Add support for data deps.

tools/gn/ninja_writer.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/ninja_writer.h"
 
 #include "tools/gn/location.h"
 #include "tools/gn/ninja_build_writer.h"
 #include "tools/gn/ninja_toolchain_writer.h"
 
@@ skipping 11 matching lines @@
   return writer.WriteRootBuildfiles();
 }
 
 bool NinjaWriter::WriteRootBuildfiles() {
   // Categorize all targets by toolchain.
   typedef std::map<Label, std::vector<const Target*> > CategorizedMap;
   CategorizedMap categorized;
 
   std::vector<const Target*> all_targets;
   build_settings_->target_manager().GetAllTargets(&all_targets);
+  if (all_targets.empty()) {
+    Err(Location(), "No targets.",
+        "I could not find any targets to write, so I'm doing nothing.")
+        .PrintToStdout();
+    return false;
+  }
   for (size_t i = 0; i < all_targets.size(); i++) {
     categorized[all_targets[i]->label().GetToolchainLabel()].push_back(
         all_targets[i]);
   }
 
   Label default_label =
       build_settings_->toolchain_manager().GetDefaultToolchainUnlocked();
 
   // Write out the toolchain buildfiles, and also accumulate the set of
   // all settings and find the list of targets in the default toolchain.
   std::vector<const Settings*> all_settings;
   const std::vector<const Target*>* default_targets = NULL;
   for (CategorizedMap::const_iterator i = categorized.begin();
        i != categorized.end(); ++i) {
     const Settings* settings;
     {
       base::AutoLock lock(build_settings_->item_tree().lock());
       Err ignored;
       settings =
           build_settings_->toolchain_manager().GetSettingsForToolchainLocked(
               LocationRange(), i->first, &ignored);
     }
     if (i->first == default_label)
       default_targets = &i->second;
     all_settings.push_back(settings);
-    if (!NinjaToolchainWriter::RunAndWriteFile(settings, i->second))
+    if (!NinjaToolchainWriter::RunAndWriteFile(settings, i->second)) {
+      Err(Location(),
+          "Couldn't open toolchain buildfile(s) for writing").PrintToStdout();
       return false;
+    }
   }
 
   // Write the root buildfile.
-  return NinjaBuildWriter::RunAndWriteFile(build_settings_, all_settings,
-                                           *default_targets);
+  if (!NinjaBuildWriter::RunAndWriteFile(build_settings_, all_settings,
+                                         *default_targets)) {
+    Err(Location(),
+        "Couldn't open toolchain buildfile(s) for writing").PrintToStdout();
+    return false;
+  }
+  return true;
 }