Support length operation without opening file.

runtime/bin/file_macos.cc

Index: runtime/bin/file_macos.cc
diff --git a/runtime/bin/file_macos.cc b/runtime/bin/file_macos.cc
index e491f34f900f39d2e27c3d962bb96a844a72d7cf..98d93e617ce6fc335b592a22f0d1b4d01b2e71ae 100644
--- a/runtime/bin/file_macos.cc
+++ b/runtime/bin/file_macos.cc
@@ -94,14 +94,11 @@ bool File::Flush() {
 
 off_t File::Length() {
   ASSERT(handle_->fd() >= 0);
-  off_t position = TEMP_FAILURE_RETRY(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 (TEMP_FAILURE_RETRY(fstat(handle_->fd(), &st)) == 0) {
+    return st.st_size;
   }
-  off_t result = TEMP_FAILURE_RETRY(lseek(handle_->fd(), 0, SEEK_END));
-  TEMP_FAILURE_RETRY(lseek(handle_->fd(), position, SEEK_SET));
-  return result;
+  return -1;
 }
 
 
@@ -169,6 +166,15 @@ bool File::Delete(const char* name) {
 }
 
 
+off_t File::LengthFromName(const char* name) {
+  struct stat st;
+  if (TEMP_FAILURE_RETRY(stat(name, &st)) == 0) {
+    return st.st_size;
+  }
+  return -1;
+}
+
+
 bool File::IsAbsolutePath(const char* pathname) {
   return (pathname != NULL && pathname[0] == '/');
 }