Provide 3.13t & 3.14t images

[lysnikolaou]

Since #947 has been closed by the OP, I thought I'd reopen this here, since the free-threaded build is now a fully-supported build as of PEP 779.

Is there an appetite for providing 3.14t (and maybe also 3.13t, but certainly not necessary) images? If yes, we can certainly offer support with doing the necessary changes for that.

[tianon]

I think #947 (comment) is still the most reflective of our current state (the build matrix is large, the compilation is heavy, and this is essentially asking us to double it all for 3.13+):

We could shrink the impact by doing "clever" things like only supporting "nogil" on a limited set of architectures, a limited set of base distributions, etc, but ultimately any choice we make there is definitely going to cause some amount of friction (both for us in maintaining and explaining the line, and for users who get to navigate the line).

We could also consider just including both in the default images, but I doubt that's going to be a popular choice either (and doesn't decrease the compilation burden, just obfuscates it). Ultimately it would be really interesting if this could be an entirely runtime determined value (or some way of having two binaries compiled concurrently and using linking cleverly so that they share most of the final binary size in shared libraries), but that's probably also a tall order.

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#947 (comment)

Technically, it is possible to re-enable the GIL on the free-threaded build by setting the environment variable PYTHON_GIL=1. I don't know if it's intended for something like this.

This is really interesting (because then while we'd still have a large matrix, half of it would be simply FROM python:freethreading [what a mouthful 😭] + ENV PYTHON_GIL 1), but does make me wonder if this is actually really supported/recommended usage -- if it is, shouldn't core be adjusted to always compile with freethreading enabled and install the binary as python3.14t with a python3.14 symlink or small script wrapper that automatically enables the classic GIL mode? Then we'd have the best of both worlds with no real overhead and only a single image. 🤔