Add directory extraction to GN path handling.

tools/gn/filesystem_utils.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/filesystem_utils.h"
 
 #include <algorithm>
 
 #include "base/file_util.h"
 #include "base/logging.h"
@@ skipping 702 matching lines @@
 }
 
 SourceDir GetOutputDirForSourceDir(const Settings* settings,
                                    const SourceDir& source_dir) {
   SourceDir toolchain = GetToolchainOutputDir(settings);
 
   std::string ret;
   toolchain.SwapValue(&ret);
   ret.append("obj/");
 
-  // The source dir should be source-absolute, so we trim off the two leading
-  // slashes to append to the toolchain object directory.
-  DCHECK(source_dir.is_source_absolute());
-  ret.append(&source_dir.value()[2], source_dir.value().size() - 2);
+  if (source_dir.is_source_absolute()) {
+    // The source dir is source-absolute, so we trim off the two leading
+    // slashes to append to the toolchain object directory.
+    ret.append(&source_dir.value()[2], source_dir.value().size() - 2);
+  }
+  // (Put system-absolute stuff in the root obj directory.)
 
   return SourceDir(SourceDir::SWAP_IN, &ret);
 }
 
 SourceDir GetGenDirForSourceDir(const Settings* settings,
                                 const SourceDir& source_dir) {
   SourceDir toolchain = GetToolchainGenDir(settings);
 
   std::string ret;
   toolchain.SwapValue(&ret);
 
-  // The source dir should be source-absolute, so we trim off the two leading
-  // slashes to append to the toolchain object directory.
-  DCHECK(source_dir.is_source_absolute());
-  ret.append(&source_dir.value()[2], source_dir.value().size() - 2);
+  if (source_dir.is_source_absolute()) {
+    // The source dir should be source-absolute, so we trim off the two leading
+    // slashes to append to the toolchain object directory.
+    DCHECK(source_dir.is_source_absolute());
+    ret.append(&source_dir.value()[2], source_dir.value().size() - 2);
+  }
+  // (Put system-absolute stuff in the root gen directory.)
 
   return SourceDir(SourceDir::SWAP_IN, &ret);
 }
 
 SourceDir GetTargetOutputDir(const Target* target) {
   return GetOutputDirForSourceDir(target->settings(), target->label().dir());
 }
 
 SourceDir GetTargetGenDir(const Target* target) {
   return GetGenDirForSourceDir(target->settings(), target->label().dir());
 }
 
 SourceDir GetCurrentOutputDir(const Scope* scope) {
   return GetOutputDirForSourceDir(scope->settings(), scope->GetSourceDir());
 }
 
 SourceDir GetCurrentGenDir(const Scope* scope) {
   return GetGenDirForSourceDir(scope->settings(), scope->GetSourceDir());
 }