Document shared cache dir

[sabik]

What's the problem this feature will solve?

Background: pip caches various information in the pip cache dir. Potentially, a user could configure this to reside on a shared filesystem (network mount, container/VM bound volume). Some attention appears to be paid to making this configuration work in the source (building wheel, then moving it into place) and issues with this configuration appear to be accepted in the project (for example, #11012 or #1969).

However, there is no documentation on whether configuring the cache dir on a shared filesystem is supported, or any limits or caveats of this configuration.

Describe the solution you'd like

Document (probably in https://pip.pypa.io/en/latest/topics/caching/) that configuring the cache dir on a shared filesystem is supported, as well as any limits or caveats of such configuration.

Alternative Solutions

As an alternative, document (probably in https://pip.pypa.io/en/latest/topics/caching/) that configuring the cache dir on a shared filesystem is not supported.

Additional context

Some of the limitation or caveats of configuring the cache dir on a shared filesystem may also apply to one user running multiple instances of pip on the same machine on separate environments.

Code of Conduct

• I agree to follow the PSF Code of Conduct.

[uranusjr]

Sounds reasonable, but we’ll need someone with real experience using shared cache directory to do this. As far as I know, no maintainers do this in real life, we only react to bug reports.