ZDDC/zddc/internal/apps/singleflight.go

package apps

import "sync"

// singleflightGroup deduplicates concurrent calls keyed by string. If N
// goroutines call Do(key, fn) before the first one returns, fn runs once
// and all callers receive the same (val, err).
//
// Hand-rolled to avoid pulling in golang.org/x/sync — we only need the
// 30-line core, not Forget/DoChan. Pattern is the standard one.
type singleflightGroup struct {
	mu sync.Mutex
	m  map[string]*sfCall
}

type sfCall struct {
	done chan struct{}
	val  any
	err  error
}

func (g *singleflightGroup) Do(key string, fn func() (any, error)) (any, error) {
	g.mu.Lock()
	if g.m == nil {
		g.m = make(map[string]*sfCall)
	}
	if c, ok := g.m[key]; ok {
		g.mu.Unlock()
		<-c.done
		return c.val, c.err
	}
	c := &sfCall{done: make(chan struct{})}
	g.m[key] = c
	g.mu.Unlock()

	c.val, c.err = fn()
	close(c.done)

	g.mu.Lock()
	delete(g.m, key)
	g.mu.Unlock()
	return c.val, c.err
}