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// Copyright 2015 The LUCI Authors.
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
package retry
import (
// Stop is a sentinel value returned by a Iterator to indicate that no more
// attempts should be made.
const Stop time.Duration = -1
// Callback is a callback function that Retry will invoke every time an
// attempt fails prior to sleeping.
type Callback func(error, time.Duration)
// LogCallback builds a Callback which logs a Warning with the opname, error
// and delay.
func LogCallback(c context.Context, opname string) Callback {
return func(err error, delay time.Duration) {
logging.ErrorKey: err,
"opname": opname,
"delay": delay,
}.Warningf(c, "operation failed transiently")
// Iterator describes a stateful implementation of retry logic.
type Iterator interface {
// Returns the next retry delay, or Stop if no more retries should be made.
Next(context.Context, error) time.Duration
// Factory is a function that produces an independent Iterator instance.
// Since each Iterator is mutated as it is iterated through, this is used to
// produce a fresh Iterator for a new round of retries. Unless the caller is
// fully aware of what they're doing, this should not return the an Iterator
// instance more than once.
type Factory func() Iterator
// Retry executes a function 'fn'. If the function returns an error, it will
// be re-executed according to a retry plan.
// If a Factory is supplied, it will be called to generate a single retry
// Iterator for this Retry round. If nil, Retry will execute the target function
// exactly once regardless of return value.
// If the supplied context is canceled, retry will stop executing. Retry will
// not execute the supplied function at all if the context is canceled when
// Retry is invoked.
// If 'callback' is not nil, it will be invoked if an error occurs (prior to
// sleeping).
func Retry(ctx context.Context, f Factory, fn func() error, callback Callback) (err error) {
var it Iterator
if f != nil {
it = f()
timer := clock.NewTimer(ctx)
defer timer.Stop()
for {
// If we've been cancelled, don't call fn.
if err := ctx.Err(); err != nil {
return err
// Execute the function.
err = fn()
if err == nil || it == nil {
// If we've been cancelled, don't call Next or callback.
if ctx.Err() != nil {
return err
delay := it.Next(ctx, err)
if delay == Stop {
// Notify our observer that we are retrying.
if callback != nil {
callback(err, delay)
if delay > 0 {
if tr := <-timer.GetC(); tr.Incomplete() {
// Context was canceled.
return tr.Err