GN: Add optional default_args variable to .gn.

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.
 
+  default_args [optional]
+      Scope containing the default overrides for declared arguments. These
+      overrides take precedence over the default values specified in the
+      declare_args() block, but can be overriden using --args or the
+      args.gn file.
+
+      This is intended to be used when subprojects declare arguments with
+      default values that need to be changed for whatever reason.
+
 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"
 
   secondary_source = "//build/config/temporary_buildfiles/"
+
+  default_args = {
+    # Default to release builds for this project.
+    is_debug = false
+    is_component_build = false
+  }
 )";
 
 namespace {
 
 const base::FilePath::CharType kGnFile[] = FILE_PATH_LITERAL(".gn");
 
 base::FilePath FindDotFile(const base::FilePath& current_dir) {
   base::FilePath try_this_file = current_dir.Append(kGnFile);
   if (base::PathExists(try_this_file))
     return try_this_file;
@@ skipping 119 matching lines @@
 const char Setup::kBuildArgFileName[] = "args.gn";
 
 Setup::Setup()
     : build_settings_(),
       loader_(new LoaderImpl(&build_settings_)),
       builder_(loader_.get()),
       root_build_file_("//BUILD.gn"),
       check_public_headers_(false),
       dotfile_settings_(&build_settings_, std::string()),
       dotfile_scope_(&dotfile_settings_),
+      default_args_(nullptr),
       fill_arguments_(true) {
   dotfile_settings_.set_toolchain_label(Label());
 
   build_settings_.set_item_defined_callback(
       base::Bind(&ItemDefinedCallback, scheduler_.task_runner(), &builder_));
 
   loader_->set_complete_callback(base::Bind(&DecrementWorkCount));
   // The scheduler's task runner wasn't created when the Loader was created, so
   // we need to set it now.
   loader_->set_task_runner(scheduler_.task_runner());
@@ skipping 23 matching lines @@
   if (!FillBuildDir(build_dir, !force_create))
     return false;
 
   // Check for unused variables in the .gn file.
   Err err;
   if (!dotfile_scope_.CheckForUnusedVars(&err)) {
     err.PrintToStdout();
     return false;
   }
 
+  // Apply project-specific default (if specified).
+  // Must happen before FillArguments().
+  if (default_args_) {
+    Scope::KeyValueMap overrides;
+    default_args_->GetCurrentScopeValues(&overrides);
+    build_settings_.build_args().AddArgOverrides(overrides);
+  }
+
   if (fill_arguments_) {
     if (!FillArguments(*cmdline))
       return false;
   }
   FillPythonPath(*cmdline);
 
   return true;
 }
 
 bool Setup::Run() {
@@ skipping 418 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));
   }
 
+  // Fill optional default_args.
+  const Value* default_args_value =
+      dotfile_scope_.GetValue("default_args", true);
+  if (default_args_value) {
+    if (!default_args_value->VerifyTypeIs(Value::SCOPE, &err)) {
+      err.PrintToStdout();
+      return false;
+    }
+
+    default_args_ = default_args_value->scope_value();
+  }
+
   return true;
 }