go runtime_error ai_generated true

panic: sync: WaitGroup is reused before previous Wait has returned

ID: go/waitgroup-add-after-wait

Also available as: JSON · Markdown · 中文
80%Fix Rate
89%Confidence
0Evidence
2024-03-22First Seen

Version Compatibility

VersionStatusIntroducedDeprecatedNotes
1.0+ active — — —

Root Cause

wg.Add() is called concurrently with wg.Wait(). The WaitGroup contract requires all Add calls that increment the counter to happen-before the Wait that observes them.

generic

中文

wg.Add() 与 wg.Wait() 并发调用。WaitGroup 约定要求所有增加计数器的 Add 调用必须先于观察到它们的 Wait。

Workarounds

  1. 97% success 
    var wg sync.WaitGroup
for _, item := range items {
    wg.Add(1)
    go func(it Item) {
        defer wg.Done()
        process(it)
    }(item)
}
wg.Wait()
  2. 96% success 
    g, ctx := errgroup.WithContext(context.Background())
for _, item := range items {
    item := item
    g.Go(func() error { return process(ctx, item) })
}
if err := g.Wait(); err != nil { return err }

Dead Ends

Common approaches that don't work:

  1. 80% fail

    The goroutine may start after Wait() already returned, so the counter never increments in time; still racy.

  2. 95% fail

    Timing-based synchronization is nondeterministic and fails under load or on fast machines.