code/controllers/subsystem/garbage.dm

Debugging GC issues In order to debug `qdel()` failures, there are several tools available. To enable these tools, define `TESTING` in _compile_options.dm. First is a verb called "Find References", which lists every refererence to an object in the world. This allows you to track down any indirect or obfuscated references that you might have missed. Complementing this is another verb, "qdel() then Find References". This does exactly what you'd expect; it calls `qdel()` on the object and then it finds all references remaining. This is great, because it means that `Destroy()` will have been called before it starts to find references, so the only references you'll find will be the ones preventing the object from `qdel()`ing gracefully. If you have a datum or something you are not destroying directly (say via the singulo), the next tool is `QDEL_HINT_FINDREFERENCE`. You can return this in `Destroy()` (where you would normally `return ..()`), to print a list of references once it enters the GC queue. Finally is a verb, "Show qdel() Log", which shows the deletion log that the garbage subsystem keeps. This is helpful if you are having race conditions or need to review the order of deletions. Note that for any of these tools to work `TESTING` must be defined. By using these methods of finding references, you can make your life far, far easier when dealing with `qdel()` failures.
/datum/qdel_item	Qdel Item: Holds statistics on each type that passes thru qdel Holds the type as a string for this type Total number of times it's passed thru qdel. Total amount of milliseconds spent processing this type's Destroy() Times it was queued for soft deletion but failed to soft delete. Different from failures because it also includes QDEL_HINT_HARDDEL deletions Total amount of milliseconds spent hard deleting this type. Highest time spent hard_deleting this in ms. Number of times hard deletes took longer than the configured threshold Number of times it's not respected force=TRUE Number of times it's not even bother to give a qdel hint Number of times it's slept in its destroy
Flags related to this type's trip thru qdel.
/proc/qdel	Should be treated as a replacement for the 'del' keyword.