Support length operation without opening file.

runtime/bin/file_linux.cc

Index: runtime/bin/file_linux.cc
diff --git a/runtime/bin/file_linux.cc b/runtime/bin/file_linux.cc
index f04234fbea7a6ef9e3743ebf2324052456605c62..03dd50a1077758f4cf2f0058dbe35640f819bf23 100644
--- a/runtime/bin/file_linux.cc
+++ b/runtime/bin/file_linux.cc
@@ -93,14 +93,11 @@ 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;
 }
 
 
@@ -168,6 +165,15 @@ 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] == '/');
 }