Document strategies for sharing the cache

[zanieb]

Per https://github.com/astral-sh/uv/issues/5581#issuecomment-2258832686, sharing the cache across users can be useful for system-wide uv installations.

[stas00]

Here is the summary so far of what needs to be done to make uv work with a shared cache - this helps to save a ton of disk space and inodes when there are many users on a system and files can be shared.

1. add to ~/.bashrc or ~/.profile of each user:

umask 000 # file creation so that all users's files are a+rw or a+rwx
export UV_LINK_MODE=copy # hardlinking doesn't work with different owners

2. Then create a shared cache folder:

   sudo mkdir -p /data/cache/uv
   sudo chmod a+rwx /data/cache/uv

3. Then each user redirecting their local cache to the shared one:

   rm -r ~/.cache/uv
   ln -s /data/cache/uv ~/.cache/uv

and this requires uv>=0.2.32 to work

Done.

[zanieb]

Thanks! If you're setting environment variables anyway, you can use UV_CACHE_DIR and skip the symbolic link.

[stas00]

oh, that's super-useful, then I don't have to go after users and making sure they installed a symlink.

Thank you very much, @zanieb!

[paveldikov]

Worth mentioning (or even discussing!) the security implications of this. (cache poisoning can be a huge problem)

[MattiasDC]

If you can't or don't want to use umask (as it will change behavior it for all files you create, not only uv cache). You can also use ACL rules on the cache directory, which overrides umask configuration.

[rabin-io]

What if you have many users WRITING to the cache? e.g. starting from a clean folder, and invoking many CI/test jobs at the same time which might want to download their files.