golang.org/x/sync v0.17.0

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 pkg:golang/golang.org/x/sync@v0.17.0
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:golang/golang.org/x/sync@v0.17.0

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:sha256:82eddf6e18460bad3708d1bfbdbed5b0536adf408a79a9c2c21e64e68faba31c","contract":["errgroup.WithContext cancels derived context and returns error on first subtask failure","errgroup.Group with SetLimit limits maximum concurrent task executions","semaphore.NewWeighted manages weighted access with TryAcquire, Acquire, and Release","singleflight.Group deduplicates concurrent executions for identical keys"],"goal":"verify pkg:golang/golang.org/x/sync@v0.17.0","kind":"HOW","packages":["pkg:golang/golang.org/x/sync@v0.17.0"],"schemaVersion":1},"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@v0.17.0"],"schemaVersion":1,"verifierAdapter":"golang@1"}

module example.com/sample

go 1.26.6

require golang.org/x/sync v0.17.0

golang.org/x/sync v0.17.0 h1:l60nONMj9l5drqw6jlhIELNv9I0A4OFgRsG9k2oT9Ug=
golang.org/x/sync v0.17.0/go.mod h1:9KTHXmSnoGruLpwFjVSX0lNNA75CykiMECbovNTZqGI=

package main

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

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

// TaskRunner coordinates concurrent tasks with concurrency limits using errgroup.
type TaskRunner struct {
	concurrency int
}

// NewTaskRunner creates a TaskRunner with the given concurrency limit.
func NewTaskRunner(concurrency int) *TaskRunner {
	return &TaskRunner{concurrency: concurrency}
}

// Run executes tasks concurrently, returning the first error encountered if any.
func (r *TaskRunner) Run(ctx context.Context, tasks []func(context.Context) error) error {
	g, gCtx := errgroup.WithContext(ctx)
	if r.concurrency > 0 {
		g.SetLimit(r.concurrency)
	}

	for _, task := range tasks {
		t := task
		g.Go(func() error {
			return t(gCtx)
		})
	}

	return g.Wait()
}

// ResourceLimiter manages access to weighted resources using semaphore.Weighted.
type ResourceLimiter struct {
	sem *semaphore.Weighted
}

// NewResourceLimiter creates a limiter with max capacity.
func NewResourceLimiter(capacity int64) *ResourceLimiter {
	return &ResourceLimiter{sem: semaphore.NewWeighted(capacity)}
}

// Acquire requests weight from the limiter, blocking until available or context canceled.
func (l *ResourceLimiter) Acquire(ctx context.Context, weight int64) error {
	return l.sem.Acquire(ctx, weight)
}

// TryAcquire attempts non-blocking acquisition of weight.
func (l *ResourceLimiter) TryAcquire(weight int64) bool {
	return l.sem.TryAcquire(weight)
}

// Release returns weight to the limiter.
func (l *ResourceLimiter) Release(weight int64) {
	l.sem.Release(weight)
}

// WorkDeduplicator deduplicates concurrent executions for matching keys using singleflight.Group.
type WorkDeduplicator struct {
	group singleflight.Group
}

// Do executes fn for the key or awaits an existing in-flight execution.
func (d *WorkDeduplicator) Do(key string, fn func() (any, error)) (any, error, bool) {
	return d.group.Do(key, fn)
}

func main() {
	runner := NewTaskRunner(3)
	var processed int32

	err := runner.Run(context.Background(), []func(context.Context) error{
		func(ctx context.Context) error {
			atomic.AddInt32(&processed, 1)
			return nil
		},
		func(ctx context.Context) error {
			atomic.AddInt32(&processed, 1)
			return nil
		},
	})
	if err != nil {
		fmt.Printf("runner error: %v\n", err)
		return
	}

	limiter := NewResourceLimiter(5)
	if limiter.TryAcquire(2) {
		fmt.Println("Acquired 2 units")
		limiter.Release(2)
	}

	dedup := &WorkDeduplicator{}
	val, _, shared := dedup.Do("item-key", func() (any, error) {
		return "computed-result", nil
	})

	fmt.Printf("Result: %v, Shared: %v, Processed tasks: %d\n", val, shared, processed)
}

