to pthread emulation at this point, we are
disabling the std::thread based assertion checking for WeakPtr on MinGW.

MozReview-Commit-ID: BmHo70n6AuK

--HG--
extra : rebase_source : 08495775b2925a797c8098216911d30c0b79ef3d
2017-01-13 13:50:21 -06:00

284 lines
8.6 KiB
C++

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
/* Weak pointer functionality, implemented as a mixin for use with any class. */
/**
* SupportsWeakPtr lets you have a pointer to an object 'Foo' without affecting
* its lifetime. It works by creating a single shared reference counted object
* (WeakReference) that each WeakPtr will access 'Foo' through. This lets 'Foo'
* clear the pointer in the WeakReference without having to know about all of
* the WeakPtrs to it and allows the WeakReference to live beyond the lifetime
* of 'Foo'.
*
* PLEASE NOTE: This weak pointer implementation is not thread-safe.
*
* Note that when deriving from SupportsWeakPtr you should add
* MOZ_DECLARE_WEAKREFERENCE_TYPENAME(ClassName) to the public section of your
* class, where ClassName is the name of your class.
*
* The overhead of WeakPtr is that accesses to 'Foo' becomes an additional
* dereference, and an additional heap allocated pointer sized object shared
* between all of the WeakPtrs.
*
* Example of usage:
*
* // To have a class C support weak pointers, inherit from
* // SupportsWeakPtr<C>.
* class C : public SupportsWeakPtr<C>
* {
* public:
* MOZ_DECLARE_WEAKREFERENCE_TYPENAME(C)
* int mNum;
* void act();
* };
*
* C* ptr = new C();
*
* // Get weak pointers to ptr. The first time a weak pointer
* // is obtained, a reference counted WeakReference object is created that
* // can live beyond the lifetime of 'ptr'. The WeakReference
* // object will be notified of 'ptr's destruction.
* WeakPtr<C> weak = ptr;
* WeakPtr<C> other = ptr;
*
* // Test a weak pointer for validity before using it.
* if (weak) {
* weak->mNum = 17;
* weak->act();
* }
*
* // Destroying the underlying object clears weak pointers to it.
* delete ptr;
*
* MOZ_ASSERT(!weak, "Deleting |ptr| clears weak pointers to it.");
* MOZ_ASSERT(!other, "Deleting |ptr| clears all weak pointers to it.");
*
* WeakPtr is typesafe and may be used with any class. It is not required that
* the class be reference-counted or allocated in any particular way.
*
* The API was loosely inspired by Chromium's weak_ptr.h:
* http://src.chromium.org/svn/trunk/src/base/memory/weak_ptr.h
*/
#ifndef mozilla_WeakPtr_h
#define mozilla_WeakPtr_h
#include "mozilla/ArrayUtils.h"
#include "mozilla/Assertions.h"
#include "mozilla/Attributes.h"
#include "mozilla/RefCounted.h"
#include "mozilla/RefPtr.h"
#include "mozilla/TypeTraits.h"
#include <string.h>
// Weak referencing is not implemeted as thread safe. When a WeakPtr
// is created or dereferenced on thread A but the real object is just
// being Released() on thread B, there is a possibility of a race
// when the proxy object (detail::WeakReference) is notified about
// the real object destruction just between when thread A is storing
// the object pointer locally and is about to add a reference to it.
//
// Hence, a non-null weak proxy object is considered to have a single
// "owning thread". It means that each query for a weak reference,
// its dereference, and destruction of the real object must all happen
// on a single thread. The following macros implement assertions for
// checking these conditions.
//
// We disable this on MinGW. MinGW has two threading models: win32
// API based, which disables std::thread; and POSIX based which
// enables it but requires an emulation library (winpthreads).
// Rather than attempting to switch to pthread emulation at this point,
// we are disabling the std::thread based assertion checking.
//
// In the future, to enable it we could
// a. have libgcc/stdc++ support win32 threads natively
// b. switch to POSIX-based threading in MinGW with pthread emulation
// c. refactor it to not use std::thread
#if !defined(__MINGW32__) && (defined(DEBUG) || (defined(NIGHTLY_BUILD) && !defined(MOZ_PROFILING)))
#include <thread>
#define MOZ_WEAKPTR_DECLARE_THREAD_SAFETY_CHECK \
std::thread::id _owningThread; \
