Add output reference to gen written files.

tools/gn/ninja_build_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_build_writer.h"
 
 #include <fstream>
 #include <map>
 
 #include "base/command_line.h"
@@ skipping 125 matching lines @@
   }
 
   NinjaBuildWriter gen(build_settings, all_settings, default_toolchain,
                        default_toolchain_targets, file, depfile);
   return gen.Run(err);
 }
 
 void NinjaBuildWriter::WriteNinjaRules() {
   out_ << "rule gn\n";
   out_ << "  command = " << GetSelfInvocationCommand(build_settings_) << "\n";
-  out_ << "  description = Regenerating ninja files\n\n";
+  out_ << "  description = Regenerating ninja files\n";
+  out_ << "  restat = 1\n\n";
 
-  // This rule will regenerate the ninja files when any input file has changed.
-  out_ << "build build.ninja: gn\n"
+  // This rule will regenerate the ninja files when any input file has changed,
+  // or is missing.
+  out_ << "build build.ninja";
+
+  // Other files read by the build.
+  std::vector<SourceFile> written_files = g_scheduler->GetWrittenFiles();
+  for (const auto& written_file : written_files)
+    out_ << " " << FilePathToUTF8(build_settings_->GetFullPath(written_file));
+
+  out_ << ": gn\n"
        << "  generator = 1\n"
        << "  depfile = build.ninja.d\n";
 
   // Input build files. These go in the ".d" file. If we write them as
   // dependencies in the .ninja file itself, ninja will expect the files to
   // exist and will error if they don't. When files are listed in a depfile,
   // missing files are ignored.
   dep_out_ << "build.ninja:";
   std::vector<base::FilePath> input_files;
   g_scheduler->input_file_manager()->GetAllPhysicalInputFileNames(&input_files);
@@ skipping 153 matching lines @@
   EscapeOptions ninja_escape;
   ninja_escape.mode = ESCAPE_NINJA;
 
   // Escape for special chars Ninja will handle.
   std::string escaped = EscapeString(phony_name, ninja_escape, nullptr);
 
   out_ << "build " << escaped << ": phony ";
   path_output_.WriteFile(out_, target_file);
   out_ << std::endl;
 }