github.com/cenkalti/backoff/v5 v5.0.3: WithNotify

Clean-room public code sample — generation instructions

Write a brand-new, minimal, self-contained code sample in this clean-room directory.
Do not copy, paraphrase, or reference any existing project source. Work only from this spec.

A csx.json manifest scaffold already exists. Do not recreate it from memory. Preserve its case.goal, packages and symbols; fill its empty case.contract with exact assertions and correct its environment, commands and verifierAdapter for the files you generate.

Goal: verify github.com/cenkalti/backoff/v5.WithNotify in pkg:golang/github.com/cenkalti/backoff/v5@v5.0.3
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:golang/github.com/cenkalti/backoff/v5@v5.0.3
Demonstrate these symbols/APIs:
  - github.com/cenkalti/backoff/v5.WithNotify

Rules:
  - One focused purpose; the smallest project that proves the goal.
  - Include a contract test (test/contract.*) that runs OFFLINE and exits 0 exactly when the goal behavior works.
  - Pin every dependency with a lockfile so resolution is reproducible.
  - No secrets, credentials, or tokens. No real URLs (only example.com or localhost). No absolute paths.
  - No personal names, emails, company names, or project identifiers of any kind.
  - No binaries and no generated output (node_modules, dist, target, venv, .git, .env).
  - Keep it under 200 files and 256KB packed.

{"case":{"caseId":"case:sha256:6741d729e104b3a24f48813ef2f8261233888940837162397e8aa11491c3a9cc","contract":["WithNotify callback is invoked on each retryable failure with the error and backoff duration before the next retry","WithNotify callback receives the exact error returned by the failed operation attempt","WithNotify is not invoked when an operation succeeds on the initial attempt","WithNotify is not invoked when an operation returns a Permanent error on the initial attempt","WithNotify callback receives the computed delay duration matching the configured backoff policy"],"goal":"verify github.com/cenkalti/backoff/v5.WithNotify in pkg:golang/github.com/cenkalti/backoff/v5@v5.0.3","kind":"HOW","packages":["pkg:golang/github.com/cenkalti/backoff/v5@v5.0.3"],"schemaVersion":1,"symbols":["github.com/cenkalti/backoff/v5.WithNotify"]},"contractCommand":["go","run","./test"],"environment":{"arch":"x64","distro":"ubuntu","ecosystem":"golang","libc":"glibc","libcVersion":"2.39","os":"linux","osVersionBucket":"24","packageManager":"go","schemaVersion":1},"license":"MIT-0","packages":["pkg:golang/github.com/cenkalti/backoff/v5@v5.0.3"],"schemaVersion":1,"subject":"pkg:golang/github.com/cenkalti/backoff/v5@v5.0.3","symbols":["github.com/cenkalti/backoff/v5.WithNotify"],"verifierAdapter":"golang@1"}

module example.com/sample

go 1.24.0

require github.com/cenkalti/backoff/v5 v5.0.3

github.com/cenkalti/backoff/v5 v5.0.3 h1:ZN+IMa753KfX5hd8vVaMixjnqRZ3y8CuJKRKj1xcsSM=
github.com/cenkalti/backoff/v5 v5.0.3/go.mod h1:rkhZdG3JZukswDf7f0cwqPNk4K0sa+F97BxZthm/crw=

package main

import (
	"context"
	"fmt"
	"time"

	"github.com/cenkalti/backoff/v5"
)

func main() {
	ctx := context.Background()
	attempts := 0

	operation := func() (string, error) {
		attempts++
		if attempts < 3 {
			return "", fmt.Errorf("temporary network timeout (attempt %d)", attempts)
		}
		return "connected successfully", nil
	}

	result, err := backoff.Retry(
		ctx,
		operation,
		backoff.WithBackOff(&backoff.ZeroBackOff{}),
		backoff.WithMaxTries(5),
		backoff.WithNotify(func(err error, d time.Duration) {
			fmt.Printf("[Notify] Operation failed: %v, retrying after %v\n", err, d)
		}),
	)
	if err != nil {
		fmt.Printf("Operation failed: %v\n", err)
		return
	}

	fmt.Printf("Result: %s (after %d attempts)\n", result, attempts)
}

{
  "schemaVersion": 1,
  "goal": "verify github.com/cenkalti/backoff/v5.WithNotify in pkg:golang/github.com/cenkalti/backoff/v5@v5.0.3",
  "kind": "HOW",
  "packages": [
    "pkg:golang/github.com/cenkalti/backoff/v5@v5.0.3"
  ],
  "symbols": [
    "github.com/cenkalti/backoff/v5.WithNotify"
  ]
}

package main

import (
	"context"
	"errors"
	"fmt"
	"os"
	"time"

	"github.com/cenkalti/backoff/v5"
)

type fixedBackOff struct {
	delay time.Duration
}

func (b *fixedBackOff) NextBackOff() time.Duration {
	return b.delay
}

func (b *fixedBackOff) Reset() {}

func main() {
	ctx := context.Background()

