Absolute path fixes for gn.

tools/gn/source_dir.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/source_dir.h"
 
 #include "base/logging.h"
 #include "tools/gn/filesystem_utils.h"
 #include "tools/gn/source_file.h"
 
@@ skipping 64 matching lines @@
     NormalizePath(&ret.value_);
     return ret;
   }
 
   if (!source_root.empty()) {
     std::string absolute =
         FilePathToUTF8(Resolve(UTF8ToFilePath(source_root)).AppendASCII(
             p.as_string()).value());
     NormalizePath(&absolute);
     if (!MakeAbsolutePathRelativeIfPossible(source_root, absolute,
-                                            &ret.value_))
-      ret.value_ = absolute;
+                                            &ret.value_)) {
+#if defined(OS_WIN)
+      // On Windows we'll accept "C:\foo" as an absolute path, which we want
+      // to convert to "/C:..." here.
+      if (absolute[0] != '/')
+        ret.value_ = "/";
+#endif
+      ret.value_.append(absolute.data(), absolute.size());
+    }
     return ret;
   }
 
   // With no source_root_, there's nothing we can do about
   // e.g. p=../../../path/to/file and value_=//source and we'll
   // errornously return //file.
   ret.value_.reserve(value_.size() + p.size());
   ret.value_.assign(value_);
   ret.value_.append(p.data(), p.size());
 
@@ skipping 30 matching lines @@
     if (!EndsWithSlash(ret.value_))
       ret.value_.push_back('/');
     return ret;
   }
 
   if (!source_root.empty()) {
     std::string absolute =
         FilePathToUTF8(Resolve(UTF8ToFilePath(source_root)).AppendASCII(
             p.as_string()).value());
     NormalizePath(&absolute);
-    if (!MakeAbsolutePathRelativeIfPossible(source_root, absolute, &ret.value_))
-      ret.value_ = absolute;
+    if (!MakeAbsolutePathRelativeIfPossible(source_root, absolute,
+                                            &ret.value_)) {
+#if defined(OS_WIN)
+      if (absolute[0] != '/')  // See the file case for why we do this check.
+        ret.value_ = "/";
+#endif
+      ret.value_.append(absolute.data(), absolute.size());
+    }
     if (!EndsWithSlash(ret.value_))
       ret.value_.push_back('/');
     return ret;
   }
 
   ret.value_.reserve(value_.size() + p.size());
   ret.value_.assign(value_);
   ret.value_.append(p.data(), p.size());
 
   NormalizePath(&ret.value_);
@@ skipping 22 matching lines @@
   // String the double-leading slash for source-relative paths.
   converted.assign(&value_[2], value_.size() - 2);
   return source_root.Append(UTF8ToFilePath(converted))
       .NormalizePathSeparatorsTo('/');
 }
 
 void SourceDir::SwapValue(std::string* v) {
   value_.swap(*v);
   AssertValueSourceDirString(value_);
 }