Fix: prevent monsters from changing gear while reading a spellbook.

A spellcasting monster's multi-turn spellbook read should be a focused
activity, but its gear-management machinery could fire mid-read. In
movemon(), the I_SPECIAL gear handler (re-equip armor, re-eval weapon)
runs before dochug()/use_misc() advances the read, so a pet that picked
up or was handed better gear would swap it and burn the turn, delaying
the read by a round. Throwing gear directly at a tame monster took the
gear immediately by a separate route. Enforce mutual exclusion: while
a read is in progress, defer the gear change until it finishes or is
interrupted, reusing the existing I_SPECIAL deferral path.

* src/mon.c: gate the I_SPECIAL gear handler on the monster not
  currently reading (msp_reading != 0). The test sits before the
  I_SPECIAL clear, so the flag stays set and the deferred wear/wield
  resumes once the read completes.
* src/dothrow.c: when a tame monster catches thrown gear mid-read,
  pocket the item and call check_gear_next_turn() instead of
  wielding/wearing immediately, so the gated movemon() handler equips
  it after the read.

Author: k21971

doc/evilhack-changelog.md

@@ -5488,4 +5488,5 @@ The following changes to date are:
 - Prevent Yendorian army types and guards from spawning as aasimar
 - Add a dmonsfree detector for monsters freed without m_detach
 - Monster re-wield a better weapon gained outside of combat
+- Fix: prevent monsters from changing gear while reading a spellbook
 

src/dothrow.c

@@ -2123,12 +2123,18 @@ struct obj *obj; /* thrownobj or kickedobj or uwep */
             pline("%s catches %s.", Monnam(mon), the(xname(obj)));
             obj_extract_self(obj);
             (void) mpickobj(mon,obj);
-            if (attacktype(mon->data, AT_WEAP)
-                && mon->weapon_check == NEED_WEAPON) {
-                mon->weapon_check = NEED_HTH_WEAPON;
-                (void) mon_wield_item(mon);
+            /* don't break a spellbook read to gear up; defer the
+               wield/wear until the monster finishes reading */
+            if (has_emsp(mon) && EMSP(mon)->msp_reading != 0) {
+                check_gear_next_turn(mon);
+            } else {
+                if (attacktype(mon->data, AT_WEAP)
+                    && mon->weapon_check == NEED_WEAPON) {
+                    mon->weapon_check = NEED_HTH_WEAPON;
+                    (void) mon_wield_item(mon);
+                }
+                m_dowear(mon, FALSE);
             }
-            m_dowear(mon, FALSE);
             newsym(mon->mx, mon->my);
             return 1;
         }

src/mon.c

@@ -1606,8 +1606,13 @@ movemon()
 
         /* after losing equipment, try to put on replacement;
            skip monsters parked at (0,0) since m_dowear_type() may call
-           begin_burn() which needs get_obj_location() to succeed */
-        if ((mtmp->misc_worn_check & I_SPECIAL) && mtmp->mx) {
+           begin_burn() which needs get_obj_location() to succeed; also
+           skip a monster concentrating on reading a spellbook so it
+           does not swap armor or weapons mid-read -- I_SPECIAL stays
+           set and the change is deferred until the read finishes or is
+           interrupted */
+        if ((mtmp->misc_worn_check & I_SPECIAL) && mtmp->mx
+            && !(has_emsp(mtmp) && EMSP(mtmp)->msp_reading != 0)) {
             long oldworn;
 
             /* hostiles only try to equip things if they think hero isn't