Teach the mojo_shell --trace-startup flag to gather data from services

shell/desktop/main.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <stdio.h>
 #include <string.h>
 
 #include <algorithm>
 #include <iostream>
 
@@ skipping 73 matching lines @@
 #if !defined(NDEBUG) || !defined(ENABLE_PROFILING)
     LOG(ERROR) << "Profiling requires is_debug=false and "
                << "enable_profiling=true. Continuing without profiling.";
 // StartProfiling() and StopProfiling() are a no-op.
 #endif
     base::debug::StartProfiling("mojo_shell.pprof");
   }
 
   // We want the shell::Context to outlive the MessageLoop so that pipes are all
   // gracefully closed / error-out before we try to shut the Context down.
-  shell::Context shell_context;
+  shell::Context shell_context(&tracer);
   {
     base::MessageLoop message_loop;
-    if (!shell_context.Init()) {
-      Usage();
-      return 1;
-    }
-
     if (trace_startup) {
       message_loop.PostDelayedTask(
           FROM_HERE, base::Bind(&shell::Tracer::StopAndFlushToFile,
                                 base::Unretained(&tracer), "mojo_shell.trace"),
           base::TimeDelta::FromSeconds(5));
     }
 
+    if (!shell_context.Init()) {

[viettrungluu, 2015/04/27 17:04:11]

I don't know if the sketchy failure case is compatible with the post task above.
(I suppose it probably is, strictly speaking, but it only becomes even more
sketchy.)

[jamesr, 2015/04/27 23:46:57]

I think it's OK - even here the message_loop goes away before the shell_context
so if the task were to run somehow it wouldn't see any dead objects

+      Usage();
+      return 1;
+    }
+
     // The mojo_shell --args-for command-line switch is handled specially
     // because it can appear more than once. The base::CommandLine class
     // collapses multiple occurrences of the same switch.
     for (int i = 1; i < argc; i++)
       ApplyApplicationArgs(&shell_context, argv[i]);
 
     message_loop.PostTask(
         FROM_HERE, base::Bind(&shell::RunCommandLineApps, &shell_context));
     message_loop.Run();
 
     // Must be called before |message_loop| is destroyed.
     shell_context.Shutdown();
   }
 
   if (command_line.HasSwitch(switches::kCPUProfile))
     base::debug::StopProfiling();
   if (trace_startup)
     tracer.StopAndFlushToFile("mojo_shell.trace");
   return 0;
 }