golang.org/x/sync v0.21.0

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@v0.21.0
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:golang/golang.org/x/sync@v0.21.0

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:sha256:203843b4dee2dba9cece56fb74805650607f7d16cad1871930b7f2bf03b8419e","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@v0.21.0","kind":"HOW","packages":["pkg:golang/golang.org/x/sync@v0.21.0"],"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@v0.21.0"],"schemaVersion":1,"subject":"pkg:golang/golang.org/x/sync@v0.21.0","verifierAdapter":"golang@1"}

module example.com/sample

go 1.26.6

require golang.org/x/sync v0.21.0

golang.org/x/sync v0.21.0 h1:HLII4xRRTtCRkxYp4HNFF0Js/Og6q2i++KXbg0gHCwM=
golang.org/x/sync v0.21.0/go.mod h1:9xrNwdLfx4jkKbNva9FpL6vEN7evnE43NNNJQ2LF3+0=

package main

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

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

// TaskRunner demonstrates concurrent task execution with limit and context cancellation using errgroup.
type TaskRunner struct {
	limit int
}

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

// RunTasks executes tasks concurrently up to the configured limit.
func (r *TaskRunner) RunTasks(ctx context.Context, tasks []func(context.Context) error) error {
	g, gCtx := errgroup.WithContext(ctx)
	if r.limit > 0 {
		g.SetLimit(r.limit)
	}

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

	return g.Wait()
}

// ResourcePool manages weighted resource tokens using a semaphore.
type ResourcePool struct {
	sem *semaphore.Weighted
}

// NewResourcePool initializes a pool with a maximum token capacity.
func NewResourcePool(capacity int64) *ResourcePool {
	return &ResourcePool{sem: semaphore.NewWeighted(capacity)}
}

// AcquireTokens acquires n tokens from the pool, blocking until available or context canceled.
func (p *ResourcePool) AcquireTokens(ctx context.Context, n int64) error {
	return p.sem.Acquire(ctx, n)
}

// TryAcquireTokens attempts to acquire n tokens without blocking.
func (p *ResourcePool) TryAcquireTokens(n int64) bool {
	return p.sem.TryAcquire(n)
}

// ReleaseTokens releases n tokens back to the pool.
func (p *ResourcePool) ReleaseTokens(n int64) {
	p.sem.Release(n)
}

// CacheLoader deduplicates concurrent requests for the same key using singleflight.
type CacheLoader struct {
	group singleflight.Group
}

// Load executes the fn for the given key, suppressing duplicate concurrent executions.
func (c *CacheLoader) Load(key string, fn func() (any, error)) (any, error, bool) {
	v, err, shared := c.group.Do(key, fn)
	return v, err, shared
}

func main() {
	runner := NewTaskRunner(2)
	var count int32

	err := runner.RunTasks(context.Background(), []func(context.Context) error{
		func(ctx context.Context) error {
			atomic.AddInt32(&count, 1)
			return nil
		},
		func(ctx context.Context) error {
			atomic.AddInt32(&count, 1)
			return nil
		},
	})

	if err != nil {
		fmt.Printf("Task execution error: %v\n", err)
		return
	}

	pool := NewResourcePool(3)
	if pool.TryAcquireTokens(2) {
		fmt.Println("Acquired 2 tokens successfully")
		pool.ReleaseTokens(2)
	}

	loader := &CacheLoader{}
	val, _, shared := loader.Load("test-key", func() (any, error) {
		return "computed-value", nil
	})
	fmt.Printf("Loaded value: %v (shared: %v), total completed tasks: %d\n", val, shared, count)
}

{
  "schemaVersion": 1,
  "goal": "verify pkg:golang/golang.org/x/sync@v0.21.0",
  "kind": "HOW",
  "packages": [
    "pkg:golang/golang.org/x/sync@v0.21.0"
  ]
}

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")
}