GN: Makes GN output deterministic

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 176 matching lines @@
        << "  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);
-  for (const auto& input_file : input_files)
-    dep_out_ << " " << FilePathToUTF8(input_file);
+
+  // Put the files into a set so that the output is sorted and hence

[M-A Ruel, 2015/12/07 00:20:16]

Why not just std::sort(input_files.begin(), input_files.end());

That would be faster than creating a second structure in memory.

[Zachary Forman, 2015/12/07 03:18:17]

Interestingly, the two vectors seem to contain a non-deterministic number of
targets, and either one might contain duplicates.
The purpose of the std::set here is both to sort the output and remove
duplicates.

This could be better commented, so I'll clarify that in the comment :).

+  // deterministic.
+  std::set<base::FilePath> file_set(input_files.begin(), input_files.end());
 
   // Other files read by the build.
-  std::vector<base::FilePath> other_files = g_scheduler->GetGenDependencies();
-  for (const auto& other_file : other_files)
-    dep_out_ << " " << FilePathToUTF8(other_file);
+  input_files = g_scheduler->GetGenDependencies();
+
+  file_set.insert(input_files.begin(), input_files.end());
+
+  for (const auto& input_file : file_set)
+    dep_out_ << " " << FilePathToUTF8(input_file);
 
   out_ << std::endl;
 }
 
 void NinjaBuildWriter::WriteLinkPool() {
   out_ << "pool link_pool\n"
        << "  depth = " << default_toolchain_->concurrent_links() << std::endl
        << std::endl;
 }
 
@@ skipping 134 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;
 }