From dc381d0560985c048426856ec417277158279c1b Mon Sep 17 00:00:00 2001
From: PatR <rankin@nethack.org>
Date: Fri, 5 Aug 2022 14:36:11 -0700
Subject: [PATCH] discard migrating monsters when entering endgame

When the hero enters the planes branch, all the rest of the dungeon
gets discarded since it can no longer be reached.  At the time that
that takes place, throw away any migrating monsters waiting to arrive
on any of those levels.
---
 include/extern.h |  1 +
 src/do.c         |  2 ++
 src/dog.c        | 22 ++++++++++++++++++++++
 3 files changed, 25 insertions(+)

diff --git a/include/extern.h b/include/extern.h
index 96fa68f13..43488aa24 100644
--- a/include/extern.h
+++ b/include/extern.h
@@ -600,6 +600,7 @@ extern void mon_arrive(struct monst *, int);
 extern void mon_catchup_elapsed_time(struct monst *, long);
 extern void keepdogs(boolean);
 extern void migrate_to_level(struct monst *, coordxy, coordxy, coord *);
+extern void discard_migrating_mons(void);
 extern int dogfood(struct monst *, struct obj *);
 extern boolean tamedog(struct monst *, struct obj *);
 extern void abuse_dog(struct monst *);
diff --git a/src/do.c b/src/do.c
index 8bb2aab5b..5daeecbd5 100644
--- a/src/do.c
+++ b/src/do.c
@@ -1463,6 +1463,8 @@ goto_level(
         /* mark #overview data for all dungeon branches as uninteresting */
         for (l_idx = 0; l_idx < g.n_dgns; ++l_idx)
             remdun_mapseen(l_idx);
+        /* get rid of monsters scheduled to migrate to discarded levels */
+        discard_migrating_mons();
     }
 
     if (Is_rogue_level(newlevel) || Is_rogue_level(&u.uz))
diff --git a/src/dog.c b/src/dog.c
index 064cbd1a5..a984f03db 100644
--- a/src/dog.c
+++ b/src/dog.c
@@ -843,6 +843,28 @@ migrate_to_level(
         vision_recalc(0);
 }
 
+/* when entering the endgame, levels from the dungeon and its branches are
+   discarded because they can't be reached again; do the same for monsters
+   scheduled to migrate to those levels */
+void
+discard_migrating_mons(void)
+{
+    struct monst *mtmp, **mprev;
+    d_level mdest;
+
+    for (mprev = &g.migrating_mons; (mtmp = *mprev) != 0; ) {
+        mdest.dnum = mtmp->mux;
+        mdest.dlevel = mtmp->muy;
+        /* the Wizard is kept regardless of location so that he is
+           ready to be brought back; nothing should be scheduled to
+           migrate to the endgame but if we find such, we'll keep it */
+        if (!mtmp->iswiz && !In_endgame(&mdest))
+            *mprev = mtmp->nmon; /* remove mtmp from migrating_mons */
+        else
+            mprev = &mtmp->nmon; /* keep mtmp on migrating_mons */
+    }
+}
+
 /* return quality of food; the lower the better */
 /* fungi will eat even tainted food */
 int