Require build directories exist for some GN commands.

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>
@@ skipping 207 matching lines @@
       base::Bind(&ItemDefinedCallback, scheduler_.main_loop(), builder_));
 
   // The scheduler's main loop wasn't created when the Loader was created, so
   // we need to set it now.
   loader_->set_main_loop(scheduler_.main_loop());
 }
 
 Setup::~Setup() {
 }
 
-bool Setup::DoSetup(const std::string& build_dir) {
+bool Setup::DoSetup(const std::string& build_dir, bool force_create) {
   CommandLine* cmdline = CommandLine::ForCurrentProcess();
 
   scheduler_.set_verbose_logging(cmdline->HasSwitch(kSwitchVerbose));
   if (cmdline->HasSwitch(kTimeSwitch) ||
       cmdline->HasSwitch(kTracelogSwitch))
     EnableTracing();
 
   ScopedTrace setup_trace(TraceItem::TRACE_SETUP, "DoSetup");
 
   if (!FillSourceDir(*cmdline))
     return false;
   if (!RunConfigFile())
     return false;
   if (!FillOtherConfig(*cmdline))
     return false;
-  if (!FillBuildDir(build_dir))  // Must be after FillSourceDir to resolve.
+
+  // Must be after FillSourceDir to resolve.
+  if (!FillBuildDir(build_dir, !force_create))
     return false;
+
   if (fill_arguments_) {
     if (!FillArguments(*cmdline))
       return false;
   }
   FillPythonPath();
 
   return true;
 }
 
 bool Setup::Run() {
@@ skipping 175 matching lines @@
     root_path = dotfile_name_.DirName();
   }
 
   if (scheduler_.verbose_logging())
     scheduler_.Log("Using source root", FilePathToUTF8(root_path));
   build_settings_.SetRootPath(root_path);
 
   return true;
 }
 
-bool Setup::FillBuildDir(const std::string& build_dir) {
+bool Setup::FillBuildDir(const std::string& build_dir, bool require_exists) {
   SourceDir resolved =
       SourceDirForCurrentDirectory(build_settings_.root_path()).
           ResolveRelativeDir(build_dir);
   if (resolved.is_null()) {
     Err(Location(), "Couldn't resolve build directory.",
         "The build directory supplied (\"" + build_dir + "\") was not valid.").
         PrintToStdout();
     return false;
   }
 
   if (scheduler_.verbose_logging())
     scheduler_.Log("Using build dir", resolved.value());
+
+  if (require_exists) {
+    base::FilePath build_dir_path = build_settings_.GetFullPath(resolved);
+    if (!base::PathExists(build_dir_path.Append(
+            FILE_PATH_LITERAL("build.ninja")))) {
+      Err(Location(), "Not a build directory.",
+          "This command requires an existing build directory. I interpreted "
+          "your input\n\"" + build_dir + "\" as:\n  " +
+          FilePathToUTF8(build_dir_path) +
+          "\nwhich doesn't seem to contain a previously-generated build.")
+          .PrintToStdout();
+      return false;
+    }
+  }
+
   build_settings_.SetBuildDir(resolved);
   return true;
 }
 
 void Setup::FillPythonPath() {
   // Trace this since it tends to be a bit slow on Windows.
   ScopedTrace setup_trace(TraceItem::TRACE_SETUP, "Fill Python Path");
 #if defined(OS_WIN)
   // Find Python on the path so we can use the absolute path in the build.
   const base::char16 kGetPython[] =
@@ skipping 125 matching lines @@
 }
 
 void DependentSetup::RunPreMessageLoop() {
   CommonSetup::RunPreMessageLoop();
 }
 
 bool DependentSetup::RunPostMessageLoop() {
   return CommonSetup::RunPostMessageLoop();
 }