[BattOr] Reduce StartTracing time by about four seconds.

tools/battor_agent/battor_agent.cc

Index: tools/battor_agent/battor_agent.cc
diff --git a/tools/battor_agent/battor_agent.cc b/tools/battor_agent/battor_agent.cc
index dc5a521a6f6abf591ac728d47be537568fc9e649..5f18c7bdf072f05343f67cdf8be160e1344ded42 100644
--- a/tools/battor_agent/battor_agent.cc
+++ b/tools/battor_agent/battor_agent.cc
@@ -246,6 +246,9 @@ void BattOrAgent::OnBytesSent(bool success) {
       num_read_attempts_ = 1;
       PerformAction(Action::READ_GIT_HASH);
       return;
+    case Action::RESET:
+      CompleteCommand(BATTOR_ERROR_NONE);
+      return;
     default:
       CompleteCommand(BATTOR_ERROR_UNEXPECTED_MESSAGE);
   }
@@ -406,7 +409,8 @@ void BattOrAgent::OnMessageRead(bool success,
       // Check for the empty frame the BattOr uses to indicate it's done
       // streaming samples.
       if (frame.empty()) {
-        CompleteCommand(BATTOR_ERROR_NONE);
+        // Proactively reset the BattOr to speed up the next trace.

[charliea (OOO until 10-5), 2017/05/09 19:19:06]

I think this could be made clearer: for people stumbling through this code in
the future, it may be unclear why resetting the BattOr now will speed up the
trace in the future. That requires a surprisingly significant amount of
knowledge about how BattOrs work.

Could you elaborate in the comment about why this speeds things up?

[aschulman, 2017/05/12 22:32:03]

agreed. I will do this as best as I can.

+        PerformAction(Action::RESET);
         return;
       }
 
@@ -536,6 +540,10 @@ void BattOrAgent::PerformAction(Action action) {
       connection_->ReadMessage(BATTOR_MESSAGE_TYPE_CONTROL_ACK);
       return;
 
+    case Action::RESET:
+      SendControlMessage(BATTOR_CONTROL_MESSAGE_TYPE_RESET, 0, 0);
+      return;
+
     case Action::INVALID:
       NOTREACHED();
   }