Add async trace events to trace progress of IPC messages

ipc/ipc_message.h

Index: ipc/ipc_message.h
diff --git a/ipc/ipc_message.h b/ipc/ipc_message.h
index 4603f0953748203e37edb0a964da4621df453cc3..1a2a45cb637ccb9c4d7c14c55666c1e46b1280df 100644
--- a/ipc/ipc_message.h
+++ b/ipc/ipc_message.h
@@ -8,6 +8,7 @@
 #include <string>
 
 #include "base/basictypes.h"
+#include "base/debug/trace_event.h"
 #include "base/pickle.h"
 #include "ipc/ipc_export.h"
 
@@ -49,14 +50,16 @@ class IPC_EXPORT Message : public Pickle {
   };
 
   // Bit values used in the flags field.
+  // Upper 24 bits of flags store a reference number, so this enum is limited to
+  // 8 bits.
   enum {
-    PRIORITY_MASK     = 0x0003,  // Low 2 bits of store the priority value.
-    SYNC_BIT          = 0x0004,
-    REPLY_BIT         = 0x0008,
-    REPLY_ERROR_BIT   = 0x0010,
-    UNBLOCK_BIT       = 0x0020,
-    PUMPING_MSGS_BIT  = 0x0040,
-    HAS_SENT_TIME_BIT = 0x0080,
+    PRIORITY_MASK     = 0x03,  // Low 2 bits of store the priority value.
+    SYNC_BIT          = 0x04,
+    REPLY_BIT         = 0x08,
+    REPLY_ERROR_BIT   = 0x10,
+    UNBLOCK_BIT       = 0x20,
+    PUMPING_MSGS_BIT  = 0x40,
+    HAS_SENT_TIME_BIT = 0x80,
   };
 
   virtual ~Message();
@@ -221,6 +224,11 @@ class IPC_EXPORT Message : public Pickle {
   bool dont_log() const { return dont_log_; }
 #endif
 
+  // Called at various points between send and receive to track message.
+  void TraceMessageStep() {
+    TRACE_EVENT_ASYNC_BEGIN_STEP0("ipc", "IPC", header()->flags, NULL);

[jam, 2012/08/31 23:07:55]

i'm a little worried that this might cause performance regressions? i.e. are
there any locks that this macro cause to be locked? how expensive is this
function? there are tons of ipcs.

[jbates, 2012/08/31 23:27:49]

When tracing is disabled, it boils down to two if statements on global pointer
values. No locks unless tracing is enable. We already trace every task run in
MessageLoop, for example.

+  }
+
  protected:
   friend class Channel;
   friend class MessageReplyDeserializer;