DiscardableHandle Implementation

gpu/command_buffer/common/discardable_handle.h

Index: gpu/command_buffer/common/discardable_handle.h
diff --git a/gpu/command_buffer/common/discardable_handle.h b/gpu/command_buffer/common/discardable_handle.h
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+++ b/gpu/command_buffer/common/discardable_handle.h
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+// Copyright (c) 2017 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 GPU_COMMAND_BUFFER_COMMON_DISCARDABLE_HANDLE_H_
+#define GPU_COMMAND_BUFFER_COMMON_DISCARDABLE_HANDLE_H_
+
+#include "base/memory/ref_counted.h"
+#include "base/memory/shared_memory.h"
+#include "gpu/command_buffer/common/buffer.h"
+
+namespace gpu {
+
+// DiscardableHandle provides a threadsafe object which can be transitioned
+// between one of three states:
+//  ╔════════════╗         ╔════════════╗         ╔═══════════╗
+//  ║   Locked   ║ ──────> ║  Unlocked  ║ ──────> ║  Deleted  ║
+//  ╚════════════╝         ╚════════════╝         ╚═══════════╝
+//         └───────────<──────────┘
+// Note that a DiscardableHAndle can be locked multiple times, and stores
+// a lock-count.
+//
+// In order to facilitate transfering DiscardableHandles across the command
+// buffer, a DiscardableHandle is backed by a gpu::Buffer and an offset
+// into that buffer. The handle uses a single uint32_t of data at the given
+// offset.
+class GPU_EXPORT DiscardableHandle {
+ public:
+  void InitializeWithNewData(scoped_refptr<Buffer> buffer,
+                             uint32_t byte_offset,
+                             int32_t shm_id);
+  void InitializeWithExistingData(scoped_refptr<Buffer> buffer,
+                                  uint32_t byte_offset,
+                                  int32_t shm_id);
+
+  bool Lock();
+  void Unlock();
+  bool Delete();
+
+  bool IsInitialized() const { return !!shm_id_; }
+  int32_t shm_id() const { return shm_id_; }
+  uint32_t byte_offset() const { return byte_offset_; }
+  bool IsDeleted() const;
+
+  // Test only functions.
+  bool IsLockedForTesting();
+
+ private:
+  volatile base::subtle::Atomic32* AsAtomic() const;
+
+  scoped_refptr<Buffer> buffer_;
+  uint32_t byte_offset_ = 0;
+  uint32_t shm_id_ = 0;
+};
+
+}  // namespace gpu
+
+#endif  // GPU_COMMAND_BUFFER_COMMON_DISCARDABLE_HANDLE_H_