Add discardable memory emulation for non-android/mac platforms

base/memory/discardable_memory.h

Index: base/memory/discardable_memory.h
diff --git a/base/memory/discardable_memory.h b/base/memory/discardable_memory.h
index d6e91b979d5a46fdd50e6ef07ab4f52a904bc042..f50caf8947b5d8fffc7e18bec7c71bc235420616 100644
--- a/base/memory/discardable_memory.h
+++ b/base/memory/discardable_memory.h
@@ -8,6 +8,7 @@
 #include "base/base_export.h"
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
+#include "base/threading/non_thread_safe.h"
 
 namespace base {
 
@@ -17,6 +18,12 @@ enum LockDiscardableMemoryStatus {
   DISCARDABLE_MEMORY_SUCCESS = 1
 };
 
+#if defined(OS_MACOSX) || defined(OS_ANDROID)
+#define DISCARDABLE_MEMORY_ALWAYS_SUPPORTED_NATIVELY
+#else
+#define DISCARDABLE_MEMORY_NEVER_SUPPORTED_NATIVELY
+#endif
+
 // 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
@@ -37,13 +44,17 @@ enum LockDiscardableMemoryStatus {
 //   - 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.
+//   - A discardable memory instance is not thread safe. If the need to pass
+//     ownership of a discardable memory instance from one thread to another,
+//     NonThreadSafe::DetachFromThread could be used, but this is unnecessary
+//     at the moment.
 //
 // 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 {
+class BASE_EXPORT DiscardableMemory : public NonThreadSafe {
  public:
   DiscardableMemory();
 
@@ -51,8 +62,9 @@ class BASE_EXPORT DiscardableMemory {
   // The opened file will also be closed after this.
   ~DiscardableMemory();
 
-  // Check whether the system supports discardable memory.
-  static bool Supported();
+  // Check whether the system supports discardable memory natively. Returns
+  // false if the support is emulated.
+  static bool SupportedNatively();
 
   // Initialize the DiscardableMemory object. On success, this function returns
   // true and the memory is locked. This should only be called once.
@@ -72,6 +84,8 @@ class BASE_EXPORT DiscardableMemory {
   // after every successful lock call.
   void Unlock();
 
+  bool is_locked() const { return is_locked_; }
+
   // 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;
@@ -87,6 +101,15 @@ class BASE_EXPORT DiscardableMemory {
   static void PurgeForTesting();
 
  private:
+#if !defined(DISCARDABLE_MEMORY_ALWAYS_SUPPORTED_NATIVELY)
+  friend class DiscardableMemoryProvider;
+  friend class DiscardableMemoryProviderTest;
+
+  // Used by the DiscardableMemoryProvider to allocate and free memory.
+  bool Allocate();
+  void Deallocate();
+#endif
+
 #if defined(OS_ANDROID)
   // Maps the discardable memory into the caller's address space.
   // Returns true on success, false otherwise.