kgo: call OnPartitionsCallbackBlocked concurrently

This was kinda meant to already be the case, and by not being the case
currently, you can accidentally deadlock your code. Running the callback
in a goroutine should help avoid user side deadlocks.

Relates to #1162.

Author: twmb

pkg/kgo/config.go

@@ -1877,6 +1877,9 @@ func OnPartitionsLost(onLost func(context.Context, *Client, map[string][]int32))
 // callbacks are blocked from [BlockRebalanceOnPoll]. You can use this as a
 // signal in your processing function to hurry up and unblock rebalancing
 // before your group member is kicked from the group at the session timeout.
+//
+// Since this is meant to be a signal to blocking operations, and to ensure
+// nothing can block this signal, this callback is called in a goroutine.
 func OnPartitionsCallbackBlocked(fn func(context.Context, *Client)) GroupOpt {
 	return groupOpt{func(cfg *cfg) { cfg.onBlocked = fn }}
 }

pkg/kgo/consumer.go

@@ -280,7 +280,7 @@ func (c *consumer) waitAndAddRebalance() {
 	for c.pollWaitState&math.MaxUint32 != 0 {
 		if !blockedCalled {
 			if c.cl.cfg.onBlocked != nil {
-				c.cl.cfg.onBlocked(c.cl.ctx, c.cl)
+				go c.cl.cfg.onBlocked(c.cl.ctx, c.cl)
 			}
 			blockedCalled = true
 		}