Require build directories exist for some GN commands.

tools/gn/setup.h

 // 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.
 
 #ifndef TOOLS_GN_SETUP_H_
 #define TOOLS_GN_SETUP_H_
 
 #include <vector>
 
 #include "base/basictypes.h"
@@ skipping 80 matching lines @@
  public:
   Setup();
   virtual ~Setup();
 
   // Configures the build for the current command line. On success returns
   // true. On failure, prints the error and returns false.
   //
   // The parameter is the string the user specified for the build directory. We
   // will try to interpret this as a SourceDir if possible, and will fail if is
   // is malformed.
-  bool DoSetup(const std::string& build_dir);
+  //
+  // With force_create = false, setup will fail if the build directory doesn't
+  // alreay exist with an args file in it. With force_create set to true, the
+  // directory will be created if necessary. Commands explicitly doing
+  // generation should set this to true to create it, but querying commands
+  // should set it to false to prevent creating oddly-named directories in case
+  // the user omits the build directory argument (which is easy to do).
+  bool DoSetup(const std::string& build_dir, bool force_create);
 
   // Runs the load, returning true on success. On failure, prints the error
   // and returns false. This includes both RunPreMessageLoop() and
   // RunPostMessageLoop().
   bool Run();
 
   Scheduler& scheduler() { return scheduler_; }
 
   virtual Scheduler* GetScheduler() OVERRIDE;
 
@@ skipping 20 matching lines @@
   bool FillArgsFromArgsInputFile();
 
   // Writes the build arguments to the build arg file.
   bool SaveArgsToFile();
 
   // Fills the root directory into the settings. Returns true on success.
   bool FillSourceDir(const base::CommandLine& cmdline);
 
   // Fills the build directory given the value the user has specified.
   // Must happen after FillSourceDir so we can resolve source-relative
-  // paths.
-  bool FillBuildDir(const std::string& build_dir);
+  // paths. If require_exists is false, it will fail if the dir doesn't exist.
+  bool FillBuildDir(const std::string& build_dir, bool require_exists);
 
   // Fills the python path portion of the command line. On failure, sets
   // it to just "python".
   void FillPythonPath();
 
   // Run config file.
   bool RunConfigFile();
 
   bool FillOtherConfig(const base::CommandLine& cmdline);
 
@@ skipping 48 matching lines @@
   void RunPreMessageLoop();
   bool RunPostMessageLoop();
 
   virtual Scheduler* GetScheduler() OVERRIDE;
 
  private:
   Scheduler* scheduler_;
 };
 
 #endif  // TOOLS_GN_SETUP_H_