Support length operation without opening file.

runtime/bin/file_win.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "bin/file.h"
 
 #include <fcntl.h>
 #include <io.h>
 #include <stdio.h>
 #include <string.h>
@@ skipping 72 matching lines @@
 
 
 bool File::Flush() {
   ASSERT(handle_->fd());
   return _commit(handle_->fd()) != -1;
 }
 
 
 off_t File::Length() {
   ASSERT(handle_->fd() >= 0);
-  off_t position = lseek(handle_->fd(), 0, SEEK_CUR);
-  if (position < 0) {
-    // The file is not capable of seeking. Return an error.
-    return -1;
+  struct stat st;
+  if (fstat(handle_->fd(), &st) == 0) {
+    return st.st_size;
   }
-  off_t result = lseek(handle_->fd(), 0, SEEK_END);
-  lseek(handle_->fd(), position, SEEK_SET);
-  return result;
+  return -1;
 }
 
 
 File* File::Open(const char* name, FileOpenMode mode) {
   int flags = O_RDONLY | O_BINARY;
   if ((mode & kWrite) != 0) {
     flags = (O_RDWR | O_CREAT | O_BINARY);
   }
   if ((mode & kTruncate) != 0) {
     flags = flags | O_TRUNC;
@@ skipping 39 matching lines @@
 
 bool File::Delete(const char* name) {
   int status = remove(name);
   if (status == -1) {
     return false;
   }
   return true;
 }
 
 
+off_t File::LengthFromName(const char* name) {
+  struct stat st;
+  if (stat(name, &st) == 0) {
+    return st.st_size;
+  }
+  return -1;
+}
+
+
 bool File::IsAbsolutePath(const char* pathname) {
   // Should we consider network paths?
   if (pathname == NULL) return false;
   return (strlen(pathname) > 2) &&
       (pathname[1] == ':') &&
       (pathname[2] == '\\' || pathname[2] == '/');
 }
 
 
 char* File::GetCanonicalPath(const char* pathname) {
@@ skipping 42 matching lines @@
 const char* File::StringEscapedPathSeparator() {
   return "\\\\";
 }
 
 
 File::StdioHandleType File::GetStdioHandleType(int fd) {
   // Treat all stdio handles as pipes. The Windows event handler and
   // socket code will handle the different handle types.
   return kPipe;
 }