Add async trace events to trace progress of IPC messages

ipc/ipc_message.cc

Index: ipc/ipc_message.cc
diff --git a/ipc/ipc_message.cc b/ipc/ipc_message.cc
index 9908bc7a001bd2ce3770ec1e789d8fbb4c133b20..cf3a65e077f08dc767f63150d48c848b79589419 100644
--- a/ipc/ipc_message.cc
+++ b/ipc/ipc_message.cc
@@ -4,6 +4,7 @@
 
 #include "ipc/ipc_message.h"
 
+#include "base/atomicops.h"
 #include "base/logging.h"
 #include "build/build_config.h"
 
@@ -11,6 +12,26 @@
 #include "ipc/file_descriptor_set_posix.h"
 #endif
 
+namespace {
+
+base::subtle::Atomic32 g_ref_num = 0;
+
+// Create a reference number for identifying IPC messages in traces. The return
+// values has the reference number stored in the upper 24 bits, leaving the low
+// 8 bits set to 0 for use as flags.
+inline uint32 GetRefNumUpper24() {
+  base::debug::TraceLog* trace_log = base::debug::TraceLog::GetInstance();
+  int32 pid = trace_log ? trace_log->process_id() : 0;
+  int32 count = base::subtle::NoBarrier_AtomicIncrement(&g_ref_num, 1);
+  // The 24 bit hash is composed of 14 bits of the count and 10 bits of the
+  // Process ID. With the current trace event buffer cap, the 14-bit count did
+  // not appear to wrap during a trace. Note that it is not a big deal if
+  // collisions occur, as this is only used for debugging and trace analysis.
+  return ((pid << 14) | (count & 0x3fff)) << 8;
+}
+
+}  // namespace
+
 namespace IPC {
 
 //------------------------------------------------------------------------------
@@ -20,7 +41,8 @@ Message::~Message() {
 
 Message::Message()
     : Pickle(sizeof(Header)) {
-  header()->routing = header()->type = header()->flags = 0;
+  header()->routing = header()->type = 0;
+  header()->flags = GetRefNumUpper24();
 #if defined(OS_POSIX)
   header()->num_fds = 0;
   header()->pad = 0;
@@ -32,7 +54,8 @@ Message::Message(int32 routing_id, uint32 type, PriorityValue priority)
     : Pickle(sizeof(Header)) {
   header()->routing = routing_id;
   header()->type = type;
-  header()->flags = priority;
+  DCHECK((priority & 0xffffff00) == 0);
+  header()->flags = priority | GetRefNumUpper24();
 #if defined(OS_POSIX)
   header()->num_fds = 0;
   header()->pad = 0;