Respect PAGER env var when paging in uv help command

[krishnan-chandra]

Summary

Closes #4931.

Test Plan

Tried running the following commands locally to make sure that all cases work:

unset PAGER
cargo run -- help venv

With no pager set, uv correctly finds less on the system as it did before and passes the help output to it.

---

PAGER= cargo run -- help venv

This correctly prints out to stdout and does not use any pager.

---

PAGER=most cargo run -- help venv

This correctly opens the most pager as shown below:

[charliermarsh]

Thanks for taking this on!

[zanieb]

Is it valid to set PAGER to a path or a pager program with arguments e.g. foo -x?

[krishnan-chandra]

Looks like paths are valid, but args are not:

[zanieb]

Thanks for checking! Seems like we'll need to support parsing it as a path and checking the final component name to see if it's less.

[krishnan-chandra]

Turns out I was testing the wrong thing earlier - the output failed because the Rust code didn't parse the pager args correctly, not because they were invalid. It turns out that you can pass args as part of PAGER, because this works:

PAGER='less -R' man wc

So I added in parsing for both paths and args; in the special case where the pager command is less and PAGER already contains the args for it, we use the provided args rather than trying to inject defaults (which seems sensible to me?)

[zanieb]

I refactored the implementation in bbefd11 in the hopes it will be easier to maintain.

[krishnan-chandra]

Got it, thanks for the refactor and merge!

[zanieb]

Yeah sorry for the delay! Been on my backburner for a while. Let me know if you have any thoughts or questions on the changes.

[twmr]

@krishnan-chandra For me PAGER='less' uv help doesn't work, but PAGER='less' git log does.
Tested with both 0.4.25 and 0.4.26.
Should I create a new ticket for that?

[krishnan-chandra]

Can you elaborate on what "doesn't work" means in this context? What is the issue that you're seeing?

[twmr]

Sure, for me the output is not paged at all. So it looks like if no pager is set.
As I said above it works with git. (OS: Ubuntu 24.04 and zsh)

[zanieb]

uv help doesn't page because it's really short. Does PAGER='less' uv help run page?

[twmr]

Thx @zanieb! The output of uv help run is indeed paged.