Add basic tracing support to the gtest launcher

base/test/launcher/test_launcher.cc

Index: base/test/launcher/test_launcher.cc
diff --git a/base/test/launcher/test_launcher.cc b/base/test/launcher/test_launcher.cc
index cfd08fbc49ca6d89c121076eddc3c5d60b125550..74ddebb349a5fd97cc253fca333f612bf772a311 100644
--- a/base/test/launcher/test_launcher.cc
+++ b/base/test/launcher/test_launcher.cc
@@ -31,6 +31,7 @@
 #include "base/strings/stringprintf.h"
 #include "base/strings/utf_string_conversions.h"
 #include "base/test/gtest_util.h"
+#include "base/test/launcher/test_launcher_tracer.h"
 #include "base/test/launcher/test_results_tracker.h"
 #include "base/test/sequenced_worker_pool_owner.h"
 #include "base/test/test_switches.h"
@@ -90,6 +91,9 @@ LazyInstance<std::map<ProcessHandle, CommandLine> > g_live_processes
     = LAZY_INSTANCE_INITIALIZER;
 LazyInstance<Lock> g_live_processes_lock = LAZY_INSTANCE_INITIALIZER;
 
+// Performance trace generator.
+LazyInstance<TestLauncherTracer> g_tracer = LAZY_INSTANCE_INITIALIZER;
+
 #if defined(OS_POSIX)
 // Self-pipe that makes it possible to do complex shutdown handling
 // outside of the signal handler.
@@ -253,6 +257,8 @@ int LaunchChildTestProcessWithOptions(
     TimeDelta timeout,
     const TestLauncher::GTestProcessLaunchedCallback& launched_callback,
     bool* was_timeout) {
+  TimeTicks start_time(TimeTicks::Now());
+
 #if defined(OS_POSIX)
   // Make sure an option we rely on is present - see LaunchChildGTestProcess.
   DCHECK(options.new_process_group);
@@ -344,6 +350,9 @@ int LaunchChildTestProcessWithOptions(
     g_live_processes.Get().erase(process.Handle());
   }
 
+  g_tracer.Get().RecordProcessExecution(

[Primiano Tucci (use gerrit), 2016/07/01 11:34:29]

If this is really what you need it's ok. But if you need more events you can
consider having:
-  a TestLauncherTracer::ScopedEvent object which records one 'B' event in the
ctor and one 'B' event in the dtor
- A method g_tracer.RecordScopedEvent() which returns the ScopedEvent object. 
 so you can just add to your code like we do with TRACE_EVENT macros without
having to diff the time by hand.
But if this is really what you want, it's probably ok as it is.

[Paweł Hajdan Jr., 2016/07/01 12:47:32]

Yeah, it's OK for now. I can take another look if I need something more
advanced.

+      start_time, TimeTicks::Now() - start_time);
+
   return exit_code;
 }
 
@@ -1034,6 +1043,13 @@ void TestLauncher::MaybeSaveSummaryAsJSON() {
       LOG(ERROR) << "Failed to save test launcher output summary.";
     }
   }
+  if (command_line->HasSwitch(switches::kTestLauncherTrace)) {
+    FilePath trace_path(command_line->GetSwitchValuePath(
+        switches::kTestLauncherTrace));
+    if (!g_tracer.Get().Dump(trace_path)) {
+      LOG(ERROR) << "Failed to save test launcher trace.";

[Primiano Tucci (use gerrit), 2016/07/01 11:34:30]

maybe move this error logging to the Dump method and make it void. Not sure in
how many other cases you really care about the success return code and want to
Dump() but ignore the eventual failure.

[Paweł Hajdan Jr., 2016/07/01 12:47:32]

This is consistent with above code. I think as an interface, it's preferred to
have bool return value rather than void and log inside.

+    }
+  }
 }
 
 void TestLauncher::OnLaunchTestProcessFinished(