An obvious bug in Dungeon Crawl: Stone Soup that players failed to recognize for two weeks, and what it demonstrates about "cognitive decoupling"

From e0bdd66d849c596fceac82094b8595117c4c4f9e Mon Sep 17 00:00:00 2001
From: Neil Moore <neil@s-z.org>
Date: Fri, 6 Mar 2015 11:31:13 -0500
Subject: [PATCH] Fix a crash with cleaving + infusion vs spectral weapons.

If we cleaved from the SW's caster (killing it) into the SW, the call
sequence melee_attack::handle_phase_hit() -> attack::calc_damage ->()
attack::player_stab() -> alert_nearby_monsters() could would cause the
SW to notice the dead caster and cease to exist.  But then we check the
vanished SW's AC for infusion damage immediately afterwards, leading
to a crash:
http://dobrazupa.org/morgue/ihatethisgame/crash-ihatethisgame-20150302-235727.txt

Instead, calculate the infusion damage first, before calling
calc_damage.

Also, for robustness, add an extra check to prevent vampires trying to
drain already-reset monsters like these.  This check isn't currently
necessary, because _player_vampire_draws_blood happens to think that
reset monsters have no blood.
---
crawl-ref/source/melee_attack.cc | 20 ++++++++++++++------
1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/crawl-ref/source/melee_attack.cc b/crawl-ref/source/melee_attack.cc
index d0528b7280b..e370089659b 100644
--- a/crawl-ref/source/melee_attack.cc
+++ b/crawl-ref/source/melee_attack.cc
@@ -412,6 +412,7 @@ bool melee_attack::handle_phase_hit()
        }
    }

+    damage_done = 0;
    // Slimify does no damage and serves as an on-hit effect, handle it
    if (attacker->is_player() && you.duration[DUR_SLIMIFY]
        && mon_can_be_slimified(defender->as_monster())
@@ -419,17 +420,12 @@ bool melee_attack::handle_phase_hit()
    {
        // Bail out after sliming so we don't get aux unarmed and
        // attack a fellow slime.
-        damage_done = 0;
        slimify_monster(defender->as_monster());
        you.duration[DUR_SLIMIFY] = 0;

        return false;
    }

-    // This does more than just calculate the damage, it also sets up
-    // messages, etc.
-    damage_done = calc_damage();
-
    if (attacker->is_player() && you.duration[DUR_INFUSION])
    {
        if (enough_mp(1, true, false))
@@ -443,12 +439,18 @@ bool melee_attack::handle_phase_hit()

            if (hurt > 0)
            {
-                damage_done += hurt;
+                damage_done = hurt;
                dec_mp(1);
            }
        }
    }

+    // This does more than just calculate the damage, it also sets up
+    // messages, etc. It also wakes nearby creatures on a failed stab,
+    // meaning it could have made the attacked creature vanish. That
+    // will be checked early in player_monattack_hit_effects
+    damage_done += calc_damage();
+
    bool stop_hit = false;
    // Check if some hit-effect killed the monster.
    if (attacker->is_player())
@@ -1879,6 +1881,12 @@ bool melee_attack::player_monattk_hit_effects()
{
    player_weapon_upsets_god();

+    // Don't even check vampire bloodletting if the monster has already
+    // been reset (for example, a spectral weapon who noticed in
+    // player_stab_check that it shouldn't exist anymore).
+    if (defender->type == MONS_NO_MONSTER)
+        return false;
+
    // Thirsty vampires will try to use a stabbing situation to draw blood.
    if (you.species == SP_VAMPIRE && you.hunger_state < HS_SATIATED
        && damage_done > 0 && stab_attempt && stab_bonus > 0