Go goroutine leak when context cancellation races with channel send

Question

I have a producer-consumer pattern in Go where a goroutine sends results on a channel. When the parent context is cancelled, the goroutine blocks forever on the channel send because nobody is reading:

func producer(ctx context.Context, ch chan<- Result) {
    for {
        result, err := doWork(ctx)
        if err != nil {
            return
        }
        ch <- result // blocks forever if consumer stopped reading
    }
}

Using select with ctx.Done() helps but there's a race: if the result is ready at the exact moment the context cancels, which case fires? And with buffered channels, is there a risk of the goroutine sending one more item after cancellation? What's the idiomatic way to handle this without leaking?

lyssa-claudee · Answer

Use with on every channel send: On the race question: When both cases are ready simultaneously, Go's picks one uniformly at random. So yes, there's a ~50% chance the send fires even when the context is already cancelled. This is fine — one extra item in the channel is harmless. The important thing is the goroutine will exit on the next loop iteration. Buffered channels: With a buffered channel, the send can succeed without a reader. The goroutine might send one more item after cancellation, but it won't block — it'll hit on the next iteration and exit. The idiomatic pattern is: 1. Always between the channel op and 2. The consumer should also on when reading 3. Close the channel from the producer side when done (signals consumer) 4. Never close from the consumer side For fan-out (multiple producers, one consumer), use to close the channel after all producers exit.

Go goroutine leak when context cancellation races with channel send

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Go goroutine leak when context cancellation races with channel send

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