change backup to use os.Link

renderer/renderer.go

@@ -163,12 +163,10 @@ func AtomicWrite(path string, createDestDirs bool, contents []byte, perms os.Fil
 	}
 
 	// If we got this far, it means we are about to save the file. Copy the
-	// current contents of the file onto disk (if it exists) so we have a backup.
+	// current file so we have a backup. Note that os.Link preserves the Mode.
 	if backup {
-		if _, err := os.Stat(path); !os.IsNotExist(err) {
-			if err := copyFile(path, path+".bak"); err != nil {
-				return err
-			}
+		if err := os.Link(path, path+".bak"); err != nil {
+			log.Printf("[WARN] (runner) could not backup %q: %v", path, err)
 		}
 	}
 
@@ -178,33 +176,3 @@ func AtomicWrite(path string, createDestDirs bool, contents []byte, perms os.Fil
 
 	return nil
 }
-
-// copyFile copies the file at src to the path at dst. Any errors that occur
-// are returned.
-func copyFile(src, dst string) error {
-	s, err := os.Open(src)
-	if err != nil {
-		return err
-	}
-	defer s.Close()
-
-	stat, err := s.Stat()
-	if err != nil {
-		return err
-	}
-
-	d, err := os.OpenFile(dst, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, stat.Mode())
-	if err != nil {
-		return err
-	}
-	if _, err := io.Copy(d, s); err != nil {
-		d.Close()
-		return err
-	}
-	if err := d.Close(); err != nil {
-		return err
-	}
-
-	// io.Copy can restrict file permissions based on umask.
-	return os.Chmod(dst, stat.Mode())
-}