[Python-Dev] Re: [PythonLabs] Re: [Python-checkins]python/dist/src/Modules gcmodule.c,2.33.6.5,2.33.6.6 (original) (raw)

Tim Peters tim_one@email.msn.com
Sat, 5 Apr 2003 03:15:40 -0500

[Jeremy Hylton]

We've got the first version of boom nailed, but we've got the same problem in handlefinalizers(). The version of boom below doesn't blow up until the second time the hasfinalizer() is called.

I don't understand the logic in handlefinalizers(), though. If the objects are all in the finalizers list, why do we call hasfinalizer() a second time? Shouldn't everything has a finalizer at that point?

Nope -- the parenthetical

/* Handle uncollectable garbage (cycles with finalizers). */

comment is incomplete. The earlier call to move_finalizer_reachable() also put everything reachable only from trash cycles with finalizers into the list. So, e.g., if the trash graph is like

A<->B->C

and A has a finalizer but B and C don't, they're all in the finalizers list (at this point) regardless. But B and C aren't stopping the blob from getting collected, and we're trying to do the user a favor by putting only A (the troublemaker) into gc.garbage. It's an approximation, though. For example, if A and C both had finalizers, A and C would both be put into gc.garbage, despite that C's finalizer isn't stopping anything from getting collected.

The comments are apparently a bit out of synch with the code, because 17 months ago all instance objects in the finalizers list were put into gc.garbage (regardless of whether they had del). The checkin comment for rev 2.28 sez the del change was needed to fix a bug; but I'm too groggy to dig more now.