golang.org/x/sync v0.21.0: singleflight.Group

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/singleflight.Group 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/singleflight.Group

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:sha256:23a096ba01bba7abc98f1285e5751a1140b27ea1dae3a32859682dd91d11144d","contract":["singleflight.Group.Do suppresses duplicate concurrent calls for the same key and returns shared result","singleflight.Group.Do returns function error and marks shared true for concurrent callers","singleflight.Group.DoChan returns channel delivering Result struct with Val, Err and Shared flags","singleflight.Group.Forget removes active key so subsequent calls execute function again"],"goal":"verify golang.org/x/sync/singleflight.Group 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/singleflight.Group"]},"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/singleflight.Group"],"verifierAdapter":"golang@1"}

module sample

go 1.25.0

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 sample

import (
	"sync/atomic"

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

// GroupCoordinator wraps singleflight.Group to coordinate duplicate execution suppression.
type GroupCoordinator struct {
	group singleflight.Group
	calls atomic.Int64
}

// NewGroupCoordinator returns a new GroupCoordinator instance.
func NewGroupCoordinator() *GroupCoordinator {
	return &GroupCoordinator{}
}

// Group returns the underlying singleflight.Group instance.
func (c *GroupCoordinator) Group() *singleflight.Group {
	return &c.group
}

// Executions returns the total count of executed underlying worker functions.
func (c *GroupCoordinator) Executions() int64 {
	return c.calls.Load()
}

// Do wraps singleflight.Group.Do, tracking execution invocations.
func (c *GroupCoordinator) Do(key string, fn func() (any, error)) (any, error, bool) {
	return c.group.Do(key, func() (any, error) {
		c.calls.Add(1)
		return fn()
	})
}

// DoChan wraps singleflight.Group.DoChan, returning a channel of singleflight.Result.
func (c *GroupCoordinator) DoChan(key string, fn func() (any, error)) <-chan singleflight.Result {
	return c.group.DoChan(key, func() (any, error) {
		c.calls.Add(1)
		return fn()
	})
}

// Forget delegates to singleflight.Group.Forget to invalidate an in-flight key.
func (c *GroupCoordinator) Forget(key string) {
	c.group.Forget(key)
}

{
  "schemaVersion": 1,
  "goal": "verify golang.org/x/sync/singleflight.Group 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/singleflight.Group"
  ]
}

package main

import (
	"errors"
	"fmt"
	"os"
	"sync"
	"time"

	"sample"
)

func main() {
	coordinator := sample.NewGroupCoordinator()

	// Assertion 1: singleflight.Group.Do suppresses duplicate concurrent calls for the same key and returns shared result
	{
		const callers = 10
		entered := make(chan struct{})
		release := make(chan struct{})
		results := make([]any, callers)
		errs := make([]error, callers)
		shareds := make([]bool, callers)
		var wg sync.WaitGroup

		for i := 0; i < callers; i++ {
			wg.Add(1)
			go func(idx int) {
				defer wg.Done()
				v, err, shared := coordinator.Do("shared-key", func() (any, error) {
					close(entered)
					<-release
					return "computed-data", nil
				})
				results[idx] = v
				errs[idx] = err
				shareds[idx] = shared
			}(i)
		}

		<-entered
		time.Sleep(25 * time.Millisecond)
		close(release)
		wg.Wait()

		for i := 0; i < callers; i++ {
			if results[i] != "computed-data" {
				fmt.Fprintf(os.Stderr, "FAIL: caller %d expected result 'computed-data', got %v\n", i, results[i])
				os.Exit(1)
			}
			if errs[i] != nil {
				fmt.Fprintf(os.Stderr, "FAIL: caller %d expected nil error, got %v\n", i, errs[i])
				os.Exit(1)
			}
		}

		sharedCount := 0
		for _, s := range shareds {
			if s {
				sharedCount++
			}
		}
		if sharedCount == 0 {
			fmt.Fprintf(os.Stderr, "FAIL: expected duplicate callers to have shared=true, got 0\n")
			os.Exit(1)
		}

		if coordinator.Executions() != 1 {
			fmt.Fprintf(os.Stderr, "FAIL: expected 1 execution for shared key, got %d\n", coordinator.Executions())
			os.Exit(1)
		}
	}

