python to support SQLite extensions

[Jelle Licht]

Hello folks,

I want to enable support for loadable extensions in python's sqlite3
module. This should be enabled by passing
`--enable-loadable-sqlite-extensions' to `./configure'.

I've spend an embarassing amount of time on this, so perhaps someone out
there understands python better than I do and can elucidate me.

This does depend on our SQLite package needing support for loadable
extensions, but it seems it does.

I've tried the following custom package definition:

--8<---------------cut here---------------start------------->8---
(define-public python/sqlite-extensions
  (package
    (inherit python)
    (arguments
     (substitute-keyword-arguments (package-arguments python)
       ((#:configure-flags flags '())
        `(cons "--enable-loadable-sqlite-extensions" ,flags))))))
--8<---------------cut here---------------end--------------->8---

... which seems to 'work', according to the build log:
--8<---------------cut here---------------start------------->8---
[snip]
checking for --enable-loadable-sqlite-extensions... yes
[snip]
--8<---------------cut here---------------end--------------->8---

Sadly, even with my freshly built python3, I get the following:

--8<---------------cut here---------------start------------->8---
#> import sqlite3
#> conn=sqlite3.connect("/tmp/some-db.sqlite")
#> conn.enable_load_extension(True)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'sqlite3.Connection' object has no attribute 
'enable_load_extension'
--8<---------------cut here---------------end--------------->8---

Does anybody perhaps know why my python is correctly receiving (and
subsequently ignoring) this configure flag? It seems that *something*
happens between the 'configure' script receiving the flag, and the file
in `Modules/_sqlite/connection.c' being built, it seems
SQLITE_OMIT_LOAD_EXTENSION is set to "1".

This (seemingly) happens in setup.py, lines 1432 and on:
--8<---------------cut here---------------start------------->8---
 if '--enable-loadable-sqlite-extensions' not in 
sysconfig.get_config_var("CONFIG_ARGS"):
     sqlite_defines.append(("SQLITE_OMIT_LOAD_EXTENSION", "1"))
--8<---------------cut here---------------end--------------->8---

Indeed, in *any* python3 I can find on my Guix System, I see:

--8<---------------cut here---------------start------------->8---
#> import sysconfig
#> sysconfig.get_config_var("CONFIG_ARGS")
'--with-system-ffi'
--8<---------------cut here---------------end--------------->8---

Something weird is going on!

I have no clue how to continue finding why this happens, but for my
personal problem I can just patch python's setup.py file.

Thanks for any pointers,
 - Jelle

NB, this configure option happens to be enabled by default in many other
distro's (among which Nix [1]), so it might actually make sense to also
set things up in such a way on our next core-updates cycle, once the
flag actually does something.

[1] https://github.com/NixOS/nixpkgs/pull/67472/files