avoid race between compaction and table stop

Also add a debug-only compaction-manager-side assertion that tests
that no new compaction tasks were submitted for a table that is being
removed (debug-only because not constant-time).

Fixes #9448.

Signed-off-by: Michael Livshin <michael.livshin@scylladb.com>
Message-Id: <20211007110416.159110-1-michael.livshin@scylladb.com>
(cherry picked from commit e88891a8af)

sstables/compaction_manager.cc

@@ -860,6 +860,9 @@ future<> compaction_manager::remove(column_family* cf) {
     return do_for_each(*tasks_to_stop, [this, cf] (auto& task) {
         return this->task_stop(task);
     }).then([this, cf, tasks_to_stop] {
+#ifdef DEBUG
+        assert(std::find_if(_tasks.begin(), _tasks.end(), [cf] (auto& task) { return task->compacting_cf == cf; }) == _tasks.end());
+#endif
         _compaction_locks.erase(cf);
     });
 }

table.cc

@@ -953,8 +953,8 @@ void table::try_trigger_compaction() noexcept {
 }
 
 void table::do_trigger_compaction() {
-    // But only submit if we're not locked out
-    if (!_compaction_disabled) {
+    // But not if we're locked out or stopping
+    if (!_compaction_disabled && !_async_gate.is_closed()) {
         _compaction_manager.submit(this);
     }
 }