Fix: polymorphed monsters ignored their terrain until their next move.

A monster polymorphed into a form that cannot survive its location
kept standing there until its next action: newcham() never re-checked
terrain, and minliquid() only runs at the start of each monster's own
move. A flying pit fiend polymorphed into a revenant over open air
hung in midair, and a salamander in lava polymorphed into a gnoll
witherling did not burn - for slow forms the gap lasted several turns.

newcham() now calls minliquid() on the surviving monster just before
returning, so an unsuitable new form drowns, burns, or plummets
immediately (or teleports clear, for survivors), with the usual kill
attribution. The hero's polymon() already works this way via
spoteffects(). Shape changes made inside death processing or level
restoration go through a new newcham_deferred() wrapper that defers
the check to the monster's next move, since running minliquid() there
could re-enter monster death handling. Because newcham() can now kill
the monster much more often, every caller was audited and the
unguarded ones hardened:

* mon.c:
  - newcham() tail-calls minliquid() unless defer_minliquid is set;
    newcham_deferred() (save/restore, safe under nesting) is used by
    the mondead() vampshifter/changeling and Izchak reverts, both
    vamp_stone() reverts, restore_cham(), icequeenrevive(), and
    become_flayer()'s Izchak revert.
  - mcalcdistress(): skip the rest of a monster's per-turn upkeep if
    decide_to_shapeshift() left it dead.
  - meatmetal()/meatobj()/meatcorpse(): report a death from eating
    polyfodder through the existing "ptr == NULL means it died" idiom
    instead of returning "ate and still alive".
  - become_flayer(): bail out with MM_AGR_DIED if the new mind flayer
    dies (flight does not protect in the Plane of Water's water).
  - rescham(): stop processing a chameleon that died reverting.

* mhitm.c: AD_DGST digestion transforms now return
  MM_DEF_DIED | MM_AGR_DIED instead of running grow_up() on a dead
  eater.

* mhitu.c: passiveum() AD_SLIM returns 2 (attacker died) when turning
  the attacker into a green slime kills it.

* muse.c: the wand/potion self-polymorph and polymorph-trap handlers
  return 1 (died) rather than 2 (did something) when the new form
  dies, so pet movement does not keep processing the dead monster and
  re-pocket its dropped inventory (object chain corruption).

* trap.c: mintrap() POLY_TRAP_SET sets trapkilled if the new form
  dies; poly traps can sit on puddles or sewage, which are lethal to
  iron golem and long worm forms.

Author: k21971

doc/evilhack-changelog.md

@@ -5542,4 +5542,5 @@ The following changes to date are:
 - Fix: paranoid swim prompt fired while flying via amulet of flying
 - Fix: paying an angry shopkeeper away from his shop hit an impossible
 - Fix: crystal plate mail made its wearer immune to disintegration
+- Fix: polymorphed monsters ignored their terrain until their next move
 

src/mhitm.c

@@ -3078,6 +3078,10 @@ struct obj **ootmp; /* to return worn armor for caller to disintegrate */
             } else if (pd == &mons[PM_NURSE]) {
                 magr->mhp = magr->mhpmax;
             }
+            /* the transform might have killed the eater (new form
+               unable to survive water, lava, or open air) */
+            if (DEADMONSTER(magr))
+                return (MM_DEF_DIED | MM_AGR_DIED);
         }
         /* caveat: above digestion handling doesn't keep `pa' up to date */
 

src/mhitu.c

@@ -5088,7 +5088,11 @@ struct attack *mattk;
                 if (!munslime(mtmp, FALSE) && !DEADMONSTER(mtmp)) {
                     if (newcham(mtmp, &mons[PM_GREEN_SLIME], FALSE,
                                 (boolean) (canseemon(mtmp))))
-                    mtmp->mstrategy &= ~STRAT_WAITFORU;
+                        mtmp->mstrategy &= ~STRAT_WAITFORU;
+                    /* the transform might have killed it (green slime
+                       unable to survive water, lava, or open air) */
+                    if (DEADMONSTER(mtmp))
+                        return 2;
                 }
             }
         }

src/mon.c

@@ -20,6 +20,10 @@ STATIC_VAR boolean rise_msg, disintegested, player_killed;
 #define ACCIDENT_COLLISION 2 /* involuntary knockback collision */
 STATIC_VAR int accidental_bump;
 STATIC_VAR struct monst *pet_killer; /* pet that killed monster, for livelog */
+/* suppress newcham()'s immediate terrain check when a shape change is
+   part of death processing or level restoration; those monsters get the
+   regular minliquid() check on their next move instead */
+STATIC_VAR boolean defer_minliquid;
 
