Fix gossip shutdown hang caused by blocking stop signal

[remo-lab]

This PR fixes a shutdown hang in the gossip channel state management logic.

Problem

stateInfoCache.Stop() was using a blocking send on an unbuffered channel:

func (cache *stateInfoCache) Stop() {
    cache.stopChan <- struct{}{}
}

If Stop() was called while the background goroutine was executing Purge(), the goroutine wasn’t listening on stopChan, causing the send to block indefinitely.
This could hang gossipChannel.Stop() and prevent peers from shutting down cleanly, especially under load.

Fix

The stop signaling was updated to use a non-blocking, idempotent close pattern:

1. Added a sync.Once guard to stateInfoCache
2. Replaced the blocking send with a guarded close(stopChan)

func (cache *stateInfoCache) Stop() {
    cache.stopOnce.Do(func() {
        close(cache.stopChan)
    })
}

Why this works

• close() never blocks, unlike a channel send
• sync.Once ensures the channel is closed exactly once
• Closing the channel reliably unblocks the background goroutine
• This matches the shutdown pattern already used elsewhere in Fabric (e.g. messageStoreImpl.Stop())

Testing

All channel-level tests pass, including shutdown-related coverage:

go test ./gossip/gossip/channel/... -v

This change is minimal, localized, and only affects shutdown behavior by removing the possibility of an indefinite block.

---

[remo-lab]

Hi @C0rWin @denyeart @yacovm ,
This fixes a shutdown hang in the gossip channel caused by a blocking stop signal and switches it to a non-blocking, idempotent close pattern.

Whenever you have time, I’d really appreciate a review. Thanks!

[yacovm]

There is an easier way of fixing it, without adding the sync.Once.

Just do:

select {
    case <- cache.stopChan:
         return
   default:
          close(cache.stopChan)
}

[remo-lab]

Hi @yacovm ,
I’ve updated the fix based on your feedback.
Switched to a simple select-based channel close instead of the blocking send, and kept the struct unchanged (no sync.Once).
I’ve tested this locally and all channel tests are passing.