golang.org/x/sync v0.21.0: semaphore.Weighted

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/semaphore.Weighted in 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
Demonstrate these symbols/APIs:
  - golang.org/x/sync/semaphore.Weighted

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:sha256:1021db8ef6aba49c8f36dd3be50158f205dceb542596e6ed447497c66c9efd71","contract":["semaphore.NewWeighted creates a weighted semaphore with specified capacity","semaphore.Weighted.TryAcquire acquires available weight without blocking","semaphore.Weighted.TryAcquire returns false when requested weight exceeds capacity","semaphore.Weighted.Release restores capacity allowing subsequent Acquire to succeed","semaphore.Weighted.Acquire acquires weight when capacity is available","semaphore.Weighted.Acquire returns context cancellation error when context is cancelled"],"goal":"verify golang.org/x/sync/semaphore.Weighted in pkg:golang/golang.org/x/sync@v0.21.0","kind":"HOW","packages":["pkg:golang/golang.org/x/sync@v0.21.0"],"schemaVersion":1,"symbols":["golang.org/x/sync/semaphore.Weighted"]},"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","symbols":["golang.org/x/sync/semaphore.Weighted"],"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"
	"os"

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

func main() {
	ctx := context.Background()
	sem := semaphore.NewWeighted(2)

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

	if !sem.TryAcquire(1) {
		fmt.Fprintf(os.Stderr, "failed to try-acquire remaining slot\n")
		os.Exit(1)
	}
	defer sem.Release(1)

	fmt.Println("semaphore.Weighted acquired and released successfully")
}

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

package main

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

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

func main() {
	if err := runTests(); err != nil {
		fmt.Fprintf(os.Stderr, "Contract test failed: %v\n", err)
		os.Exit(1)
	}
	fmt.Println("All contract assertions passed.")
}

func runTests() error {
	// 1. semaphore.NewWeighted creates a weighted semaphore with specified capacity
	sem := semaphore.NewWeighted(5)
	if sem == nil {
		return fmt.Errorf("semaphore.NewWeighted returned nil")
	}

	// 2. semaphore.Weighted.TryAcquire acquires available weight without blocking
	if !sem.TryAcquire(3) {
		return fmt.Errorf("expected TryAcquire(3) to succeed on capacity 5")
	}

	// 3. semaphore.Weighted.TryAcquire returns false when requested weight exceeds capacity
	if sem.TryAcquire(3) {
		return fmt.Errorf("expected TryAcquire(3) to fail when only 2 remains")
	}
	if !sem.TryAcquire(2) {
		return fmt.Errorf("expected TryAcquire(2) to succeed with exact remaining capacity")
	}
	if sem.TryAcquire(1) {
		return fmt.Errorf("expected TryAcquire(1) to fail on fully saturated semaphore")
	}

	// 4. semaphore.Weighted.Release restores capacity allowing subsequent Acquire to succeed
	sem.Release(2)
	if !sem.TryAcquire(2) {
		return fmt.Errorf("expected TryAcquire(2) to succeed after releasing 2")
	}
	sem.Release(5) // release all 5 held (3 + 2)

	// 5. semaphore.Weighted.Acquire acquires weight when capacity is available
	ctx := context.Background()
	if err := sem.Acquire(ctx, 5); err != nil {
		return fmt.Errorf("Acquire(5) failed on empty semaphore: %w", err)
	}

	// 6. semaphore.Weighted.Acquire returns context cancellation error when context is cancelled
	cancelCtx, cancel := context.WithCancel(context.Background())
	cancel() // cancel immediately
	err := sem.Acquire(cancelCtx, 1)
	if err == nil {
		return fmt.Errorf("expected Acquire to fail on cancelled context, but succeeded")
	}
	if !errors.Is(err, context.Canceled) {
		return fmt.Errorf("expected context.Canceled error, got: %w", err)
	}

	// Clean up acquired weight
	sem.Release(5)

	return nil
}