Allow overriding script executable in .gn file

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 80 matching lines @@
         exec_script_whitelist = [
           "//base/BUILD.gn",
           "//build/my_config.gni",
         ]
 
   root [optional]
       Label of the root build target. The GN build will start by loading the
       build file containing this target name. This defaults to "//:" which will
       cause the file //BUILD.gn to be loaded.
 
+  script_executable [optional]
+      Path to specific Python executable or potentially a different language
+      interpreter that is used to execute scripts in action targets and
+      exec_script calls.
+
   secondary_source [optional]
       Label of an alternate directory tree to find input files. When searching
       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.
 
@@ skipping 196 matching lines @@
     return false;
   if (!RunConfigFile())
     return false;
   if (!FillOtherConfig(*cmdline))
     return false;
 
   // Must be after FillSourceDir to resolve.
   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);
+  if (!FillPythonPath(*cmdline))
+    return false;
+
+  // Check for unused variables in the .gn file.
+  Err err;
+  if (!dotfile_scope_.CheckForUnusedVars(&err)) {
+    err.PrintToStdout();
+    return false;
+  }
 
   return true;
 }
 
 bool Setup::Run() {
   RunPreMessageLoop();
   if (!scheduler_.Run())
     return false;
   return RunPostMessageLoop();
 }
@@ skipping 272 matching lines @@
           "\nwhich doesn't seem to contain a previously-generated build.")
           .PrintToStdout();
       return false;
     }
   }
 
   build_settings_.SetBuildDir(resolved);
   return true;
 }
 
-void Setup::FillPythonPath(const base::CommandLine& cmdline) {
+bool Setup::FillPythonPath(const base::CommandLine& cmdline) {
   // Trace this since it tends to be a bit slow on Windows.
   ScopedTrace setup_trace(TraceItem::TRACE_SETUP, "Fill Python Path");
+  const Value* value = dotfile_scope_.GetValue("script_executable", true);
   if (cmdline.HasSwitch(switches::kScriptExecutable)) {
     build_settings_.set_python_path(
         cmdline.GetSwitchValuePath(switches::kScriptExecutable));
+  } else if (value) {
+    Err err;
+    if (!value->VerifyTypeIs(Value::STRING, &err)) {
+      err.PrintToStdout();
+      return false;
+    }
+    build_settings_.set_python_path(
+        base::FilePath(UTF8ToFilePath(value->string_value())));
   } else {
 #if defined(OS_WIN)
     base::FilePath python_path = FindWindowsPython();
     if (python_path.empty()) {
       scheduler_.Log("WARNING", "Could not find python on path, using "
           "just \"python.exe\"");
       python_path = base::FilePath(kPythonExeName);
     }
     build_settings_.set_python_path(python_path.NormalizePathSeparatorsTo('/'));
 #else
     build_settings_.set_python_path(base::FilePath("python"));
 #endif
   }
+  return true;
 }
 
 bool Setup::RunConfigFile() {
   if (scheduler_.verbose_logging())
     scheduler_.Log("Got dotfile", FilePathToUTF8(dotfile_name_));
 
   dotfile_input_file_.reset(new InputFile(SourceFile("//.gn")));
   if (!dotfile_input_file_->Load(dotfile_name_)) {
     Err(Location(), "Could not load dotfile.",
         "The file \"" + FilePathToUTF8(dotfile_name_) + "\" couldn't be loaded")
@@ skipping 129 matching lines @@
     if (!arg_file_template_value->VerifyTypeIs(Value::STRING, &err)) {
       err.PrintToStdout();
       return false;
     }
     SourceFile path(arg_file_template_value->string_value());
     build_settings_.set_arg_file_template_path(path);
   }
 
   return true;
 }