[battor agent] Make BattOr agent block until the BattOrAgent is created

tools/battor_agent/battor_agent_bin.cc

Index: tools/battor_agent/battor_agent_bin.cc
diff --git a/tools/battor_agent/battor_agent_bin.cc b/tools/battor_agent/battor_agent_bin.cc
index 5cfce66a7b6492e795a36cacf8431941e366c80a..291f2d877d8c5b2c8b25cf023f5c806d536417af 100644
--- a/tools/battor_agent/battor_agent_bin.cc
+++ b/tools/battor_agent/battor_agent_bin.cc
@@ -141,10 +141,13 @@ class BattOrAgentBin : public BattOrAgent::Listener {
       ExitFromThreadStartFailure(kIoThreadName);
     }
 
+    base::WaitableEvent done(base::WaitableEvent::ResetPolicy::AUTOMATIC,
+                             base::WaitableEvent::InitialState::NOT_SIGNALED);

[aschulman, 2016/06/15 14:51:03]

I'd like to see a comment explaining why this particular waitable event is
needed since this was needed to fix a particularly nasty bug that wasn't obvious
when you first wrote the code for this thread.

     io_thread_.task_runner()->PostTask(
         FROM_HERE,
         base::Bind(&BattOrAgentBin::CreateAgent, base::Unretained(this), path,
-                   base::ThreadTaskRunnerHandle::Get()));
+                   base::ThreadTaskRunnerHandle::Get(), &done));
+    done.Wait();
   }
 
   // Performs any cleanup necessary after the BattOr binary is done running.
@@ -272,7 +275,8 @@ class BattOrAgentBin : public BattOrAgent::Listener {
   // and deleting it, MUST happen on the IO thread.
   void CreateAgent(
       const std::string& path,
-      scoped_refptr<base::SingleThreadTaskRunner> ui_thread_task_runner) {
+      scoped_refptr<base::SingleThreadTaskRunner> ui_thread_task_runner,
+      base::WaitableEvent* done) {
     // In Chrome, we already have a file thread running. Because the Chrome
     // serial library relies on having it available, we have to spin up our own.
     if (!file_thread_.Start())
@@ -280,6 +284,7 @@ class BattOrAgentBin : public BattOrAgent::Listener {
 
     agent_.reset(new BattOrAgent(path, this, file_thread_.task_runner(),
                                  ui_thread_task_runner));
+    done->Signal();
   }
 
   // Postable task for deleting the BattOrAgent. See the comment for