errgroup: use consistent read for SetLimit panic

justinmayhew · gopherbot · commit 2a180e22fddc · 2025-12-03T16:24:39.000-08:00
This could produce a confusing panic message if the second len(g.sem) call returns 0. Avoid that by reading it once. Change-Id: Ibdb4963f90921bc20427b3f1e2de410638f6cb6b Reviewed-on: https://go-review.googlesource.com/c/sync/+/726280 Reviewed-by: Alan Donovan <adonovan@google.com> Auto-Submit: Alan Donovan <adonovan@google.com> Reviewed-by: Sean Liao <sean@liao.dev> LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
diff --git a/errgroup/errgroup.go b/errgroup/errgroup.go
@@ -144,8 +144,8 @@ func (g *Group) SetLimit(n int) {
 		g.sem = nil
 		return
 	}
-	if len(g.sem) != 0 {
-		panic(fmt.Errorf("errgroup: modify limit while %v goroutines in the group are still active", len(g.sem)))
+	if active := len(g.sem); active != 0 {
+		panic(fmt.Errorf("errgroup: modify limit while %v goroutines in the group are still active", active))
 	}
 	g.sem = make(chan token, n)
 }

Original file line number	Diff line number	Diff line change
`@@ -144,8 +144,8 @@ func (g *Group) SetLimit(n int) {`
`144`	`144`	`g.sem = nil`
`145`	`145`	`return`
`146`	`146`	`}`
`147`		`- if len(g.sem) != 0 {`
`148`		`- panic(fmt.Errorf("errgroup: modify limit while %v goroutines in the group are still active", len(g.sem)))`
	`147`	`+ if active := len(g.sem); active != 0 {`
	`148`	`+ panic(fmt.Errorf("errgroup: modify limit while %v goroutines in the group are still active", active))`
`149`	`149`	`}`
`150`	`150`	`g.sem = make(chan token, n)`
`151`	`151`	`}`