Re-land: base: Replace PurgeAndTruncate with Shrink function.

base/memory/discardable_shared_memory.h

 // Copyright 2014 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_SHARED_MEMORY_H_
 #define BASE_MEMORY_DISCARDABLE_SHARED_MEMORY_H_
 
 #include "base/base_export.h"
 #include "base/logging.h"
 #include "base/memory/shared_memory.h"
 #include "base/threading/thread_collision_warner.h"
 #include "base/time/time.h"
 
 #if DCHECK_IS_ON()
 #include <set>
 #endif
 
 namespace base {
 

[danakj, 2015/03/25 17:55:00]

If might be simpler to just #define DISCARDABLE_SHARED_MEMORY_SHRINKING here
instead of in the gypi file (does that work for the GN build?)

[reveman, 2015/03/25 18:26:42]

Sure, that works.

Having the define in build/ makes sense if we'd like to have some code that
doesn't need to include this file behave differently based on if this is
supported or not. We might need that in the future but we can keep it in here
for now.

 // Platform abstraction for discardable shared memory.
 //
 // This class is not thread-safe. Clients are responsible for synchronizing
 // access to an instance of this class.
 class BASE_EXPORT DiscardableSharedMemory {
  public:
   enum LockResult { SUCCESS, PURGED, FAILED };
 
   DiscardableSharedMemory();
 
@@ skipping 56 matching lines @@
   // if locked or the actual last usage timestamp if unlocked. It is often
   // necessary to call this function twice for the object to successfully be
   // purged. First call, updates |last_known_usage_|. Second call, successfully
   // purges the object using the updated |last_known_usage_|.
   // Note: there is no guarantee that multiple calls to this function will
   // successfully purge object. DiscardableSharedMemory object might be locked
   // or another thread/process might be able to lock and unlock it in between
   // each call.
   bool Purge(Time current_time);
 
-  // Purge and release as much memory as possible to the OS.
-  // Note: The amount of memory that can be released to the OS is platform
-  // specific. Best case, all but one page is released. Worst case, nothing
-  // is released.
-  bool PurgeAndTruncate(Time current_time);
-
   // Returns true if memory is still resident.
   bool IsMemoryResident() const;
 
   // Closes the open discardable memory segment.
   // It is safe to call Close repeatedly.
   void Close();
 
   // Shares the discardable memory segment to another process. Attempts to
   // create a platform-specific |new_handle| which can be used in a remote
   // process to access the discardable memory segment. |new_handle| is an
   // output parameter to receive the handle for use in the remote process.
   // Returns true on success, false otherwise.
   bool ShareToProcess(ProcessHandle process_handle,
                       SharedMemoryHandle* new_handle) {
     return shared_memory_.ShareToProcess(process_handle, new_handle);
   }
 
+#if defined(DISCARDABLE_SHARED_MEMORY_SHRINKING)
+  // Release as much memory as possible to the OS. The change in size will
+  // be reflected by the return value of mapped_size().
+  void Shrink();
+#endif
+
  private:
   // Virtual for tests.
   virtual Time Now() const;
 
   SharedMemory shared_memory_;
   size_t mapped_size_;
   size_t locked_page_count_;
 #if DCHECK_IS_ON()
   std::set<size_t> locked_pages_;
 #endif
   // Implementation is not thread-safe but still usable if clients are
   // synchronized somehow. Use a collision warner to detect incorrect usage.
   DFAKE_MUTEX(thread_collision_warner_);
   Time last_known_usage_;
 
   DISALLOW_COPY_AND_ASSIGN(DiscardableSharedMemory);
 };
 
 }  // namespace base
 
 #endif  // BASE_MEMORY_DISCARDABLE_SHARED_MEMORY_H_