Makes the BattOrAgent asynchronous

I confirmed with simonhatch@ that this made sense given the way that
the Chromium TracingController works. The agent will be communicating
over a serial connection, which probably shouldn't be done
synchronously.

BUG=542837

Review URL: https://codereview.chromium.org/1407033008

Cr-Commit-Position: refs/heads/master@{#357144}

tools/battor_agent/BUILD.gn

@@ -8,6 +8,7 @@ executable("battor_agent") {
   ]
   deps = [
     ":battor_agent_lib",
+    "//base",
   ]
 }
 

tools/battor_agent/battor_agent.cc

@@ -6,30 +6,38 @@
 
 #include <iostream>
 
+#include "base/callback.h"
+#include "base/memory/scoped_ptr.h"
+
 namespace battor {
 
 BattOrAgent::BattOrAgent(const std::string& path) : path_(path) {
-  // TODO: Open up a serial connection with the BattOr.
+  // TODO(charliea): Open up a serial connection with the BattOr.
 }
 
 BattOrAgent::~BattOrAgent() {
-  // TODO: Close the serial connection with the BattOr.
+  // TODO(charliea): Close the serial connection with the BattOr.
 }
 
 void BattOrAgent::StartTracing() {
-  // TODO: Tell the BattOr to start tracing.
+  // TODO(charliea): Tell the BattOr to start tracing.
 }
 
-void BattOrAgent::StopTracing(std::string* out_trace) {
-  // TODO: Tell the BattOr to stop tracing.
+void BattOrAgent::StopTracing(std::string* trace_output,
+                              const base::Closure& callback) {
+  // TODO(charliea): Tell the BattOr to stop tracing.
+  *trace_output = "battor trace output";
+  callback.Run();
 }
 
-void BattOrAgent::RecordClockSyncMarker(const std::string& marker) {
-  // TODO: Tell the BattOr to record the specified clock sync marker.
+void BattOrAgent::RecordClockSyncMarker(const std::string& marker,
+                                        const base::Closure& callback) {
+  // TODO(charliea): Tell the BattOr to record the specified clock sync marker.
+  callback.Run();
 }
 
 void BattOrAgent::IssueClockSyncMarker() {
-  // TODO: Tell atrace to issue a clock sync marker.
+  // TODO(charliea): Tell atrace to issue a clock sync marker.
 }
 
 }  // namespace battor

tools/battor_agent/battor_agent.h

@@ -4,7 +4,9 @@
 
 #include <string>
 
+#include "base/callback_forward.h"
 #include "base/macros.h"
+#include "base/memory/scoped_ptr.h"
 
 namespace battor {
 
@@ -13,17 +15,20 @@ class BattOrAgent {
   explicit BattOrAgent(const std::string& path);
   virtual ~BattOrAgent();
 
-  // Tells the BattOr to start tracing.
+  // Tells the BattOr (using a best-effort signal) to start tracing.
   void StartTracing();
 
-  // Tells the BattOr to stop tracing and returns the trace contents.
-  void StopTracing(std::string* out_trace);
+  // Tells the BattOr to stop tracing and write the trace output to
+  // the specified location and calls the callback when complete.
+  void StopTracing(std::string* trace_output, const base::Closure& callback);
 
-  // Tells the BattOr to record a clock sync marker in its own trace log.
-  void RecordClockSyncMarker(const std::string& marker);
+  // Tells the BattOr to record a clock sync marker in its own trace
+  // log and calls the callback when complete.
+  void RecordClockSyncMarker(const std::string& marker,
+                             const base::Closure& callback);
 
-  // Tells the BattOr to issue clock sync markers to all other tracing
-  // agents that it's connected to.
+  // Tells the BattOr (using a best-effort signal) to issue clock sync
+  // markers to all other tracing agents that it's connected to.
   void IssueClockSyncMarker();
 
   // Returns whether the BattOr is able to record clock sync markers

tools/battor_agent/battor_agent_bin.cc

@@ -2,8 +2,16 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// This file provides a thin binary wrapper around the BattOr Agent
+// library. This binary wrapper provides a means for non-C++ tracing
+// controllers, such as Telemetry and Android Systrace, to issue high-level
+// tracing commands to the BattOr..
+
 #include <iostream>
 
+#include "base/bind.h"
+#include "base/memory/scoped_ptr.h"
+#include "base/synchronization/waitable_event.h"
 #include "tools/battor_agent/battor_agent.h"
 
 using std::cout;
@@ -12,12 +20,16 @@ using std::string;
 
 namespace {
 
+// An event used to signal that the BattOr Agent has finished executing its
+// command.
+base::WaitableEvent g_stop_tracing_complete_event(false, false);
+
 void PrintUsage() {
   cout << "Usage: battor_agent <command> <arguments>" << endl << endl
        << "Commands:" << endl << endl
        << "  StartTracing <path>" << endl
        << "  StopTracing <path>" << endl
-       << "  SupportsExplicitClockSync <path>" << endl
+       << "  SupportsExplicitClockSync" << endl
        << "  RecordClockSyncMarker <path> <marker>" << endl
        << "  IssueClockSyncMarker <path>" << endl
        << "  Help" << endl;
@@ -36,6 +48,10 @@ bool GetArg(int argnum, int argc, char* argv[], string* value) {
   return true;
 }
 
+void OnCommandComplete() {
+  g_stop_tracing_complete_event.Signal();
+}
+
 } // namespace
 
 int main(int argc, char* argv[]) {
@@ -55,10 +71,12 @@ int main(int argc, char* argv[]) {
     if (!GetArg(2, argc, argv, &path))
       return 1;
 
-    string out_trace;
     cout << "Calling StopTracing()" << endl;
-    battor::BattOrAgent(path).StopTracing(&out_trace);
-    cout << out_trace << endl;
+    std::string trace_output;
+    battor::BattOrAgent(path)
+        .StopTracing(&trace_output, base::Bind(&OnCommandComplete));
+    g_stop_tracing_complete_event.Wait();
+    cout << trace_output << endl;
   } else if (cmd == "SupportsExplicitClockSync") {
     cout << "Calling SupportsExplicitClockSync" << endl;
     cout << battor::BattOrAgent::SupportsExplicitClockSync() << endl;
@@ -69,7 +87,11 @@ int main(int argc, char* argv[]) {
 
     cout << "Marker: " << marker << endl;
     cout << "Calling RecordClockSyncMarker()" << endl;
-    battor::BattOrAgent(path).RecordClockSyncMarker(marker);
+    // TODO(charliea): Write the  time to STDOUT
+    battor::BattOrAgent(path)
+        .RecordClockSyncMarker(marker, base::Bind(&OnCommandComplete));
+    g_stop_tracing_complete_event.Wait();
+    // TODO(charliea): Write the time to STDOUT
   } else if (cmd == "IssueClockSyncMarker") {
     string path;
     if (!GetArg(2, argc, argv, &path))