home: Handle empty HOME on unix

[utkarshgupta137]

What does this PR try to resolve?

Contrary to the documentation, if the HOME environment variable is empty on Unix, then an empty Path is returned. This is because std::env::home_dir doesn't handle this case.
I've copied the code from standard library & added a .filter(|s| !s.is_empty()) check. The rest of the code is the same.
Also added a test similar to Windows.

How should we test and review this PR?

The current home_dir function doesn't handle an empty HOME environment variable, with this PR, it should.

Additional information

Related discussion: rust-lang/rust#71684 (comment)

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[bors]

☔ The latest upstream changes (presumably #12021) made this pull request unmergeable. Please resolve the merge conflicts.

[ehuss]

Can you provide some context here about why this would be changed? What is the importance of how it handles an empty HOME value? When would that happen in practice? How do other ecosystems handle this?

[utkarshgupta137]

Can you provide some context here about why this would be changed? What is the importance of how it handles an empty HOME value? When would that happen in practice? How do other ecosystems handle this?

1. The docs falsely claim that if HOME is empty, then it falls back to getpwuid_r.
2. I hope to see this crate recommended in the standard library & it came up in the linked discussion that this crate doesn't handle this case.
3. It is not very difficult to get a shell where the HOME variable is not set or set incorrectly. For eg. env -i bash.
4. A lot of crates outside of cargo & rustup use home as well, so it would be useful to fix it for the ecosystem.
5. The dirs/dirs-next crate & other "dirs" crate handle this case. These crates have been abandoned in the past, so it would be nice if this crate could handle this case.

[ehuss]

The docs falsely claim that if HOME is empty, then it falls back to getpwuid_r.

Can you say why we wouldn't just change the docs instead?

It is not very difficult to get a shell where the HOME variable is not set or set incorrectly. For eg. env -i bash.

From what I can tell, at least on my systems, that unsets the HOME variable (which should go to the fallback).

As for the ecosystem question, I was asking more along the lines of what other languages and libraries do. For example, Python doesn't handle it, and it essentially becomes unusable. go also seems to fail with an empty HOME. Are there any languages or common tools or libraries that handle this? Or is this requirement a holdover from a documentation mistake when the initial env module was added?

I'm not completely opposed to checking for this case. However, it's a lot of code to add for an edge case that doesn't seem terribly important without some motivation.

[utkarshgupta137]

The docs falsely claim that if HOME is empty, then it falls back to getpwuid_r.

Can you say why we wouldn't just change the docs instead?

It is not very difficult to get a shell where the HOME variable is not set or set incorrectly. For eg. env -i bash.

From what I can tell, at least on my systems, that unsets the HOME variable (which should go to the fallback).

As for the ecosystem question, I was asking more along the lines of what other languages and libraries do. For example, Python doesn't handle it, and it essentially becomes unusable. go also seems to fail with an empty HOME. Are there any languages or common tools or libraries that handle this? Or is this requirement a holdover from a documentation mistake when the initial env module was added?

I'm not completely opposed to checking for this case. However, it's a lot of code to add for an edge case that doesn't seem terribly important without some motivation.

Hmmm. I can't find any resources on any other languages handling this. On top of that, defining $HOME is a requirement of the POSIX spec, so I guess it should be fine to not handle this. I'll raise a new PR to fix the documentation.

@ehuss It would be nice to get your input on rust-lang/rust#71684 & rust-lang/rust#110665.

[utkarshgupta137]

@ehuss One thing to note is that this crate currently uses a fallback for Windows, but not for Unix. On Python, it doesn't use a fallback for both unix & windows. Same for Go.
While the documentation could clear that up, it is still unexpected that the 2 platforms are treated differently.