	// Assertion 1: WithNotify callback is invoked on each retryable failure with the error and backoff duration before the next retry
	{
		var notifiedErrors []error
		var notifiedDurations []time.Duration
		attempts := 0
		errTransient := errors.New("transient error")

		res, err := backoff.Retry(
			ctx,
			func() (string, error) {
				attempts++
				if attempts < 3 {
					return "", errTransient
				}
				return "success", nil
			},
			backoff.WithBackOff(&backoff.ZeroBackOff{}),
			backoff.WithNotify(func(err error, d time.Duration) {
				notifiedErrors = append(notifiedErrors, err)
				notifiedDurations = append(notifiedDurations, d)
			}),
		)

		if err != nil {
			fmt.Fprintf(os.Stderr, "Assertion 1 failed: unexpected error: %v\n", err)
			os.Exit(1)
		}
		if res != "success" {
			fmt.Fprintf(os.Stderr, "Assertion 1 failed: expected res 'success', got %q\n", res)
			os.Exit(1)
		}
		if attempts != 3 {
			fmt.Fprintf(os.Stderr, "Assertion 1 failed: expected 3 attempts, got %d\n", attempts)
			os.Exit(1)
		}
		if len(notifiedErrors) != 2 {
			fmt.Fprintf(os.Stderr, "Assertion 1 failed: expected 2 notifications, got %d\n", len(notifiedErrors))
			os.Exit(1)
		}
		for i, nErr := range notifiedErrors {
			if !errors.Is(nErr, errTransient) {
				fmt.Fprintf(os.Stderr, "Assertion 1 failed: notification %d error mismatch: %v\n", i, nErr)
				os.Exit(1)
			}
		}
	}

	// Assertion 2: WithNotify callback receives the exact error returned by the failed operation attempt
	{
		var notifiedMessages []string
		attempts := 0

		_, _ = backoff.Retry(
			ctx,
			func() (int, error) {
				attempts++
				return 0, fmt.Errorf("error on attempt %d", attempts)
			},
			backoff.WithBackOff(&backoff.ZeroBackOff{}),
			backoff.WithMaxTries(3),
			backoff.WithNotify(func(err error, d time.Duration) {
				notifiedMessages = append(notifiedMessages, err.Error())
			}),
		)

		if len(notifiedMessages) != 2 {
			fmt.Fprintf(os.Stderr, "Assertion 2 failed: expected 2 notifications, got %d\n", len(notifiedMessages))
			os.Exit(1)
		}
		if notifiedMessages[0] != "error on attempt 1" {
			fmt.Fprintf(os.Stderr, "Assertion 2 failed: expected 'error on attempt 1', got %q\n", notifiedMessages[0])
			os.Exit(1)
		}
		if notifiedMessages[1] != "error on attempt 2" {
			fmt.Fprintf(os.Stderr, "Assertion 2 failed: expected 'error on attempt 2', got %q\n", notifiedMessages[1])
			os.Exit(1)
		}
	}

	// Assertion 3: WithNotify is not invoked when an operation succeeds on the initial attempt
	{
		notifyCalled := false
		res, err := backoff.Retry(
			ctx,
			func() (string, error) {
				return "immediate success", nil
			},
			backoff.WithBackOff(&backoff.ZeroBackOff{}),
			backoff.WithNotify(func(err error, d time.Duration) {
				notifyCalled = true
			}),
		)

		if err != nil {
			fmt.Fprintf(os.Stderr, "Assertion 3 failed: unexpected error: %v\n", err)
			os.Exit(1)
		}
		if res != "immediate success" {
			fmt.Fprintf(os.Stderr, "Assertion 3 failed: expected 'immediate success', got %q\n", res)
			os.Exit(1)
		}
		if notifyCalled {
			fmt.Fprintf(os.Stderr, "Assertion 3 failed: notify was called on immediate success\n")
			os.Exit(1)
		}
	}

	// Assertion 4: WithNotify is not invoked when an operation returns a Permanent error on the initial attempt
	{
		notifyCalled := false
		permErr := errors.New("permanent error")

		_, err := backoff.Retry(
			ctx,
			func() (int, error) {
				return 0, backoff.Permanent(permErr)
			},
			backoff.WithBackOff(&backoff.ZeroBackOff{}),
			backoff.WithMaxTries(3),
			backoff.WithNotify(func(err error, d time.Duration) {
				notifyCalled = true
			}),
		)

		if !errors.Is(err, permErr) {
			fmt.Fprintf(os.Stderr, "Assertion 4 failed: expected permanent error, got %v\n", err)
			os.Exit(1)
		}
		if notifyCalled {
			fmt.Fprintf(os.Stderr, "Assertion 4 failed: notify was called on Permanent error\n")
			os.Exit(1)
		}
	}

	// Assertion 5: WithNotify callback receives the computed delay duration matching the configured backoff policy
	{
		expectedDelay := 10 * time.Millisecond
		var receivedDelays []time.Duration
		attempts := 0

		_, _ = backoff.Retry(
			ctx,
			func() (int, error) {
				attempts++
				if attempts < 3 {
					return 0, errors.New("fail")
				}
				return 100, nil
			},
			backoff.WithBackOff(&fixedBackOff{delay: expectedDelay}),
			backoff.WithNotify(func(err error, d time.Duration) {
				receivedDelays = append(receivedDelays, d)
			}),
		)

		if len(receivedDelays) != 2 {
			fmt.Fprintf(os.Stderr, "Assertion 5 failed: expected 2 notifications, got %d\n", len(receivedDelays))
			os.Exit(1)
		}
		for i, d := range receivedDelays {
			if d != expectedDelay {
				fmt.Fprintf(os.Stderr, "Assertion 5 failed: delay %d was %v, expected %v\n", i, d, expectedDelay)
				os.Exit(1)
			}
		}
	}

	fmt.Println("Contract assertions passed.")
}