{
  "schemaVersion": 1,
  "goal": "verify pkg:golang/golang.org/x/sync@v0.17.0",
  "kind": "HOW",
  "packages": [
    "pkg:golang/golang.org/x/sync@v0.17.0"
  ]
}

package main

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

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

func main() {
	// Assertion 1: errgroup.WithContext cancels derived context and returns error on first subtask failure
	{
		expectedErr := errors.New("simulated failure")
		g, ctx := errgroup.WithContext(context.Background())

		g.Go(func() error {
			return expectedErr
		})

		g.Go(func() error {
			select {
			case <-ctx.Done():
				return nil
			case <-time.After(2 * time.Second):
				return errors.New("timeout waiting for context cancellation")
			}
		})

		err := g.Wait()
		if err == nil || !errors.Is(err, expectedErr) {
			fmt.Fprintf(os.Stderr, "FAIL: expected error %v, got %v\n", expectedErr, err)
			os.Exit(1)
		}
	}

	// Assertion 2: errgroup.Group with SetLimit limits maximum concurrent task executions
	{
		var g errgroup.Group
		limit := 2
		g.SetLimit(limit)

		var currentActive int32
		var maxActive int32
		var completedTasks int32
		totalTasks := 6

		for i := 0; i < totalTasks; i++ {
			g.Go(func() error {
				curr := atomic.AddInt32(&currentActive, 1)
				for {
					max := atomic.LoadInt32(&maxActive)
					if curr > max {
						if atomic.CompareAndSwapInt32(&maxActive, max, curr) {
							break
						}
					} else {
						break
					}
				}

				time.Sleep(10 * time.Millisecond)
				atomic.AddInt32(&currentActive, -1)
				atomic.AddInt32(&completedTasks, 1)
				return nil
			})
		}

		if err := g.Wait(); err != nil {
			fmt.Fprintf(os.Stderr, "FAIL: errgroup wait error: %v\n", err)
			os.Exit(1)
		}

		if atomic.LoadInt32(&completedTasks) != int32(totalTasks) {
			fmt.Fprintf(os.Stderr, "FAIL: expected %d completed tasks, got %d\n", totalTasks, completedTasks)
			os.Exit(1)
		}

		if atomic.LoadInt32(&maxActive) > int32(limit) {
			fmt.Fprintf(os.Stderr, "FAIL: max active goroutines (%d) exceeded limit (%d)\n", maxActive, limit)
			os.Exit(1)
		}
	}

	// Assertion 3: semaphore.NewWeighted manages weighted access with TryAcquire, Acquire, and Release
	{
		sem := semaphore.NewWeighted(3)

		if !sem.TryAcquire(2) {
			fmt.Fprintf(os.Stderr, "FAIL: TryAcquire(2) failed on empty 3-weight semaphore\n")
			os.Exit(1)
		}

		if sem.TryAcquire(2) {
			fmt.Fprintf(os.Stderr, "FAIL: TryAcquire(2) should fail when only 1 weight is available\n")
			os.Exit(1)
		}

		sem.Release(2)

		ctx, cancel := context.WithTimeout(context.Background(), time.Second)
		defer cancel()

		if err := sem.Acquire(ctx, 3); err != nil {
			fmt.Fprintf(os.Stderr, "FAIL: Acquire(3) failed: %v\n", err)
			os.Exit(1)
		}
		sem.Release(3)
	}

	// Assertion 4: singleflight.Group deduplicates concurrent executions for identical keys
	{
		var sf singleflight.Group
		var callCount int32
		var wg sync.WaitGroup
		concurrency := 5
		results := make([]string, concurrency)

		barrier := make(chan struct{})

		for i := 0; i < concurrency; i++ {
			wg.Add(1)
			idx := i
			go func() {
				defer wg.Done()
				<-barrier
				v, err, _ := sf.Do("shared-key", func() (any, error) {
					atomic.AddInt32(&callCount, 1)
					time.Sleep(20 * time.Millisecond)
					return "shared-value", nil
				})
				if err != nil {
					return
				}
				results[idx] = v.(string)
			}()
		}

		close(barrier)
		wg.Wait()

		calls := atomic.LoadInt32(&callCount)
		if calls != 1 {
			fmt.Fprintf(os.Stderr, "FAIL: singleflight executed function %d times, expected 1\n", calls)
			os.Exit(1)
		}

		for i, res := range results {
			if res != "shared-value" {
				fmt.Fprintf(os.Stderr, "FAIL: result[%d] = %q, expected %q\n", i, res, "shared-value")
				os.Exit(1)
			}
		}
	}

	fmt.Println("PASS: golang.org/x/sync contract passed")
}