golang.org/x/sync v0.0.0-20200625203802-6e8e738ad208: errgroup.WithContext

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 golang.org/x/sync/errgroup.WithContext in pkg:golang/golang.org/x/sync@v0.0.0-20200625203802-6e8e738ad208
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:golang/golang.org/x/sync@v0.0.0-20200625203802-6e8e738ad208
Demonstrate these symbols/APIs:
  - golang.org/x/sync/errgroup.WithContext

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:f665235edaea9dc8793fd8471c992ef25442269196929fcdd2e46d54576cc585","contract":["errgroup.WithContext returns a non-nil *errgroup.Group and a derived context.Context that inherits values from the parent context","errgroup.WithContext runs concurrent functions registered via g.Go, returns nil from g.Wait when all return nil, and cancels the derived context when g.Wait returns","errgroup.WithContext cancels the derived context when any goroutine returns a non-nil error and g.Wait returns the first non-nil error","Canceling the parent context passed to errgroup.WithContext cancels the derived context"],"goal":"verify golang.org/x/sync/errgroup.WithContext in pkg:golang/golang.org/x/sync@v0.0.0-20200625203802-6e8e738ad208","kind":"HOW","packages":["pkg:golang/golang.org/x/sync@v0.0.0-20200625203802-6e8e738ad208"],"schemaVersion":1,"symbols":["golang.org/x/sync/errgroup.WithContext"]},"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/golang.org/x/sync@v0.0.0-20200625203802-6e8e738ad208"],"schemaVersion":1,"subject":"pkg:golang/golang.org/x/sync@v0.0.0-20200625203802-6e8e738ad208","symbols":["golang.org/x/sync/errgroup.WithContext"],"verifierAdapter":"golang@1"}

module example.com/sample

go 1.21

require golang.org/x/sync v0.0.0-20200625203802-6e8e738ad208

golang.org/x/sync v0.0.0-20200625203802-6e8e738ad208 h1:qwRHBd0NqMbJxfbotnDhm2ByMI1Shq4Y6oRJo21SGJA=
golang.org/x/sync v0.0.0-20200625203802-6e8e738ad208/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=

{
  "schemaVersion": 1,
  "goal": "verify golang.org/x/sync/errgroup.WithContext in pkg:golang/golang.org/x/sync@v0.0.0-20200625203802-6e8e738ad208",
  "kind": "HOW",
  "packages": [
    "pkg:golang/golang.org/x/sync@v0.0.0-20200625203802-6e8e738ad208"
  ],
  "symbols": [
    "golang.org/x/sync/errgroup.WithContext"
  ]
}

package main

import (
	"context"
	"errors"
	"fmt"
	"os"
	"sync/atomic"

	"golang.org/x/sync/errgroup"
)

func fail(msg string, args ...any) {
	fmt.Fprintf(os.Stderr, "FAIL: "+msg+"\n", args...)
	os.Exit(1)
}

func main() {
	// 1. errgroup.WithContext returns a non-nil *errgroup.Group and a derived context.Context.
	type ctxKey string
	parentCtx := context.WithValue(context.Background(), ctxKey("trace"), "ok")
	g1, ctx1 := errgroup.WithContext(parentCtx)
	if g1 == nil {
		fail("expected non-nil *errgroup.Group from WithContext")
	}
	if ctx1 == nil {
		fail("expected non-nil derived context from WithContext")
	}
	if got := ctx1.Value(ctxKey("trace")); got != "ok" {
		fail("expected derived context to inherit parent value, got %v", got)
	}

	// 2. Successful goroutines complete, g.Wait() returns nil, and Wait() cancels the derived context on return.
	var sum int32
	for i := int32(1); i <= 3; i++ {
		val := i
		g1.Go(func() error {
			atomic.AddInt32(&sum, val)
			return nil
		})
	}
	if err := g1.Wait(); err != nil {
		fail("expected nil error from Wait when all tasks succeed, got %v", err)
	}
	if got := atomic.LoadInt32(&sum); got != 6 {
		fail("expected sum 6 from concurrent tasks, got %d", got)
	}
	select {
	case <-ctx1.Done():
	default:
		fail("expected derived context to be canceled after Wait returns")
	}

	// 3. A non-nil error from a goroutine cancels the derived context and is returned by Wait().
	expectedErr := errors.New("worker failure")
	g2, ctx2 := errgroup.WithContext(context.Background())
	ready := make(chan struct{})
	var canceledObserved int32

	g2.Go(func() error {
		close(ready)
		<-ctx2.Done()
		if errors.Is(ctx2.Err(), context.Canceled) {
			atomic.StoreInt32(&canceledObserved, 1)
		}
		return ctx2.Err()
	})
	g2.Go(func() error {
		<-ready
		return expectedErr
	})

	if err := g2.Wait(); !errors.Is(err, expectedErr) {
		fail("expected Wait to return %v, got %v", expectedErr, err)
	}
	if atomic.LoadInt32(&canceledObserved) != 1 {
		fail("expected sibling goroutine to observe context cancellation")
	}

	// 4. Canceling the parent context propagates cancellation to the derived context.
	cancelParent, cancel := context.WithCancel(context.Background())
	g3, ctx3 := errgroup.WithContext(cancelParent)
	g3.Go(func() error {
		<-ctx3.Done()
		return ctx3.Err()
	})
	cancel()
	if err := g3.Wait(); !errors.Is(err, context.Canceled) {
		fail("expected Wait to return context.Canceled after parent cancel, got %v", err)
	}

	fmt.Println("PASS")
}