sequencing issue: dismounting from dying steed

Reported by entrez:   if a trap killed hero's steed and dismounting
was fatal for the hero (probably by falling onto the same trap),
impossible "dmonsfree: 1 removed doesn't match 0 pending" warning
occurred during game-over cleanup.

Move the dismount calls in mondead() and mongone() from before their
m_detach() call to the end of m_detach() itself.  This led to a
cascade of problems and attempted fixes until finally zeroing in on
place_monster()'s sanity checks and dismount_steed()'s attempts to
work-around one of them.

This reverts the convoluted hack from four years ago in commit
be327d9822 and deals with the issue in
a simpler way.  After that, the new dismount_steed() placement at
end of m_detach() works cleanly.

doc/fixes3-7-0.txt

@@ -1075,6 +1075,8 @@ if punished and iron ball was cursed and wielded--so welded to hand--falling
 if player's run-time config file had OPTIONS=role:Val and the environment had
 	NETHACKOPTIONS='role:!Val' the hero would be a Val instead of !Val
 sleeping or unconscious hero attacked by Medusa would meet her gaze
+if a trap killed both the hero's steed and the hero an impossible "dmonsfree:
+	N+1 removed doesn't match N pending" warning could occur
 
 
 Fixes to 3.7.0-x Problems that Were Exposed Via git Repository

src/hack.c

@@ -2897,7 +2897,8 @@ spoteffects(boolean pick)
                                     : (time_left < 10L) ? 1
                                       : 0]);
     }
-    if ((mtmp = m_at(u.ux, u.uy)) && !u.uswallow) {
+
+    if ((mtmp = m_at(u.ux, u.uy)) != 0 && !u.uswallow) {
         mtmp->mundetected = mtmp->msleeping = 0;
         switch (mtmp->data->mlet) {
         case S_PIERCER:

src/mon.c

@@ -2465,6 +2465,11 @@ m_detach(
         mtmp->mstate |= MON_DETACH;
         iflags.purge_monsters++;
     }
+
+    /* hero is thrown from his steed when it dies or gets genocided */
+    if (mtmp == u.usteed)
+        dismount_steed(DISMOUNT_GENERIC);
+    return;
 }
 
 /* give a life-saved monster a reasonable mhpmax value in case it has
@@ -2636,10 +2641,6 @@ mondead(struct monst *mtmp)
     if (mtmp->isgd && !grddead(mtmp))
         return;
 
-    /* Player is thrown from his steed when it dies */
-    if (mtmp == u.usteed)
-        dismount_steed(DISMOUNT_GENERIC);
-
     mptr = mtmp->data; /* save this for m_detach() */
     /* restore chameleon, lycanthropes to true form at death */
     if (mtmp->cham >= LOW_PM) {
@@ -2740,6 +2741,7 @@ mondead(struct monst *mtmp)
 
     if (glyph_is_invisible(levl[mtmp->mx][mtmp->my].glyph))
         unmap_object(mtmp->mx, mtmp->my);
+
     m_detach(mtmp, mptr);
     return;
 }
@@ -2834,9 +2836,6 @@ mongone(struct monst* mdef)
        his temporary corridor to/from the vault has been removed */
     if (mdef->isgd && !grddead(mdef))
         return;
-    /* hero is thrown from his steed when it disappears */
-    if (mdef == u.usteed)
-        dismount_steed(DISMOUNT_GENERIC);
     /* stuck to you? release */
     unstuck(mdef);
     /* drop special items like the Amulet so that a dismissed Kop or nurse

src/steed.c

@@ -660,19 +660,12 @@ dismount_steed(
             /* still no spot; last resort is any spot within bounds */
             (void) enexto(&steedcc, u.ux, u.uy, &mons[PM_GHOST]);
     }
-    if (!m_at(steedcc.x, steedcc.y)) {
-        if (mtmp->mhp < 1) /* make sure it isn't negative so that */
-            mtmp->mhp = 0; /* ++mhp produces a positive value     */
-        mtmp->mhp++; /* force at least one hit point, possibly resurrecting
-                      * to avoid impossible("placing defunct monst on map") */
-        place_monster(mtmp, steedcc.x, steedcc.y);
-        mtmp->mhp--; /* take the extra hit point away: cancel resurrection
-                      * if former steed has died */
-    } else {
-        impossible("Dismounting: can't place former steed on map.");
-    }
 
     if (!DEADMONSTER(mtmp)) {
+        gi.in_steed_dismounting++;
+        place_monster(mtmp, steedcc.x, steedcc.y);
+        gi.in_steed_dismounting--;
+
         /* if for bones, there's no reason to place the hero;
            we want to make room for potential ghost, so move steed */
         if (reason == DISMOUNT_BONES) {
@@ -855,7 +848,7 @@ place_monster(struct monst* mon, int x, int y)
                    minimal_monnam(mon, TRUE), x, y, mon->mstate, buf);
         x = y = 0;
     }
-    if (mon == u.usteed
+    if ((mon == u.usteed && !gi.in_steed_dismounting)
         /* special case is for convoluted vault guard handling */
         || (DEADMONSTER(mon) && !(mon->isgd && x == 0 && y == 0))) {
         describe_level(buf, 0);