NilAway reports false positive for a nil channel with context.WithCancel(context.Background())

[tyree731]

Ran NilAway on some code of ours, and got a few messages to this effect:

broadcast_receiver.go:66:10: error: Potential nil panic detected. Observed nil flow from source to dereference point:
        -> context/context.go:184:9: literal `nil` returned from `Done()` in position 0
        -> context/context.go:105:2: returned as result 0 from interface method `Context.Done()` (implemented by `emptyCtx.Done()`)
        -> broadcast_receiver.go:66:10: result 0 of `Done()` uninitialized; nil channel accessed****

The code in question is selecting on .Done() for a context. That context is created as such:

ctx, cancel := context.WithCancel(context.Background())

Said selection happening in a separate function:

	for {
		select {
		case <-b.ctx.Done():
			return
		... more cases here...
		}
	}

While it is true that context.Background() creates an emptyCtx, that which can return a nil channel, when wrapping that emptyCtx with context.WithCancel, the resulting context is replaced with a cancelCtx, cancels being propagated (needlessly) to the emptyCtx, with cancelCtx not returning a nil channel.

This to me suggests this is a false positive.

[ioppermann]

Same here. I get this false positive all over our codebase.

[dolmen]

Duplicate of #98.

[yuxincs]

Thanks for reporting such cases! We were aware of this issue (generally channels are not handled gracefully for now) and are brewing some PRs for a fix.

Closing this issue as a duplicate of #98, let's continue the discussion there!