 /* Set the pet responsible for a kill (call before monkilled) */
 void
@@ -39,6 +43,8 @@ STATIC_DCL int FDECL(pickvampshape, (struct monst *));
 STATIC_DCL boolean FDECL(isspecmon, (struct monst *));
 STATIC_DCL boolean FDECL(validspecmon, (struct monst *, int));
 STATIC_DCL struct permonst *FDECL(accept_newcham_form, (struct monst *, int));
+STATIC_DCL int FDECL(newcham_deferred,
+                     (struct monst *, struct permonst *, BOOLEAN_P));
 STATIC_DCL struct obj *FDECL(make_corpse, (struct monst *, unsigned));
 STATIC_DCL short FDECL(m_chooserace, (unsigned long));
 STATIC_OVL long FDECL(mm_2way_aggression, (struct monst *, struct monst *));
@@ -364,7 +370,7 @@ struct monst* mdef;  /* victim */
             pline("But wait!  %s transforms into his true form!",
                   Monnam(mdef));
         }
-        newcham(mdef, &mons[PM_ARCHANGEL], FALSE, FALSE);
+        newcham_deferred(mdef, &mons[PM_ARCHANGEL], FALSE);
         mdef->mcanmove = 1;
         mdef->mfrozen = 0;
         mdef->mstone = 0;
@@ -405,6 +411,9 @@ struct monst* mdef;  /* victim */
 
     /* Transform the larva (aggressor) into a mind flayer */
     if (newcham(magr, &mons[PM_MIND_FLAYER], FALSE, FALSE)) {
+        /* the transform might have killed the new mind flayer */
+        if (DEADMONSTER(magr))
+            return MM_AGR_DIED;
         /* Give the mind flayer the victim's name */
         if (victimname[0] != '\0') {
             christen_monst(magr, victimname);
@@ -1504,10 +1513,15 @@ mcalcdistress()
         }
 
         /* possibly polymorph shapechangers and lycanthropes */
-        if (mtmp->cham >= LOW_PM)
+        if (mtmp->cham >= LOW_PM) {
             decide_to_shapeshift(mtmp, (canspotmon(mtmp)
                                         || (u.uswallow && mtmp == u.ustuck))
                                           ? SHIFT_MSG : 0);
+            /* the shape change might have killed it (new form unable
+               to survive water, lava, or open air) */
+            if (DEADMONSTER(mtmp))
+                continue;
+        }
         were_change(mtmp);
 
         /* special handling for Izchak */
@@ -1836,7 +1850,8 @@ struct monst *mtmp;
                     ptr = mtmp->data;
                     if (poly) {
                         if (newcham(mtmp, (struct permonst *) 0, FALSE, FALSE))
-                            ptr = mtmp->data;
+                            ptr = DEADMONSTER(mtmp) ? (struct permonst *) 0
+                                                    : mtmp->data;
                     } else if (grow) {
                         ptr = grow_up(mtmp, (struct monst *) 0);
                     } else if (mstone) {
@@ -1969,7 +1984,8 @@ struct monst *mtmp;
             ptr = mtmp->data;
             if (poly) {
                 if (newcham(mtmp, (struct permonst *) 0, FALSE, FALSE))
-                    ptr = mtmp->data;
+                    ptr = DEADMONSTER(mtmp) ? (struct permonst *) 0
+                                            : mtmp->data;
             } else if (grow) {
                 ptr = grow_up(mtmp, (struct monst *) 0);
             } else if (heal) {
@@ -2163,7 +2179,8 @@ struct monst *mtmp;
             ptr = mtmp->data;
             if (poly) {
                 if (newcham(mtmp, (struct permonst *) 0, FALSE, FALSE))
-                    ptr = mtmp->data;
+                    ptr = DEADMONSTER(mtmp) ? (struct permonst *) 0
+                                            : mtmp->data;
             } else if (grow) {
                 ptr = grow_up(mtmp, (struct monst *) 0);
             } else if (heal) {
@@ -4355,7 +4372,7 @@ struct monst *mtmp;
         mtmp->mstun = 0;
         if (!mtmp->mpeaceful)
             hot_pursuit(mtmp);
-        newcham(mtmp, &mons[PM_ARCHANGEL], FALSE, FALSE);
+        newcham_deferred(mtmp, &mons[PM_ARCHANGEL], FALSE);
         free_erac(mtmp);
         mtmp->mhp = mtmp->mhpmax = 1500;
         if (mtmp == u.ustuck) {
@@ -4521,7 +4538,7 @@ struct monst *mtmp;
                     rloc_to(mtmp, new_xy.x, new_xy.y);
                 }
             }
-            newcham(mtmp, &mons[mndx], FALSE, FALSE);
+            newcham_deferred(mtmp, &mons[mndx], FALSE);
             if (is_changeling(mtmp))
                 mtmp->mcan = TRUE;
             if (mtmp->data == &mons[mndx])
@@ -5592,7 +5609,7 @@ struct monst *mtmp;
                 pline("%s!", buf);
                 display_nhwindow(WIN_MESSAGE, FALSE);
             }
-            newcham(mtmp, &mons[mndx], FALSE, FALSE);
+            newcham_deferred(mtmp, &mons[mndx], FALSE);
             if (mtmp->data == &mons[mndx])
                 mtmp->cham = NON_PM;
             else
@@ -5613,7 +5630,7 @@ struct monst *mtmp;
         if (mtmp->mhpmax <= 0)
             mtmp->mhpmax = 10;
         mtmp->mhp = mtmp->mhpmax;
-        (void) newcham(mtmp, &mons[mtmp->cham], FALSE, TRUE);
+        (void) newcham_deferred(mtmp, &mons[mtmp->cham], TRUE);
         newsym(mtmp->mx, mtmp->my);
         return FALSE;   /* didn't petrify */
     }
@@ -6361,6 +6378,10 @@ rescham()
         if (mcham >= LOW_PM) {
             (void) newcham(mtmp, &mons[mcham], FALSE, FALSE);
             mtmp->cham = NON_PM;
+            /* reverting might have killed it (base form unable to
+               survive water, lava, or open air) */
+            if (DEADMONSTER(mtmp))
+                continue;
         }
         if (is_were(mtmp->data) && mtmp->data->mlet != S_HUMAN)
             new_were(mtmp);
@@ -6405,7 +6426,7 @@ struct monst *mon;
         mcham = (int) mon->cham;
         if (mcham >= LOW_PM) {
             mon->cham = NON_PM;
-            (void) newcham(mon, &mons[mcham], FALSE, FALSE);
+            (void) newcham_deferred(mon, &mons[mcham], FALSE);
         } else if (is_were(mon->data) && !racial_human(mon)) {
             new_were(mon);
         }
@@ -7298,9 +7319,38 @@ boolean msg;      /* "The oldmon turns into a newmon!" */
         }
     }
 
