1 files changed, 173 insertions, 0 deletions
diff --git a/memory/volatile/VolatileBuffer.h b/memory/volatile/VolatileBuffer.h
new file mode 100644
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+++ b/memory/volatile/VolatileBuffer.h
@@ -0,0 +1,173 @@
+/* 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/. */
+
+#ifndef mozalloc_VolatileBuffer_h
+#define mozalloc_VolatileBuffer_h
+
+#include "mozilla/mozalloc.h"
+#include "mozilla/Mutex.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/RefCounted.h"
+
+/* VolatileBuffer
+ *
+ * This class represents a piece of memory that can potentially be reclaimed
+ * by the OS when not in use. As long as there are one or more
+ * VolatileBufferPtrs holding on to a VolatileBuffer, the memory will remain
+ * available. However, when there are no VolatileBufferPtrs holding a
+ * VolatileBuffer, the OS can purge the pages if it wants to. The OS can make
+ * better decisions about what pages to purge than we can.
+ *
+ * VolatileBuffers may not always be volatile - if the allocation is too small,
+ * or if the OS doesn't support the feature, or if the OS doesn't want to,
+ * the buffer will be allocated on heap.
+ *
+ * VolatileBuffer allocations are fallible. They are intended for uses where
+ * one may allocate large buffers for caching data. Init() must be called
+ * exactly once.
+ *
+ * After getting a reference to VolatileBuffer using VolatileBufferPtr,
+ * WasPurged() can be used to check if the OS purged any pages in the buffer.
+ * The OS cannot purge a buffer immediately after a VolatileBuffer is
+ * initialized. At least one VolatileBufferPtr must be created before the
+ * buffer can be purged, so the first use of VolatileBufferPtr does not need
+ * to check WasPurged().
+ *
+ * When a buffer is purged, some or all of the buffer is zeroed out. This
+ * API cannot tell which parts of the buffer were lost.
+ *
+ * VolatileBuffer and VolatileBufferPtr are threadsafe.
+ */
+
+namespace mozilla {
+
+class VolatileBuffer
+{
+  friend class VolatileBufferPtr_base;
+public:
+  MOZ_DECLARE_REFCOUNTED_TYPENAME(VolatileBuffer)
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(VolatileBuffer)
+
+  VolatileBuffer();
+
+  /* aAlignment must be a multiple of the pointer size */
+  bool Init(size_t aSize, size_t aAlignment = sizeof(void*));
+
+  size_t HeapSizeOfExcludingThis(MallocSizeOf aMallocSizeOf) const;
+  size_t NonHeapSizeOfExcludingThis() const;
+  bool OnHeap() const;
+
+protected:
+  bool Lock(void** aBuf);
+  void Unlock();
+
+private:
+  ~VolatileBuffer();
+
+  /**
+   * Protects mLockCount, mFirstLock, and changes to the volatility of our
+   * buffer.  Other member variables are read-only except in Init() and the
+   * destructor.
+   */
+  Mutex mMutex;
+
+  void* mBuf;
+  size_t mSize;
+  int mLockCount;
+#if defined(ANDROID)
+  int mFd;
+#elif defined(XP_DARWIN)
+  bool mHeap;
+#elif defined(XP_WIN)
+  bool mHeap;
+  bool mFirstLock;
+#endif
+};
+
+class VolatileBufferPtr_base {
+public:
+  explicit VolatileBufferPtr_base(VolatileBuffer* vbuf)
+    : mVBuf(vbuf)
+    , mMapping(nullptr)
+    , mPurged(false)
+  {
+    Lock();
+  }
+
+  ~VolatileBufferPtr_base() {
+    Unlock();
+  }
+
+  bool WasBufferPurged() const {
+    return mPurged;
+  }
+
+protected:
+  RefPtr<VolatileBuffer> mVBuf;
+  void* mMapping;
+
+  void Set(VolatileBuffer* vbuf) {
+    Unlock();
+    mVBuf = vbuf;
+    Lock();
+  }
+
+private:
+  bool mPurged;
+
+  void Lock() {
+    if (mVBuf) {
+      mPurged = !mVBuf->Lock(&mMapping);
+    } else {
+      mMapping = nullptr;
+      mPurged = false;
+    }
+  }
+
+  void Unlock() {
+    if (mVBuf) {
+      mVBuf->Unlock();
+    }
+  }
+};
+
+template <class T>
+class VolatileBufferPtr : public VolatileBufferPtr_base
+{
+public:
+  explicit VolatileBufferPtr(VolatileBuffer* vbuf) : VolatileBufferPtr_base(vbuf) {}
+  VolatileBufferPtr() : VolatileBufferPtr_base(nullptr) {}
+
+  VolatileBufferPtr(VolatileBufferPtr&& aOther)
+    : VolatileBufferPtr_base(aOther.mVBuf)
+  {
+    aOther.Set(nullptr);
+  }
+
+  operator T*() const {
+    return (T*) mMapping;
+  }
+
+  VolatileBufferPtr& operator=(VolatileBuffer* aVBuf)
+  {
+    Set(aVBuf);
+    return *this;
+  }
+
+  VolatileBufferPtr& operator=(VolatileBufferPtr&& aOther)
+  {
+    MOZ_ASSERT(this != &aOther, "Self-moves are prohibited");
+    Set(aOther.mVBuf);
+    aOther.Set(nullptr);
+    return *this;
+  }
+
+private:
+  VolatileBufferPtr(VolatileBufferPtr const& vbufptr) = delete;
+};
+
+} // namespace mozilla
+
+#endif /* mozalloc_VolatileBuffer_h */