feature: support --link-mode=symlink

[hutch3232]

Currently --link-mode supports copy, hardlink, and clone. I have my UV_CACHE_DIR set to a mounted drive which prevents hardlinks and cloning ("invalid cross-device link"). Symlinks would work, however. This could really increase performance for my situation, rather than copying over a bunch of files. When I work in R, I use renv which uses symlinks to a central cache successfully in the proposed way: https://github.com/rstudio/renv.

More details about the specific use-case:
I work in ephemeral docker containers. My workflow is generally to start by running uv pip sync requirements.txt which currently is triggering a copy for each package. I mount a persistent drive to my container so that my uv cache persists from session to session. Also worth noting that some collaborators mount the same drive and use the same cache, so that we benefit from each others' install time. Hopefully that isn't too risky, but it has worked well so far and works great in the aforementioned renv package in R.

Related issue:
#2103

Thanks for making this amazing project!

[charliermarsh]

I don't mind supporting this, I was kinda waiting for it to be requested.

[charliermarsh]

Should be straightforward-ish if anyone wants to submit a PR -- it'd basically be copying the hardlink code, but changing the operation we use at the end.

[danielenricocahall]

Hi @charliermarsh! Would it be a matter of swapping fs::hard_link for std::os::unix::fs::symlink (after adding a Symlink value to the enum and mapping it to a new callable?

[charliermarsh]

@danielenricocahall - Yeah that sounds roughly correct to me.

[charliermarsh]

We'll also need std::os::windows::fs::symlink_file or similar on Windows which will probably fail for most people due to how symlink permissions work on Windows, but that's fine, they just shouldn't opt-in to this mode if they haven't enabled symlinks :)

[charliermarsh]

Closed by #5208. Thanks @danielenricocahall!

[helderco]

I was about to ask for this for the same reasons and just found out it’s been released already! 🎉

[skshetry]

I wonder if uv can symlink packages directly instead of per-file (similar to virtualenv --symlink-app-data), which could make it much faster than hardlink/clone.

On macOS, you can also clone a directory, but I see that uv already does just that.

[charliermarsh]

That’s a good idea. We should probably symlink at the directory level.

[charliermarsh]

#5244