+    /* the new form might not be able to survive its current location
+       (non-flyer over water, lava, or open air); apply liquid and
+       open-air effects now instead of waiting for the monster's next
+       move, unless this shape change is part of death processing or
+       level restoration */
+    if (!DEADMONSTER(mtmp) && !defer_minliquid)
+        (void) minliquid(mtmp);
+
     return 1;
 }
 
+/* newcham() for shape changes that happen inside death processing
+   (vampshifter, changeling, and Izchak reverts) or during level
+   restoration; defers the immediate terrain check so those paths
+   cannot re-enter monster death handling. save/restore rather than
+   set/clear keeps nested shape changes (mselftouch petrification can
+   trigger vamp_stone) deferred as well */
+STATIC_OVL int
+newcham_deferred(mtmp, mdat, msg)
+struct monst *mtmp;
+struct permonst *mdat;
+boolean msg;
+{
+    boolean save_defer = defer_minliquid;
+    int result;
+
+    defer_minliquid = TRUE;
+    result = newcham(mtmp, mdat, FALSE, msg);
+    defer_minliquid = save_defer;
+    return result;
+}
+
 /* sometimes an egg will be special */
 #define BREEDER_EGG (!rn2(77))
 
@@ -7815,7 +7865,7 @@ struct monst *mtmp;
 
     mvitals[PM_KATHRYN_THE_ICE_QUEEN].died++;
     livelog_printf(LL_UMONST, "defeated %s", livelog_mon_nam(mtmp));
-    newcham(mtmp, &mons[PM_KATHRYN_THE_ENCHANTRESS], FALSE, FALSE);
+    newcham_deferred(mtmp, &mons[PM_KATHRYN_THE_ENCHANTRESS], FALSE);
 
     mtmp->mcanmove = 1;
     mtmp->mfrozen = 0;

src/muse.c

@@ -3753,7 +3753,7 @@ struct monst *mtmp;
         (void) newcham(mtmp, muse_newcham_mon(mtmp), TRUE, FALSE);
         if (oseen)
             makeknown(WAN_POLYMORPH);
-        return 2;
+        return (DEADMONSTER(mtmp)) ? 1 : 2;
     case MUSE_WAN_WISHING:
         /* wear any armor items previously wished for before
          * using another wish */
@@ -3774,7 +3774,7 @@ struct monst *mtmp;
         (void) newcham(mtmp, muse_newcham_mon(mtmp), FALSE, FALSE);
         if (oseen)
             makeknown(POT_POLYMORPH);
-        return 2;
+        return (DEADMONSTER(mtmp)) ? 1 : 2;
     case MUSE_SCR_REMOVE_CURSE:
         mreadmsg(mtmp, otmp);
         if (canseemon(mtmp)) {
@@ -3816,7 +3816,7 @@ struct monst *mtmp;
         newsym(trapx, trapy);
 
         (void) newcham(mtmp, (struct permonst *) 0, FALSE, FALSE);
-        return 2;
+        return (DEADMONSTER(mtmp)) ? 1 : 2;
     case MUSE_BULLWHIP:
         /* attempt to disarm hero */
         {

src/trap.c

@@ -3344,6 +3344,12 @@ struct monst *mtmp;
                     /* we're done with mptr but keep it up to date */
                     mptr = mtmp->data;
                 }
+                /* the transform might have killed it (new form unable
+                   to survive shallow water at the trap's location) */
+                if (DEADMONSTER(mtmp)) {
+                    trapkilled = TRUE;
+                    break;
+                }
                 /* Is the monster riding another monster? */
                 if (has_erid(mtmp)
                     && (!humanoid(mtmp->data) || bigmonst(mtmp->data))) {