atexit: implement OnError for cancelling multpart uploads

lib/atexit/atexit.go

@@ -77,3 +77,23 @@ func Run() {
 		}
 	})
 }
+
+// OnError registers fn with atexit and returns a function which
+// runs fn() if *perr != nil and deregisters fn
+//
+// It should be used in a defer statement normally so
+//
+//     defer OnError(&err, cancelFunc)()
+//
+// So cancelFunc will be run if the function exits with an error or
+// at exit.
+func OnError(perr *error, fn func()) func() {
+	handle := Register(fn)
+	return func() {
+		defer Unregister(handle)
+		if *perr != nil {
+			fn()
+		}
+	}
+
+}