fix broken migrating monster arrival

If the first monster on the migrating_mons list couldn't arrive and
was put back on the list to try again later, 'later' would happen
immediately and the program looped forever trying and failing to
bring that monster to the level.

Defer repeat attempts at migration until losedogs() has been through
the whole migrating_mons list.  mon_arrive() now populates a new
list called failed_arrivals and losedog() moves its contents, if any,
to migrating_mons prior to returning.

doc/fixes3-7-0.txt

@@ -979,6 +979,8 @@ scared hostile monster which cannot move away will attack
 prevent a fog cloud that has engulfed the hero from moving under closed doors
 allow cutting a known spider web with wielded weapon by force-fighting the web
 holes and trapdoors have a fixed exit level
+recent changes to losedogs() could result in an infinite loop when migrating
+	monsters try to arrive as hero moves to a different level
 
 
 Fixes to 3.7.0-x Problems that Were Exposed Via git Repository

src/dog.c

@@ -11,10 +11,11 @@ static int mon_leave(struct monst *);
 static boolean keep_mon_accessible(struct monst *);
 
 enum arrival {
-    Before_you = 0, /* monsters kept on migrating_mons for accessibility;
-                     * they haven't actually left their level */
-    With_you   = 1, /* pets and level followers */
-    After_you  = 2  /* regular migrating monsters */
+    Before_you =  0, /* monsters kept on migrating_mons for accessibility;
+                      * they haven't actually left their level */
+    With_you   =  1, /* pets and level followers */
+    After_you  =  2, /* regular migrating monsters */
+    Wiz_arrive = -1  /* resurrect(wizard.c) */
 };
 
 void
@@ -224,12 +225,16 @@ update_mlstmv(void)
     iter_mons(set_mon_lastmove);
 }
 
+/* note: always reset when used so doesn't need to be part of struct 'g' */
+static struct monst *failed_arrivals = 0;
+
 void
 losedogs(void)
 {
     register struct monst *mtmp, **mprev;
     int dismissKops = 0, xyloc;
 
+    failed_arrivals = 0;
     /*
      * First, scan g.migrating_mons for shopkeepers who want to dismiss Kops,
      * and scan g.mydogs for shopkeepers who want to retain kops.
@@ -279,15 +284,17 @@ losedogs(void)
         make_happy_shoppers(TRUE);
 
     /* put monsters who went onto migrating_mons in order to be accessible
-       when other levels are active back to the positions on this level;
+       when other levels are active back to their positions on this level;
        they're handled before mydogs so that monsters accompanying the
-       hero can't steal the spot that belongs to them
-       [note: mon_arrive() might fail and put mtmp back at the head of
-       migrating_mons; that doesn't interfere with our list traversal] */
+       hero can't steal the spot that belongs to them; these migraters
+       should always be able to arrive because they were present on the
+       level at the time the hero left [if they can't arrive for some
+       reason, mon_arrive() will put them on the 'failed_arrivals' list] */
     for (mprev = &g.migrating_mons; (mtmp = *mprev) != 0; ) {
         xyloc = mtmp->mtrack[0].x; /* (for legibility) */
         if (mtmp->mux == u.uz.dnum && mtmp->muy == u.uz.dlevel
             && xyloc == MIGR_EXACT_XY) {
+            /* remove mtmp from migrating_mons */
             *mprev = mtmp->nmon;
             mon_arrive(mtmp, Before_you);
         } else {
@@ -303,19 +310,36 @@ losedogs(void)
         mon_arrive(mtmp, With_you);
     }
 
-    /* time for migrating monsters to arrive;
-       monsters who belong on this level but fail to arrive get put back
-       onto the list (at head) but that doesn't interfere with traversal */
+    /* time for migrating monsters to arrive; monsters who belong on
+       this level but fail to arrive get put on the failed_arrivals list
+       temporarily [by mon_arrive()], then back onto the migrating_mons
+       list below */
     for (mprev = &g.migrating_mons; (mtmp = *mprev) != 0; ) {
         xyloc = mtmp->mtrack[0].x;
         if (mtmp->mux == u.uz.dnum && mtmp->muy == u.uz.dlevel
             && xyloc != MIGR_EXACT_XY) {
+            /* remove mtmp from migrating_mons */
             *mprev = mtmp->nmon;
+            /* note: if there's no room, it ends up on failed_arrivals list */
             mon_arrive(mtmp, After_you);
         } else {
             mprev = &mtmp->nmon;
         }
     }
+
+    /* put any monsters who couldn't arrive back on migrating_mons,
+       clearing out the temporary 'failed_arrivals' list in the process */
+    while ((mtmp = failed_arrivals) != 0) {
+        failed_arrivals = mtmp->nmon;
+        /* mon_arrive() put mtmp onto fmon, but if there wasn't room to
+           arrive, relmon() was used to take it off again; put it back now
+           because m_into_limbo() expects it to be there */
+        mtmp->nmon = fmon;
+        fmon = mtmp;
+        /* set this monster to migrate back this level if hero leaves
+           and then returns */
+        m_into_limbo(mtmp);
+    }
 }
 
 /* called from resurrect() in addition to losedogs() */
@@ -497,8 +521,13 @@ mon_arrive(struct monst *mtmp, int when)
     else
         failed_to_place = !rloc(mtmp, RLOC_NOMSG);
 
-    if (failed_to_place)
-        m_into_limbo(mtmp); /* try again next time hero comes to this level */
+    if (failed_to_place) {
+        if (when != Wiz_arrive)
+            /* losedogs() will deal with this */
+            relmon(mtmp, &failed_arrivals);
+        else
+            m_into_limbo(mtmp);
+    }
 }
 
 /* heal monster for time spent elsewhere */

src/makemon.c

@@ -1511,9 +1511,10 @@ mbirth_limit(int mndx)
 /* used for wand/scroll/spell of create monster */
 /* returns TRUE iff you know monsters have been created */
 boolean
-create_critters(int cnt,
-                struct permonst *mptr, /* usually null; used for confused reading */
-                boolean neverask)
+create_critters(
+    int cnt,
+    struct permonst *mptr, /* usually null; used for confused reading */
+    boolean neverask)
 {
     coord c;
     coordxy x, y;

src/vault.c

@@ -215,10 +215,12 @@ findgd(void)
     for (mprev = &g.migrating_mons; (mtmp = *mprev) != 0;
          mprev = &mtmp->nmon) {
         if (mtmp->isgd && on_level(&EGD(mtmp)->gdlevel, &u.uz)) {
-            /* take out of migrating_mons and place at <0,0> */
+            /* take out of migrating_mons and place at <0,0>;
+               simplified mon_arrive(); avoid that because it would
+               send mtmp into limbo if no regular map spot is available */
             *mprev = mtmp->nmon;
-            /* simplified mon_arrive(); avoid that because it would send
-               mtmp into limbo if no regular map spot is available */
+            mtmp->nmon = fmon;
+            fmon = mtmp;
             mon_track_clear(mtmp);
             mtmp->mux = u.ux, mtmp->muy = u.uy;
             mtmp->mx = mtmp->my = 0; /* not on map (note: mx is already 0) */

src/wizard.c

@@ -708,7 +708,7 @@ resurrect(void)
                     mtmp->mfrozen = 0, mtmp->mcanmove = 1;
                 if (!helpless(mtmp)) {
                     *mmtmp = mtmp->nmon;
-                    mon_arrive(mtmp, 0); /* 0: not With_you (1) */
+                    mon_arrive(mtmp, -1); /* -1: Wiz_arrive (dog.c) */
                     /* note: there might be a second Wizard; if so,
                        he'll have to wait til the next resurrection */
                     break;