Issues with iterating over lists that are mutated as we go (specifically pets)

[mattgodbolt]

There's a lot of code that loops over CHAR_DATAs in various containers with code like:

for (auto *ch = blah; ch; ch = ch->next) {...}

Some code takes defensive action "what if the the ch is removed/deleted/moved...

CHAR_DATA *ch_next;
for (auto *ch = blah; ch; ch = ch_next) {
 ch_next = ch->next;
 ...maybe do stuff that deletes `ch` or moves it out of the room or whatever...
}

This is "ok" but:

• Users have to remember which pattern to use, which is "safe" vs "I'm sure nothing will go away"...
  • probably a general iteration solution can make this easier
• Some code paths remove more than one char.

In the first case, some smart iteration will make this "go away" (mostly).

In the second case, smart iteration may be able to be taught (I have ideas)...

The second case can probably happen when:

• CHAR_DATA list is TheMoog -> TheMoog's pet -> nullptr
• MUD is shutdown.

• This code runs

  for (ch = char_list; ch != nullptr; ch = ch_next) {
      ch_next = ch->next;
  
      if (ch->desc != nullptr && ch->desc->channel() % save_every_n == save_number)
          save_char_obj(ch);
  
      if (ch == ch_quit)
          do_quit(ch, "");
  }

• in do_quit the character and their pet are removed
• the ch_next is invalid (it will point at the pet that's been removed)
• :fire: