Implement proper synchronization between HW video decode IPC and CommandBuffer.

gpu/command_buffer/common/command_buffer.h

Index: gpu/command_buffer/common/command_buffer.h
diff --git a/gpu/command_buffer/common/command_buffer.h b/gpu/command_buffer/common/command_buffer.h
index d438f7225acf9762af4a4b6e31cc78aea7deef61..83b942688e8c5f4935fc9c435536dbdd51004f9a 100644
--- a/gpu/command_buffer/common/command_buffer.h
+++ b/gpu/command_buffer/common/command_buffer.h
@@ -123,6 +123,25 @@ class CommandBuffer {
   DISALLOW_COPY_AND_ASSIGN(CommandBuffer);
 };
 
+// Synchronizing other mechanisms (such as IPC) with the CommandBuffer requires
+// inserting (writing) a token into the buffer and knowing what the last token
+// read at that point was.  ReadWriteTokens is a convenience struct for passing
+// these pairs around.  Expected usage is to compare a current token to
+// [last_token_read,last_token_written).
+struct ReadWriteTokens {
+  ReadWriteTokens(int32 read, int32 written);
+  // Required to support pickling.  Use by anything else will DCHECK in InRange.
+  ReadWriteTokens();
+
+  // Return true iff |value| is in the range described by |tokens|, accounting
+  // for (up to) one wrap-around.
+  bool InRange(int32 token) const;
+
+  // This want to be private (and const) but can't in order to support pickling.

[piman, 2011/06/28 21:24:41]

No need for this comment... coding style says that structs should have public
members. This seems perfectly fair to be public.

[Ami GONE FROM CHROMIUM, 2011/06/28 21:27:38]

My reason for the comment is that this isn't used as a struct - nobody outside
this class other than pickle code should be reading or writing these members;
instead they should be using InRange().

+  int32 last_token_read;
+  int32 last_token_written;
+};
+
 }  // namespace gpu
 
 #endif  // GPU_COMMAND_BUFFER_COMMON_COMMAND_BUFFER_H_