Improve multi-process debugging.

mojo/shell/child_process.cc

 // Copyright 2014 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 "mojo/shell/child_process.h"
 
+#include "base/base_switches.h"
 #include "base/bind.h"
 #include "base/callback_helpers.h"
 #include "base/command_line.h"
+#include "base/debug/debugger.h"
 #include "base/files/file_path.h"
 #include "base/location.h"
 #include "base/logging.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/message_loop/message_loop.h"
 #include "base/single_thread_task_runner.h"
 #include "base/synchronization/waitable_event.h"
 #include "base/threading/thread.h"
@@ skipping 252 matching lines @@
 
   DISALLOW_COPY_AND_ASSIGN(ChildControllerImpl);
 };
 
 }  // namespace
 
 int ChildProcessMain() {
   DVLOG(2) << "ChildProcessMain()";
   const base::CommandLine& command_line =
       *base::CommandLine::ForCurrentProcess();
+  if (command_line.HasSwitch(switches::kWaitForDebugger)) {
+    std::string app = command_line.GetSwitchValueASCII(switches::kApp);
+#if defined(OS_WIN)
+    MessageBox(NULL, command_line.GetSwitchValueNative(switches::kApp).c_str(),
+               command_line.GetSwitchValueNative(switches::kApp).c_str(),
+               MB_OK | MB_SETFOREGROUND);
+#else
+    LOG(ERROR) << command_line.GetSwitchValueASCII(switches::kApp)
+               << " waiting for GDB.";

[sky, 2015/04/23 21:48:54]

Can you add the process id to this output.

[jam, 2015/04/23 22:18:44]

Done.

+    base::debug::WaitForDebugger(60, true);
+#endif
+  }
+
   embedder::ScopedPlatformHandle platform_channel =
       embedder::PlatformChannelPair::PassClientHandleFromParentProcess(
           command_line);
   CHECK(platform_channel.is_valid());
 
   DCHECK(!base::MessageLoop::current());
 
   AppContext app_context;
   app_context.Init();
 
   Blocker blocker;
   app_context.controller_runner()->PostTask(
       FROM_HERE,
       base::Bind(&ChildControllerImpl::Init, base::Unretained(&app_context),
                  base::Passed(&platform_channel), blocker.GetUnblocker()));
   // This will block, then run whatever the controller wants.
   blocker.Block();
 
   app_context.Shutdown();
 
   return 0;
 }
 
 }  // namespace shell
 }  // namespace mojo