test-golang-ForgetUnshared

main.go

// You can edit this code!
// Click here and start typing.
package main

import (
	"context"
	"fmt"
	"math/rand"
	"sync"
	"time"
)


// start ----------------------- copy from src/internal/singleflight/singleflight.go ------------------------

// call is an in-flight or completed singleflight.Do call
type call struct {
	wg sync.WaitGroup

	// These fields are written once before the WaitGroup is done
	// and are only read after the WaitGroup is done.
	val interface{}
	err error

	// These fields are read and written with the singleflight
	// mutex held before the WaitGroup is done, and are read but
	// not written after the WaitGroup is done.
	dups  int
	chans []chan<- Result
}

// Group represents a class of work and forms a namespace in
// which units of work can be executed with duplicate suppression.
type Group struct {
	mu sync.Mutex       // protects m
	m  map[string]*call // lazily initialized
}

// Result holds the results of Do, so they can be passed
// on a channel.
type Result struct {
	Val    interface{}
	Err    error
	Shared bool
}

// Do executes and returns the results of the given function, making
// sure that only one execution is in-flight for a given key at a
// time. If a duplicate comes in, the duplicate caller waits for the
// original to complete and receives the same results.
// The return value shared indicates whether v was given to multiple callers.
func (g *Group) Do(key string, fn func() (interface{}, error)) (v interface{}, err error, shared bool) {
	g.mu.Lock()
	if g.m == nil {
		g.m = make(map[string]*call)
	}
	if c, ok := g.m[key]; ok {
		c.dups++
		g.mu.Unlock()
		c.wg.Wait()
		return c.val, c.err, true
	}
	c := new(call)
	c.wg.Add(1)
	g.m[key] = c
	g.mu.Unlock()

	g.doCall(c, key, fn)
	return c.val, c.err, c.dups > 0
}

// DoChan is like Do but returns a channel that will receive the
// results when they are ready. The second result is true if the function
// will eventually be called, false if it will not (because there is
// a pending request with this key).
func (g *Group) DoChan(key string, fn func() (interface{}, error)) (<-chan Result, bool) {
	ch := make(chan Result, 1)
	g.mu.Lock()
	if g.m == nil {
		g.m = make(map[string]*call)
	}
	if c, ok := g.m[key]; ok {
		c.dups++
		c.chans = append(c.chans, ch)
		g.mu.Unlock()
		return ch, false
	}
	c := &call{chans: []chan<- Result{ch}}
	c.wg.Add(1)
	g.m[key] = c
	g.mu.Unlock()

	go g.doCall(c, key, fn)

	return ch, true
}

// doCall handles the single call for a key.
func (g *Group) doCall(c *call, key string, fn func() (interface{}, error)) {
	c.val, c.err = fn()
	c.wg.Done()

	g.mu.Lock()
	delete(g.m, key)
	for _, ch := range c.chans {
		ch <- Result{c.val, c.err, c.dups > 0}
	}
	g.mu.Unlock()
}

// ForgetUnshared tells the singleflight to forget about a key if it is not
// shared with any other goroutines. Future calls to Do for a forgotten key
// will call the function rather than waiting for an earlier call to complete.
// Returns whether the key was forgotten or unknown--that is, whether no
// other goroutines are waiting for the result.
func (g *Group) ForgetUnshared(key string) bool {
	g.mu.Lock()
	defer g.mu.Unlock()
	c, ok := g.m[key]
	if !ok {
		return true
	}
	if c.dups == 0 {
		delete(g.m, key)
		return true
	}
	return false
}
// end ----------------------- copy from src/internal/singleflight/singleflight.go ------------------------


func main() {
	var g Group
	wg := sync.WaitGroup{}
	key := "aa"

	for i := 0; i <= 1000000; i++ {
		ctx, cancel := context.WithCancel(context.Background())
		uuid := rand.Intn(10000000000)
		ctx = context.WithValue(ctx, "uuid", uuid)
		wg.Add(1)

		go func(ctx context.Context) {
			defer wg.Done()

			ch, _ := g.DoChan(key, func() (interface{}, error) {
				time.Sleep(randTimeout())
				if ctx.Err() == context.Canceled {
					return nil, fmt.Errorf("callUUID=[%d] err=[%s]", uuid, ctx.Err())
				}
				return "right", nil
			})

			// randomly choose a timeout to cancel
			cancelTime := time.After(randTimeout())
			select {
			case <-cancelTime:
				// cancel only if no other goroutines share
				if g.ForgetUnshared(key) {
					cancel()
				}

			case res := <-ch:
				if res.Err != nil {
					// program should never get here!!!
					panic(fmt.Sprintf("%s currentUUId=[%d]", res.Err, uuid))
				}
			}
		}(ctx)
	}

	wg.Wait()
}

func randTimeout() time.Duration {
	max := 100
	min := 1
	return time.Duration(rand.Intn(max-min)+min) * time.Millisecond
}

If ForgetUnshared returned as expected, this code should not panic, but it did.

Is there a bug in the ForgetUnshared method? Or am I misunderstanding ForgetUnshared?