[Python-Dev] No longer enable Py_TRACE_REFS by default in debug build

[Brett Cannon]

On Wed, Apr 10, 2019 at 12:30 PM Nathaniel Smith <njs at pobox.com> wrote:

> On Wed, Apr 10, 2019, 04:04 Victor Stinner <vstinner at redhat.com> wrote:
>
>> Le mar. 9 avr. 2019 à 22:16, Steve Dower <steve.dower at python.org> a
>> écrit :
>> > What are the other changes that would be required?
>>
>> I don't know.
>>
>> > And is there another
>> > way to get the same functionality without ABI modifications?
>>
>> Py_TRACE_REFS is a double linked list of *all* Python objects. To get
>> this functionality, you need to store the list somewhere. I don't know
>> how to maintain such list outside the PyObject structure.
>>
>
> I assume these pointers get updated from some generic allocation/free
> code. Could that code instead overallocate by 16 bytes, use the first 16
> bytes to hold the pointers, and then return the PyObject* as (actual
> allocated pointer + 16)? Basically the "container_of" trick.
>
> I don't think that I ever used sys.getobjects(), whereas many projects
>> use gc.get_objects() which is also available in release builds (not
>> only in debug builds).
>
>
> Can anyone explain what pydebug builds are... for? Confession: I've never
> used them myself, and don't know why I would want to.
>

There is a bunch of extra things done in a debug build, e.g. all freed
memory is blanked out with a known pattern so it's easy to tell when you're
reading from freed memory (and thus probably messed up your refcounts). And
then various extras are tossed on to the sys module to help with things.
Basically anything people have found useful and require being compiled in
typically get clumped in under the debug build.

-Brett


>
> (I have to assume that most of Steve's Windows downloads are from folks
> who thought they were downloading a python debugger.)
>
> -n
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