Add support for customizing GN's args text.

tools/gn/setup.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/setup.h"
 
 #include <stdlib.h>
 #include <algorithm>
 #include <sstream>
 #include <utility>
@@ skipping 88 matching lines @@
       for a BUILD.gn file (or the build config file discussed above), the file
       will first be looked for in the source root. If it's not found, the
       secondary source root will be checked (which would contain a parallel
       directory hierarchy).
 
       This behavior is intended to be used when BUILD.gn files can't be checked
       in to certain source directories for whatever reason.
 
       The secondary source root must be inside the main source tree.
 
+  arg_file_template [optional]

[brettw, 2016/11/22 16:14:32]

These parameters are alphabetical. Can you put this at the top?

[Dirk Pranke, 2016/11/22 18:41:52]

Done. I thought I checked for that but clearly misread something.

+      Label of a file that contains the text that should be used as the

[brettw, 2016/11/22 16:14:32]

Label -> File name (labels are used for targets not files).

[Dirk Pranke, 2016/11/22 18:41:52]

Changed "Label" to "Path to", instead, and fixed buildconfig to do the same (it
also said label, which is where I got this from).

+      default args.gn file when you run `gn args`.
+
 Example .gn file contents
 
   buildconfig = "//build/config/BUILDCONFIG.gn"
 
   check_targets = [
     "//doom_melon/*",  # Check everything in this subtree.
     "//tools:mind_controlling_ant",  # Check this specific target.
   ]
 
   root = "//:root"
@@ skipping 612 matching lines @@
       }
       whitelist->insert(current_dir.ResolveRelativeFile(item, &err));
       if (err.has_error()) {
         err.PrintToStdout();
         return false;
       }
     }
     build_settings_.set_exec_script_whitelist(std::move(whitelist));
   }
 
+  const Value* arg_file_template_value =
+    dotfile_scope_.GetValue("arg_file_template", true);

[brettw, 2016/11/22 16:14:32]

Indent more.

[Dirk Pranke, 2016/11/22 18:41:52]

Done.

+  if (arg_file_template_value) {
+    if (!arg_file_template_value->VerifyTypeIs(Value::STRING, &err)) {
+      err.PrintToStdout();
+      return false;
+    }
+    SourceFile path(arg_file_template_value->string_value());
+    if (!base::PathExists(path.Resolve(build_settings_.root_path()))) {

[brettw, 2016/11/22 16:14:32]

I don't think we should be doing filesystem operations at this point. This will
almost never be used and you already have some error handling when it is. This
code is always called for every run.

[Dirk Pranke, 2016/11/22 18:41:52]

Okay. I moved the error handling code.

+      err = Err(Location(), arg_file_template_value->string_value() +
+                std::string(" does not exist."));
+      err.PrintToStdout();
+      return false;
+    }
+    build_settings_.set_arg_file_template_path(path);
+  }
+
   return true;
 }