Errors examining or picking up items dropped by dead NPC

[wapcaplet]

Describe the bug

After I killed NPC Omar Bender inside my base in self-defense, he dropped a bunch of items, including a radio and crossbow that emit errors when I try to examine or grab them:

 DEBUG    : Requested non-existing faction 'solo_Omar Bender'
 FUNCTION : faction* faction_manager::get(const faction_id&, bool)
 FILE     : src/faction.cpp
 LINE     : 456

followed by:

 DEBUG    : item::get_owner_name() item two-way radio has no valid nor null faction id 
 FUNCTION : std::__cxx11::string item::get_owner_name() const
 FILE     : src/item.cpp
 LINE     : 1087

Steps To Reproduce

Steps to reproduce the behavior:

1. Kill a hostile NPC
2. See if they drop any items
3. Try to examine or pick up their loot

Expected behavior

Expected all of dead NPC's items to have similar ownership/faction attributes. Or, if there is some reason his radio and crossbow are owned by another faction, at least there should be no errors trying to examine them.

Screenshots

Here are the two problematic items once I moved the corpse and clothing aside. Attempting to move them using advanced inventory manager or hauling simply does nothing (which seems fine, if these items aren't mine), but I only see errors if I try to examine or grab them:

Versions and configuration

• OS: Linux
  • OS Version: LSB Version: core-9.20170808ubuntu1-noarch:security-9.20170808ubuntu1-noarch; Distributor ID: Ubuntu; Description: Ubuntu 18.04.3 LTS; Release: 18.04; Codename: bionic;
• Game Version: 0.D-11480-gdcdef1eefc [64-bit]
• Graphics Version: Tiles
• Mods loaded: [
  Dark Days Ahead [dda],
  Disable NPC Needs [no_npc_food],
  Vehicle Additions Pack [blazemod],
  Tanks and Other Vehicles [Tanks],
  Stats Through Kills [stats_through_kills],
  Stats Through Skills [StatsThroughSkills],
  Disable Religious Texts [no_religious_Texts]
  ]

[wapcaplet]

I was able to reproduce this a couple more times in a build from master today (0.D-11818-g2c8919d95b) with these steps:

• Create a new world
• Create a new character
• Debug in invincibility and a nice gun
• Debug spawn NPCs until one is hostile
• Kill NPC and see what they dropped

Once, the faulty item was a pot helmet; another time, a Mossberg 500 Security shotgun.

I have not tried killing friendly NPCs to see if it affects them too, but I might have to take the morale penalty and do it for science.

Attaching save game with a nearby corpse; the Mossberg 500 can't be examined or picked up, and shows up in red text:

Save game:
NPCville_CDDA_savegame.zip

• OS: Linux
  • OS Version: LSB Version: core-9.20170808ubuntu1-noarch:security-9.20170808ubuntu1-noarch; Distributor ID: Ubuntu; Description: Ubuntu 18.04.3 LTS; Release: 18.04; Codename: bionic;
• Game Version: 0.D-11818-g2c8919d95b [64-bit]
• Graphics Version: Tiles
• Mods loaded: [
  Dark Days Ahead [dda],
  Disable NPC Needs [no_npc_food]
  ]

Now that I'm sure I can reproduce the issue, I will try cracking this and submit a PR if I can figure out a fix.

[wapcaplet]

I located the code that deals with factions after an NPC's death but it'll take some time for me to grok it:

Cataclysm-DDA/src/npc.cpp

Lines 2406 to 2417 in 2c8919d

	// if this NPC was the only member of a micro-faction, clean it up.
	if( my_fac ) {
	if( !is_fake() && !is_hallucination() ) {
	if( my_fac->members.size() == 1 ) {
	for( auto elem : inv_dump() ) {
	elem->remove_owner();
	elem->remove_old_owner();
	}
	}
	my_fac->remove_member( getID() );
	}
	}

It looks like this bit is part of the "lone wolf" faction system added / revised back in September 2019:

9155080#diff-529d096f96854d0d52b251607dce9f6e

My guess is the faction was deleted after the NPC's death, but one of the dropped items remained attached to that faction ID somehow.

[ghost]

Cant reproduce, but can reproduce linked issue which is a different circumstance.

[ghost]

Well, either way, the linked PR will fix all future instances of this type of bug, hopefully