bool _empty; // If it was initialized as a placeholder with mPtr = nullptr.
#define MOZ_WEAKPTR_INIT_THREAD_SAFETY_CHECK() \
do { \
_owningThread = std::this_thread::get_id(); \
_empty = !p; \
} while (false)
#define MOZ_WEAKPTR_ASSERT_THREAD_SAFETY() \
MOZ_DIAGNOSTIC_ASSERT(_empty || _owningThread == std::this_thread::get_id(), \
"WeakPtr used on multiple threads")
#define MOZ_WEAKPTR_ASSERT_THREAD_SAFETY_DELEGATED(that) \
(that)->AssertThreadSafety();
#define MOZ_WEAKPTR_THREAD_SAFETY_CHECKING 1
#else
#define MOZ_WEAKPTR_DECLARE_THREAD_SAFETY_CHECK
#define MOZ_WEAKPTR_INIT_THREAD_SAFETY_CHECK() do { } while (false)
#define MOZ_WEAKPTR_ASSERT_THREAD_SAFETY() do { } while (false)
#define MOZ_WEAKPTR_ASSERT_THREAD_SAFETY_DELEGATED(that) do { } while (false)
#endif
namespace mozilla {
template <typename T> class WeakPtr;
template <typename T> class SupportsWeakPtr;
#ifdef MOZ_REFCOUNTED_LEAK_CHECKING
#define MOZ_DECLARE_WEAKREFERENCE_TYPENAME(T) \
static const char* weakReferenceTypeName() { return "WeakReference<" #T ">"; }
#else
#define MOZ_DECLARE_WEAKREFERENCE_TYPENAME(T)
#endif
namespace detail {
// This can live beyond the lifetime of the class derived from
// SupportsWeakPtr.
template<class T>
class WeakReference : public ::mozilla::RefCounted<WeakReference<T> >
{
public:
explicit WeakReference(T* p) : mPtr(p)
{
MOZ_WEAKPTR_INIT_THREAD_SAFETY_CHECK();
}
T* get() const {
MOZ_WEAKPTR_ASSERT_THREAD_SAFETY();
return mPtr;
}
#ifdef MOZ_REFCOUNTED_LEAK_CHECKING
const char* typeName() const
{
// The first time this is called mPtr is null, so don't
// invoke any methods on mPtr.
return T::weakReferenceTypeName();
}
size_t typeSize() const { return sizeof(*this); }
#endif
#ifdef MOZ_WEAKPTR_THREAD_SAFETY_CHECKING
void AssertThreadSafety() { MOZ_WEAKPTR_ASSERT_THREAD_SAFETY(); }
#endif
private:
friend class mozilla::SupportsWeakPtr<T>;
void detach() {
MOZ_WEAKPTR_ASSERT_THREAD_SAFETY();
mPtr = nullptr;
}
T* MOZ_NON_OWNING_REF mPtr;
MOZ_WEAKPTR_DECLARE_THREAD_SAFETY_CHECK
};
} // namespace detail
template <typename T>
class SupportsWeakPtr
{
protected:
~SupportsWeakPtr()
{
static_assert(IsBaseOf<SupportsWeakPtr<T>, T>::value,
"T must derive from SupportsWeakPtr<T>");
if (mSelfReferencingWeakPtr) {
mSelfReferencingWeakPtr.mRef->detach();
}
}
private:
const WeakPtr<T>& SelfReferencingWeakPtr()
{
if (!mSelfReferencingWeakPtr) {
mSelfReferencingWeakPtr.mRef = new detail::WeakReference<T>(static_cast<T*>(this));
} else {
MOZ_WEAKPTR_ASSERT_THREAD_SAFETY_DELEGATED(mSelfReferencingWeakPtr.mRef);
}
return mSelfReferencingWeakPtr;
}
const WeakPtr<const T>& SelfReferencingWeakPtr() const
{
const WeakPtr<T>& p = const_cast<SupportsWeakPtr*>(this)->SelfReferencingWeakPtr();
return reinterpret_cast<const WeakPtr<const T>&>(p);
}
friend class WeakPtr<T>;
friend class WeakPtr<const T>;
WeakPtr<T> mSelfReferencingWeakPtr;
};
template <typename T>
class WeakPtr
{
typedef detail::WeakReference<T> WeakReference;
public:
WeakPtr& operator=(const WeakPtr& aOther)
{
mRef = aOther.mRef;
MOZ_WEAKPTR_ASSERT_THREAD_SAFETY_DELEGATED(mRef);
return *this;
}
WeakPtr(const WeakPtr& aOther)
{
// The thread safety check is performed inside of the operator= method.
*this = aOther;
}
WeakPtr& operator=(T* aOther)
{
if (aOther) {
*this = aOther->SelfReferencingWeakPtr();
} else if (!mRef || mRef->get()) {
// Ensure that mRef is dereferenceable in the uninitialized state.
mRef = new WeakReference(nullptr);
}
// The thread safety check happens inside SelfReferencingWeakPtr
// or is initialized in the WeakReference constructor.
return *this;
}
MOZ_IMPLICIT WeakPtr(T* aOther)
{
*this = aOther;
MOZ_WEAKPTR_ASSERT_THREAD_SAFETY_DELEGATED(mRef);
}
// Ensure that mRef is dereferenceable in the uninitialized state.
WeakPtr() : mRef(new WeakReference(nullptr)) {}
operator T*() const { return mRef->get(); }
T& operator*() const { return *mRef->get(); }
T* operator->() const MOZ_NO_ADDREF_RELEASE_ON_RETURN { return mRef->get(); }
T* get() const { return mRef->get(); }
private:
friend class SupportsWeakPtr<T>;
explicit WeakPtr(const RefPtr<WeakReference>& aOther) : mRef(aOther) {}
RefPtr<WeakReference> mRef;
};
} // namespace mozilla
#endif /* mozilla_WeakPtr_h */