GN threading refactor

tools/gn/command_gen.cc

Index: tools/gn/command_gen.cc
diff --git a/tools/gn/command_gen.cc b/tools/gn/command_gen.cc
index 9f6d38f01afa62c3608ca75ba787de75a18ea29d..cde677a1443d01191210272d667f171a1f529177 100644
--- a/tools/gn/command_gen.cc
+++ b/tools/gn/command_gen.cc
@@ -22,10 +22,26 @@ namespace {
 // Suppress output on success.
 const char kSwitchQuiet[] = "q";
 
-void TargetResolvedCallback(base::subtle::Atomic32* write_counter,
-                            const Target* target) {
+void BackgroundDoWrite(const Target* target, const Toolchain* toolchain) {
+  NinjaTargetWriter::RunAndWriteFile(target, toolchain);
+  g_scheduler->DecrementWorkCount();
+}
+
+// Called on the main thread.
+void ItemResolvedCallback(base::subtle::Atomic32* write_counter,
+                          scoped_refptr<Builder> builder,
+                          const Item* item) {
   base::subtle::NoBarrier_AtomicIncrement(write_counter, 1);
-  NinjaTargetWriter::RunAndWriteFile(target);
+
+  const Target* target = item->AsTarget();
+  if (target) {
+    const Toolchain* toolchain =
+        builder->GetToolchain(target->settings()->toolchain_label());
+    DCHECK(toolchain);
+    g_scheduler->IncrementWorkCount();
+    g_scheduler->ScheduleWork(
+        base::Bind(&BackgroundDoWrite, target, toolchain));
+  }
 }
 
 }  // namespace
@@ -51,15 +67,17 @@ int RunGen(const std::vector<std::string>& args) {
   // Cause the load to also generate the ninja files for each target. We wrap
   // the writing to maintain a counter.
   base::subtle::Atomic32 write_counter = 0;
-  setup->build_settings().set_target_resolved_callback(
-      base::Bind(&TargetResolvedCallback, &write_counter));
+  setup->builder()->set_resolved_callback(
+      base::Bind(&ItemResolvedCallback, &write_counter,
+                 scoped_refptr<Builder>(setup->builder())));
 
   // Do the actual load. This will also write out the target ninja files.
   if (!setup->Run())
     return 1;
 
   // Write the root ninja files.
-  if (!NinjaWriter::RunAndWriteFiles(&setup->build_settings()))
+  if (!NinjaWriter::RunAndWriteFiles(&setup->build_settings(),
+                                     setup->builder()))
     return 1;
 
   base::TimeTicks end_time = base::TimeTicks::Now();