golang.org/x/sync v0.0.0-20190423024810-112230192c58: 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@v0.0.0-20190423024810-112230192c58
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:golang/golang.org/x/sync@v0.0.0-20190423024810-112230192c58
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:sha256:7a81faf08cec55026d52ae9e091e498077aa94732f765b03d7105eadfedc83d9","contract":["errgroup.WithContext returns a non-nil *errgroup.Group and a derived context.Context that preserves values from the parent context","errgroup.WithContext runs concurrent subtasks registered via Group.Go, returns nil from Group.Wait when all subtasks return nil, and cancels the derived context when Group.Wait returns","errgroup.WithContext cancels the derived context when any subtask returns a non-nil error and Group.Wait returns that first non-nil error","Canceling the parent context passed to errgroup.WithContext propagates cancellation to the derived context"],"goal":"verify golang.org/x/sync/errgroup.WithContext in pkg:golang/golang.org/x/sync@v0.0.0-20190423024810-112230192c58","kind":"HOW","packages":["pkg:golang/golang.org/x/sync@v0.0.0-20190423024810-112230192c58"],"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@v0.0.0-20190423024810-112230192c58"],"schemaVersion":1,"subject":"pkg:golang/golang.org/x/sync@v0.0.0-20190423024810-112230192c58","symbols":["golang.org/x/sync/errgroup.WithContext"],"verifierAdapter":"golang@1"}

module example.com/sample

go 1.21

require golang.org/x/sync v0.0.0-20190423024810-112230192c58

golang.org/x/sync v0.0.0-20190423024810-112230192c58 h1:8gQV6CLnAEikrhgkHFbMAEhagSSnXWGV915qUMm9mrU=
golang.org/x/sync v0.0.0-20190423024810-112230192c58/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=

{
  "schemaVersion": 1,
  "goal": "verify golang.org/x/sync/errgroup.WithContext in pkg:golang/golang.org/x/sync@v0.0.0-20190423024810-112230192c58",
  "kind": "HOW",
  "packages": [
    "pkg:golang/golang.org/x/sync@v0.0.0-20190423024810-112230192c58"
  ],
  "symbols": [
    "golang.org/x/sync/errgroup.WithContext"
  ]
}

package main

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

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

func assert(cond bool, msg string, args ...any) {
	if !cond {
		fmt.Fprintf(os.Stderr, "ASSERTION FAILED: "+msg+"\n", args...)
		os.Exit(1)
	}
}

func main() {
	// 1. errgroup.WithContext returns a non-nil *errgroup.Group and a derived context that preserves parent values.
	type keyType string
	parentWithVal := context.WithValue(context.Background(), keyType("reqID"), "req-42")
	grp1, ctx1 := errgroup.WithContext(parentWithVal)
	assert(grp1 != nil, "WithContext must return a non-nil *errgroup.Group")
	assert(ctx1 != nil, "WithContext must return a non-nil derived context.Context")
	assert(ctx1.Value(keyType("reqID")) == "req-42", "derived context must preserve parent context values")

	// 2. Concurrent subtasks scheduled with Go complete when returning nil, Wait returns nil, and Wait cancels the derived context.
	var total int64
	for i := int64(1); i <= 4; i++ {
		delta := i * 10
		grp1.Go(func() error {
			atomic.AddInt64(&total, delta)
			return nil
		})
	}
	err := grp1.Wait()
	assert(err == nil, "Wait must return nil when all subtasks return nil, got %v", err)
	assert(atomic.LoadInt64(&total) == 100, "all concurrent subtasks must execute before Wait returns, got total=%d", total)
	select {
	case <-ctx1.Done():
		assert(errors.Is(ctx1.Err(), context.Canceled), "derived context Err after Wait must be context.Canceled, got %v", ctx1.Err())
	default:
		assert(false, "derived context must be canceled when Wait returns")
	}

	// 3. The first non-nil error returned by a subtask cancels the derived context and is returned by Wait.
	sentinelErr := errors.New("subtask failed")
	grp2, ctx2 := errgroup.WithContext(context.Background())
	started := make(chan struct{})
	var peerCanceled int32

	grp2.Go(func() error {
		close(started)
		<-ctx2.Done()
		if errors.Is(ctx2.Err(), context.Canceled) {
			atomic.StoreInt32(&peerCanceled, 1)
		}
		return ctx2.Err()
	})
	grp2.Go(func() error {
		<-started
		return sentinelErr
	})

	err = grp2.Wait()
	assert(errors.Is(err, sentinelErr), "Wait must return the first non-nil subtask error (%v), got %v", sentinelErr, err)
	assert(atomic.LoadInt32(&peerCanceled) == 1, "sibling goroutine must observe derived context cancellation upon error")

	// 4. Canceling the parent context cancels the derived context returned by WithContext.
	parentCancelable, cancelParent := context.WithCancel(context.Background())
	grp3, ctx3 := errgroup.WithContext(parentCancelable)
	grp3.Go(func() error {
		<-ctx3.Done()
		return ctx3.Err()
	})
	cancelParent()
	err = grp3.Wait()
	assert(errors.Is(err, context.Canceled), "Wait must return context.Canceled when parent context is canceled, got %v", err)
}