Save compat for the Beogh/priest polymorph bug

See e6d7efa for the bug itself.

This cleans up duplicate mid monsters for various cases that match the
conditions of this bug. Possibly this should have a minor version tag
bump, but as of right now that will mess up the ghost permastores and I
don't want to do that this close to a release.

crawl-ref/source/god-companions.cc

@@ -12,6 +12,7 @@
 #include "branch.h"
 #include "dgn-overview.h"
 #include "message.h"
+#include "mon-death.h"
 #include "mon-util.h"
 #include "religion.h"
 #include "spl-other.h"
@@ -265,4 +266,24 @@ void fixup_bad_companions()
             ++i;
     }
 }
+
+bool maybe_bad_priest_monster(monster &mons)
+{
+    // prior to e6d7efa92cb0, if a follower got polymorphed to a form that
+    // satisfied is_priest, its god got cleared. This resulted in Beogh
+    // followers potentially getting cloned on level load, resulting in
+    // duplicate mids or a corrupted mid cache depending on ordering. This is
+    // now fixed up in tag_read_level_load.
+    return mons.alive() && mons.attitude == ATT_FRIENDLY
+                        && mons.god == GOD_NAMELESS;
+}
+
+void fixup_bad_priest_monster(monster &mons)
+{
+    if (!maybe_bad_priest_monster(mons))
+        return;
+    mprf(MSGCH_ERROR, "Removing corrupted ex-follower from level: %s.",
+                                            mons.full_name(DESC_PLAIN).c_str());
+    monster_die(mons, KILL_RESET, -1, true, false);
+}
 #endif

crawl-ref/source/god-companions.h

@@ -44,4 +44,6 @@ monster* hepliaklqana_ancestor_mon();
 
 #if TAG_MAJOR_VERSION == 34
 void fixup_bad_companions();
+bool maybe_bad_priest_monster(monster &mons);
+void fixup_bad_priest_monster(monster &mons);
 #endif

crawl-ref/source/mon-transit.cc

@@ -253,6 +253,15 @@ monster* follower::place(bool near_player)
 
     if (m->find_place_to_live(near_player))
     {
+#if TAG_MAJOR_VERSION == 34
+        // fix up some potential cloned monsters for beogh chars.
+        // see comments on maybe_bad_priest monster and in tags.cc for details
+        monster *dup_m = monster_by_mid(m->mid);
+        if (dup_m && maybe_bad_priest_monster(*dup_m))
+            fixup_bad_priest_monster(*dup_m);
+        // any clones not covered under maybe_bad_priest_monster will result
+        // in duplicate mid errors.
+#endif
         dprf("Placed follower: %s", m->name(DESC_PLAIN, true).c_str());
         m->target.reset();
 

crawl-ref/source/tags.cc

@@ -6439,6 +6439,32 @@ static void tag_read_level_monsters(reader &th)
         if (!m.alive())
             continue;
 
+#if TAG_MAJOR_VERSION == 34
+        // clear duplicates of followers who got their god cleared as the result
+        // of a bad polymorph prior to e6d7efa92cb0. This only fires on level
+        // load *when there are duplicate mids*, because otherwise the clones
+        // aren't uniquely identifiable. This fix may still result in duplicate
+        // mid errors from time to time, but should never crash; saving and
+        // loading will fix up the duplicate errors. A similar check also
+        // happens in follower::place (since that runs after the level is
+        // loaded).
+        monster *dup_m = monster_by_mid(m.mid);
+        if (dup_m)
+        {
+            if (maybe_bad_priest_monster(*dup_m))
+                fixup_bad_priest_monster(*dup_m);
+            else if (maybe_bad_priest_monster(m))
+            {
+                fixup_bad_priest_monster(m);
+                env.mid_cache[dup_m->mid] = dup_m->mindex();
+                // dup_m should already be placed, so nothing else is needed.
+                continue;
+            }
+            // we could print an error on the else case, but this is already
+            // going to be handled by debug_mons_scan.
+        }
+#endif
+
         // companion_is_elsewhere checks the mid cache
         env.mid_cache[m.mid] = i;
         if (m.is_divine_companion() && companion_is_elsewhere(m.mid))