kvserver: call cb.onEnqueueResult in defer on errors

Previously, cb.onEnqueueResult was invoked inline before returning errors, which
was less robust and required explicit calls. This commit refactors the code to
invoke onEnqueueResult in a defer statement when returning a non-nil error. Note
that the function may still call cb.onEnqueueResult with non-nil errors even
when no error is returned, since we want visibility into those cases as well.

Author: wenyihu6

pkg/kv/kvserver/queue.go

@@ -839,21 +839,24 @@ func (bq *baseQueue) addInternal(
 	replicaID roachpb.ReplicaID,
 	priority float64,
 	cb processCallback,
-) (bool, error) {
+) (added bool, err error) {
+	defer func() {
+		if err != nil {
+			cb.onEnqueueResult(-1 /* indexOnHeap */, err)
+		}
+	}()
 	// NB: this is intentionally outside of bq.mu to avoid having to consider
 	// lock ordering constraints.
 	if !desc.IsInitialized() {
 		// We checked this above in MaybeAdd(), but we need to check it
 		// again for Add().
-		cb.onEnqueueResult(-1 /*indexOnHeap*/, errReplicaNotInitialized)
 		return false, errReplicaNotInitialized
 	}
 
 	bq.mu.Lock()
 	defer bq.mu.Unlock()
 
 	if bq.mu.stopped {
-		cb.onEnqueueResult(-1 /*indexOnHeap*/, errQueueStopped)
 		return false, errQueueStopped
 	}
 
@@ -866,7 +869,6 @@ func (bq *baseQueue) addInternal(
 			if log.V(3) {
 				log.Infof(ctx, "queue disabled")
 			}
-			cb.onEnqueueResult(-1 /*indexOnHeap*/, errQueueDisabled)
 			return false, errQueueDisabled
 		}
 	}