[Not for review] Discardable memory emulation

base/memory/discardable_memory.h

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_MEMORY_DISCARDABLE_MEMORY_H_
 #define BASE_MEMORY_DISCARDABLE_MEMORY_H_
 
 #include "base/base_export.h"
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 
 namespace base {
 
 enum LockDiscardableMemoryStatus {
   DISCARDABLE_MEMORY_FAILED = -1,
   DISCARDABLE_MEMORY_PURGED = 0,
   DISCARDABLE_MEMORY_SUCCESS = 1
 };
 
+#if defined(OS_MACOSX) || defined(OS_ANDROID)
+#define DISCARDABLE_MEMORY_SUPPORTED_NATIVELY
+#endif
+
+class DiscardableMemory;
+
+class BASE_EXPORT DiscardableMemoryProvider {
+ public:
+  // Returns the instance set via SetInstance, if one exists. Otherwise, it
+  // will attempt to create an instance via CreateInstance(). It is possible

[Avi (use Gerrit), 2013/06/07 14:46:53]

What is CreateInstance()?

+  // that this can return NULL.

[Avi (use Gerrit), 2013/06/07 14:46:53]

"Can" return NULL is not helpful. In what scenario _will_ this return NULL,
exactly? Is this rewrite correct?

"If no instance of a DiscardableMemoryProvider was set via GetInstance, and
discardable memory isn't available natively, this call will return NULL."

+  static DiscardableMemoryProvider* GetInstance();
+
+#if !defined(DISCARDABLE_MEMORY_SUPPORTED_NATIVELY)
+  // The caller retains ownership of the factory.
+  static void SetInstance(DiscardableMemoryProvider* provider);
+#endif
+
+  virtual DiscardableMemory* CreateDiscardableMemory() = 0;
+
+  // Check whether a purge of all discardable memory in the system is supported.
+  // Use only for testing!
+  virtual bool PurgeForTestingSupported() const = 0;
+
+  // Purge all discardable memory in the system. This call has global effects
+  // across all running processes, so it should only be used for testing!
+  virtual void PurgeForTesting() = 0;
+};
+
 // Platform abstraction for discardable memory. DiscardableMemory is used to
 // cache large objects without worrying about blowing out memory, both on mobile
 // devices where there is no swap, and desktop devices where unused free memory
 // should be used to help the user experience. This is preferable to releasing
 // memory in response to an OOM signal because it is simpler, though it has less
 // flexibility as to which objects get discarded.
 //
 // Discardable memory has two states: locked and unlocked. While the memory is
 // locked, it will not be discarded. Unlocking the memory allows the OS to
 // reclaim it if needed. Locks do not nest.
 //
 // Notes:
 //   - The paging behavior of memory while it is locked is not specified. While
 //     mobile platforms will not swap it out, it may qualify for swapping
 //     on desktop platforms. It is not expected that this will matter, as the
 //     preferred pattern of usage for DiscardableMemory is to lock down the
 //     memory, use it as quickly as possible, and then unlock it.
 //   - Because of memory alignment, the amount of memory allocated can be
 //     larger than the requested memory size. It is not very efficient for
 //     small allocations.
 //
 // References:
 //   - Linux: http://lwn.net/Articles/452035/
 //   - Mac: http://trac.webkit.org/browser/trunk/Source/WebCore/platform/mac/PurgeableBufferMac.cpp
 //          the comment starting with "vm_object_purgable_control" at
 //            http://www.opensource.apple.com/source/xnu/xnu-792.13.8/osfmk/vm/vm_object.c
 class BASE_EXPORT DiscardableMemory {
  public:
-  DiscardableMemory();
-
   // If the discardable memory is locked, the destructor will unlock it.
   // The opened file will also be closed after this.
-  ~DiscardableMemory();
-
-  // Check whether the system supports discardable memory.
-  static bool Supported();
+  virtual ~DiscardableMemory();
 
   // Initialize the DiscardableMemory object. On success, this function returns
   // true and the memory is locked. This should only be called once.
   // This call could fail because of platform-specific limitations and the user
   // should stop using the DiscardableMemory afterwards.
-  bool InitializeAndLock(size_t size);
+  virtual bool InitializeAndLock(size_t size) = 0;
 
   // Lock the memory so that it will not be purged by the system. Returns
   // DISCARDABLE_MEMORY_SUCCESS on success. If the return value is
   // DISCARDABLE_MEMORY_FAILED then this object should be discarded and
   // a new one should be created. If the return value is
   // DISCARDABLE_MEMORY_PURGED then the memory is present but any data that
   // was in it is gone.
-  LockDiscardableMemoryStatus Lock() WARN_UNUSED_RESULT;
+  virtual LockDiscardableMemoryStatus Lock() WARN_UNUSED_RESULT = 0;
 
   // Unlock the memory so that it can be purged by the system. Must be called
   // after every successful lock call.
-  void Unlock();
+  virtual void Unlock() = 0;
 
   // Return the memory address held by this object. The object must be locked
   // before calling this. Otherwise, this will cause a DCHECK error.
-  void* Memory() const;
-
-  // Testing utility calls.
-
-  // Check whether a purge of all discardable memory in the system is supported.
-  // Use only for testing!
-  static bool PurgeForTestingSupported();
-
-  // Purge all discardable memory in the system. This call has global effects
-  // across all running processes, so it should only be used for testing!
-  static void PurgeForTesting();
-
- private:
-#if defined(OS_ANDROID)
-  // Maps the discardable memory into the caller's address space.
-  // Returns true on success, false otherwise.
-  bool Map();
-
-  // Unmaps the discardable memory from the caller's address space.
-  void Unmap();
-
-  // Reserve a file descriptor. When reaching the fd limit, this call returns
-  // false and initialization should fail.
-  bool ReserveFileDescriptor();
-
-  // Release a file descriptor so that others can reserve it.
-  void ReleaseFileDescriptor();
-#endif  // OS_ANDROID
-
-  void* memory_;
-  size_t size_;
-  bool is_locked_;
-#if defined(OS_ANDROID)
-  int fd_;
-#endif  // OS_ANDROID
-
-  DISALLOW_COPY_AND_ASSIGN(DiscardableMemory);
+  virtual void* Memory() const = 0;
 };
 
 }  // namespace base
 
 #endif  // BASE_MEMORY_DISCARDABLE_MEMORY_H_