	// Assertion 2: singleflight.Group.Do returns function error and marks shared true for concurrent callers
	{
		const errCallers = 5
		enteredErr := make(chan struct{})
		releaseErr := make(chan struct{})
		expectedErr := errors.New("simulated upstream failure")
		errs := make([]error, errCallers)
		shareds := make([]bool, errCallers)
		var wg sync.WaitGroup

		for i := 0; i < errCallers; i++ {
			wg.Add(1)
			go func(idx int) {
				defer wg.Done()
				_, err, shared := coordinator.Do("error-key", func() (any, error) {
					close(enteredErr)
					<-releaseErr
					return nil, expectedErr
				})
				errs[idx] = err
				shareds[idx] = shared
			}(i)
		}

		<-enteredErr
		time.Sleep(25 * time.Millisecond)
		close(releaseErr)
		wg.Wait()

		for i := 0; i < errCallers; i++ {
			if !errors.Is(errs[i], expectedErr) {
				fmt.Fprintf(os.Stderr, "FAIL: caller %d expected error %v, got %v\n", i, expectedErr, errs[i])
				os.Exit(1)
			}
		}

		if coordinator.Executions() != 2 {
			fmt.Fprintf(os.Stderr, "FAIL: expected 2 total executions, got %d\n", coordinator.Executions())
			os.Exit(1)
		}
	}

	// Assertion 3: singleflight.Group.DoChan returns channel delivering Result struct with Val, Err and Shared flags
	{
		ch := coordinator.DoChan("chan-key", func() (any, error) {
			return "async-data", nil
		})

		select {
		case res := <-ch:
			if res.Val != "async-data" {
				fmt.Fprintf(os.Stderr, "FAIL: expected Result.Val 'async-data', got %v\n", res.Val)
				os.Exit(1)
			}
			if res.Err != nil {
				fmt.Fprintf(os.Stderr, "FAIL: expected Result.Err nil, got %v\n", res.Err)
				os.Exit(1)
			}
		case <-time.After(2 * time.Second):
			fmt.Fprintf(os.Stderr, "FAIL: timed out waiting on DoChan result channel\n")
			os.Exit(1)
		}

		if coordinator.Executions() != 3 {
			fmt.Fprintf(os.Stderr, "FAIL: expected 3 total executions, got %d\n", coordinator.Executions())
			os.Exit(1)
		}
	}

	// Assertion 4: singleflight.Group.Forget removes active key so subsequent calls execute function again
	{
		enteredForget := make(chan struct{})
		releaseForget1 := make(chan struct{})
		releaseForget2 := make(chan struct{})

		var wg sync.WaitGroup
		wg.Add(1)
		go func() {
			defer wg.Done()
			_, _, _ = coordinator.Do("forget-key", func() (any, error) {
				close(enteredForget)
				<-releaseForget1
				return "first", nil
			})
		}()

		<-enteredForget
		// In-flight call is blocked. Forget active key.
		coordinator.Forget("forget-key")

		secondStarted := make(chan struct{})
		wg.Add(1)
		go func() {
			defer wg.Done()
			_, _, _ = coordinator.Do("forget-key", func() (any, error) {
				close(secondStarted)
				<-releaseForget2
				return "second", nil
			})
		}()

		select {
		case <-secondStarted:
			// Second call started execution independently because the key was forgotten
		case <-time.After(2 * time.Second):
			fmt.Fprintf(os.Stderr, "FAIL: second call did not start independently after Forget\n")
			os.Exit(1)
		}

		close(releaseForget1)
		close(releaseForget2)
		wg.Wait()

		if coordinator.Executions() != 5 {
			fmt.Fprintf(os.Stderr, "FAIL: expected 5 total executions, got %d\n", coordinator.Executions())
			os.Exit(1)
		}
	}

	fmt.Println("PASS: all singleflight.Group contract assertions passed")
}