[COMMIT seastar master] reactor: fix crash during metrics gathering

[Commit Bot]

From: Pavel Melkozerov <p.melk...@yadro.com>
Committer: Avi Kivity <a...@scylladb.com>
Branch: master

reactor: fix crash during metrics gathering

The crash is caused by statistics request while scheduling_group is
removed. The tasks_pending metric gathering triggers
reactor::pending_task_count() call. This call causes null pointer
derefence as the task queue pointer has been reset in
reactor::destroy_scheduling_group()

---
diff --git a/src/core/reactor.cc b/src/core/reactor.cc
--- a/src/core/reactor.cc
+++ b/src/core/reactor.cc
@@ -2545,7 +2545,9 @@ uint64_t
reactor::pending_task_count() const {
uint64_t ret = 0;
for (auto&& tq : _task_queues) {
- ret += tq->_q.size();
+ if (tq) {
+ ret += tq->_q.size();
+ }
}
return ret;
}