[Python-Dev] Showstopper

[Tim Peters]

[Tim]
> allocating containers "in the middle" of something else is
> actually rare.

[Neil Schemenauer]
> It looks like you and Guido are working on a solution to avoid
> doing this.  Wouldn't it be better to change the GC so that it
> was okay to do that?  Specifically, I'm thinking of making
> collection only happen between opcodes.  Perhaps that is too
> large of a change to make before the release.

Changing PyDict_Next() and PyDict_SetItem() to avoid gratuitous reshuffling
is a worthy goal regardless (merely traversing a dict simply "should not"
resize it, and has caused problems independent of gc), so I'm solidly +1 on
those.

Loops using PyDict_Next() to mutate values of existing keys can also cause
__del__ methods to execute (because of decref'ing the old values), so there
are non-gc vulnerabilities there too we haven't really addressed -- and then
even switching to "between opcodes" gc wouldn't stop the problems unique to
gc (since __del__ methods go back to the eval loop).

But "between opcodes" sounds like a promising idea to me to short-circuit the
mass of other potential problems that can't be triggered by just decref'ing
things.  Where's the PEP <wink>?