npc_destroy: Defer NPC destruction via timer

055-1 _nodes.txt will call `destroy;` from within OnInit, that is during an
iteration of the global `ev_db`. Previously, concurrent modification
invalidated this iteration, resulting in a crash.

This still nullifes `oid`, dequeues all timers, and effectively calls
`builtin_end`.

`npc_data::deletion_pending` is extended to include a third state.
In addition to no deletion happening, and indicating when `npc_free` is
currently on the stack, it now also tracks whether the NPC is about to be
deleted by a timer.

This means that an NPC which is about to be deleted is still blocked from
triggering new events, much like an NPC actively being deleted would.

Starting or continuing an NPC dialog with an NPC that does not, or is about to
no longer exist, is now also blocked.

1 files changed, 5 insertions, 1 deletions

diff --git a/src/map/map.hpp b/src/map/map.hpp
index 7cf43d5..56d07a5 100644
--- a/src/map/map.hpp
+++ b/src/map/map.hpp
@@ -360,7 +360,11 @@ struct npc_data : block_list
     Opt0 option;
     short flag;
 
-    bool deletion_pending;
+    enum {
+        NOT_DELETING = 0,
+        DELETION_QUEUED = 1,
+        DELETION_ACTIVE = 2
+    } deletion_pending;
 
     Array<Timer, MAX_EVENTTIMER> eventtimer;