Fix GN deadlock on error handling.

tools/gn/scheduler.cc

 // Copyright (c) 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 "tools/gn/scheduler.h"
 
 #include "base/bind.h"
 #include "base/command_line.h"
 #include "base/strings/string_number_conversions.h"
 #include "tools/gn/standard_out.h"
@@ skipping 25 matching lines @@
 }
 
 Scheduler::~Scheduler() {
   if (!has_been_shutdown_)
     pool_->Shutdown();
   g_scheduler = NULL;
 }
 
 bool Scheduler::Run() {
   runner_.Run();
-  base::AutoLock lock(lock_);
+  bool local_is_failed;
+  {
+    base::AutoLock lock(lock_);
+    local_is_failed = is_failed();
+    has_been_shutdown_ = true;
+  }
+  // Don't do this inside the lock since it will block on the workers, which
+  // may be in turn waiting on the lock.
   pool_->Shutdown();
-  has_been_shutdown_ = true;
-  return !is_failed();
+  return !local_is_failed;
 }
 
 void Scheduler::Log(const std::string& verb, const std::string& msg) {
   if (base::MessageLoop::current() == &main_loop_) {
     LogOnMainThread(verb, msg);
   } else {
     // The run loop always joins on the sub threads, so the lifetime of this
     // object outlives the invocations of this function, hence "unretained".
     main_loop_.PostTask(FROM_HERE,
                         base::Bind(&Scheduler::LogOnMainThread,
                                    base::Unretained(this), verb, msg));
   }
 }
 
 void Scheduler::FailWithError(const Err& err) {
   DCHECK(err.has_error());
   {
     base::AutoLock lock(lock_);
 
-    if (is_failed_)
+    if (is_failed_ || has_been_shutdown_)
       return;  // Ignore errors once we see one.
     is_failed_ = true;
   }
 
   if (base::MessageLoop::current() == &main_loop_) {
     FailWithErrorOnMainThread(err);
   } else {
     // The run loop always joins on the sub threads, so the lifetime of this
     // object outlives the invocations of this function, hence "unretained".
     main_loop_.PostTask(FROM_HERE,
@@ skipping 50 matching lines @@
 void Scheduler::DoWork(const base::Closure& closure) {
   closure.Run();
   DecrementWorkCount();
 }
 
 void Scheduler::OnComplete() {
   // Should be called on the main thread.
   DCHECK(base::MessageLoop::current() == main_loop());
   runner_.Quit();
 }