Fix: paying an angry shopkeeper away from his shop hit an impossible.

dopay() assumes an angry shopkeeper can never have a live bill while
the hero is not their shop's resident customer: the "after your hide /
after blood" branch only handles the robbed total, and everything
past it requires shkp == resident, else impossible("dopay: not to
shopkeeper?"). That invariant leaks. Angering a shk by attacking them
(rile_shk()/setmangry()) does not touch the bill, and u_left_shop()
returns early without rob_shop() when the shk is not inside their shop,
so a hero who steps out while the angry shk is already outside
chasing keeps billct/debit live. Pressing 'p' next to that shk then
took the irate-shk shortcut straight to the impossible (a panic under
the fuzzer's impossible-escalation mode). The hero-inside variant,
shk adjacent just beyond their door, reached it too. The same latent
assumption exists upstream in 3.6.7 and 5.0.

* src/shk.c: factor the "past due" conversion out of make_angry_shk()
  into new past_due_charges() (bill, debit, and loan fold into
  robbed, offset by credit, then setpaid() clears them); dopay() now
  settles the books with it when a non-resident angry shk still has
  billct or debit, so payment falls through to the existing "after
  blood, not money" compensation branch and pacifies the shk. The
  impossible remains as a safety net.

Author: k21971

doc/evilhack-changelog.md

@@ -5540,4 +5540,5 @@ The following changes to date are:
 - Fix: pet kills of unique monsters were missing from the livelog
 - Tweak vampiric monk loadout at game start
 - Fix: paranoid swim prompt fired while flying via amulet of flying
+- Fix: paying an angry shopkeeper away from his shop hit an impossible
 

src/shk.c

@@ -72,6 +72,7 @@ STATIC_DCL long FDECL(get_pricing_units, (struct obj *));
 STATIC_DCL boolean NDECL(angry_shk_exists);
 STATIC_DCL void FDECL(rile_shk, (struct monst *));
 STATIC_DCL void FDECL(rouse_shk, (struct monst *, BOOLEAN_P));
+STATIC_DCL void FDECL(past_due_charges, (struct monst *));
 STATIC_DCL void FDECL(remove_damage, (struct monst *, BOOLEAN_P));
 STATIC_DCL void FDECL(sub_one_frombill, (struct obj *, struct monst *));
 STATIC_DCL void FDECL(add_one_tobill, (struct obj *, BOOLEAN_P,
@@ -1238,15 +1239,11 @@ struct monst *shkp;
     ESHK(shkp)->following = 1;
 }
 
-/* Used when the shkp is teleported or falls (ox == 0) out of his shop, or
-   when the player is not on a costly_spot and he damages something inside
-   the shop.  These conditions must be checked by the calling function. */
-/*ARGSUSED*/
-void
-make_angry_shk(shkp, ox, oy)
+/* an angry shk no longer does business normally; fold any pending
+   transactions into the amount owed for robbery */
+STATIC_OVL void
+past_due_charges(shkp)
 struct monst *shkp;
-xchar ox UNUSED; /* <ox,oy> predate 'noit_Monnam()', let alone Shknam() */
-xchar oy UNUSED;
 {
     struct eshk *eshkp = ESHK(shkp);
 
@@ -1259,6 +1256,19 @@ xchar oy UNUSED;
         /* billct, debit, loan, and credit will be cleared by setpaid */
         setpaid(shkp);
     }
+}
+
+/* Used when the shkp is teleported or falls (ox == 0) out of his shop, or
+   when the player is not on a costly_spot and he damages something inside
+   the shop. These conditions must be checked by the calling function */
+/*ARGSUSED*/
+void
+make_angry_shk(shkp, ox, oy)
+struct monst *shkp;
+xchar ox UNUSED; /* <ox,oy> predate 'noit_Monnam()', let alone Shknam() */
+xchar oy UNUSED;
+{
+    past_due_charges(shkp);
 
     pline("%s %s!", Shknam(shkp), !ANGRY(shkp) ? "gets angry" : "is furious");
     hot_pursuit(shkp);
@@ -1438,6 +1448,15 @@ dopay()
         return 1;
     }
 
+    /* an angry shk who isn't tending shop couldn't collect when the
+       transactions went past due (hero and/or shk outside the shop);
+       settle the books now so payment proceeds as compensation */
+    if (shkp != resident && ANGRY(shkp)
+        && (eshkp->billct || eshkp->debit)) {
+        past_due_charges(shkp);
+        ltmp = eshkp->robbed;
+    }
+
     /* ltmp is still eshkp->robbed here */
     if (!eshkp->billct && !eshkp->debit) {
         umoney = money_cnt(invent);