TempDir: Provide an API to not delete-dir-on-drop without consuming the TempDir

[vincentdephily]

The current API offers into_path(self) -> PathBuf which is nice and clean (not sure why PathBuf instead of Path, but I can live with that), but forces you to decide ahead of time if you want a temporary or persisted path.

I'd like to change the behaviour at runtime, but am hitting the following snags:

• If I call into_path() near the end of my scope, it might not get called because of asserts that triggered earlyer (precisely when I'd have liked to keep the tempdir).
• If I call it at the beginning of my scope, I either run into PathBuf/TempDir type mismatches, or into temporary value does not live long enough problems:

let tempdir = TempDir::new().unwrap();
// Doesn't compile because of lifetime issues
let cwd = match std::env::var("ASSERTFS_KEEP_TEMP") {
    Ok(_) => tempdir.into_path().as_path(),
    Err(_) => tempdir.path(),
};
assert!(dostuff(cwd).is_ok());
// Too late: assert has fired already.
if std::env::var("ASSERTFS_KEEP_TEMP").is_ok() {
    tempdir.into_path();
}

Perhaps I missed an easy solution, but I doubt it would be as easy as having some kind of delete_on_drop(bool) API:

let tempdir = TempDir::new()
                      .unwrap()
                      .delete_on_drop(std::env::var("ASSERTFS_KEEP_TEMP").is_ok());
assert!(dostuff(cwd).is_ok());

Workaround

let tempdir = TempDir::new().unwrap();
let mut cwd = PathBuf::new();
match env::var("ASSERTFS_KEEP_TEMP") {
    Ok(_) => cwd.push(&tempdir.into_path()),
    Err(_) => cwd.push(tempdir.path()),
}
assert!(dostuff(cwd).is_ok());

[epage]

Could you describe the motivation for keeping the temp dir?

[vincentdephily]

I'm running unit tests that create files in the tempdir. Most of the time I want them deleted, but if tests start failing I want to look at the files to figure out what happened.

Even outside of unittesting, there are many tools that implement some kind of --keep-temp-files option, so this seems to me like a fairly common need.

[vincentdephily]

For what it's worth, I eventually managed to make my usecase work with the current API:

let tempdir = TempDir::new().unwrap();
let mut cwd = PathBuf::new();
match env::var("ASSERTFS_KEEP_TEMP") {
    Ok(_) => cwd.push(&tempdir.into_path()),
    Err(_) => cwd.push(tempdir.path()),
}
assert!(dostuff(cwd).is_ok());

It works but it's not elegant (copying the path like this seems like a hack), and took me a while to come up with. An obvious TempDir API to do the same would still be welcome.

[epage]

Thanks for describing that.

For tests, that hasn't been as important to me because they have been things I could always exercise with the [bin] target but that isn't true in all cases.

[epage]

I think another important aspect for the user, whether its done inside of assert_fs or the tests, is to print the path in case the failure message doesn't have it.

[vincentdephily]

Yes, I wouldn't mind replacing my println!("{:?}", tempdir.path()) with println!("{}", tempdir)... but that's a different issue ;)

[epage]

I think I'm going to give making a newtype for TempDir a try which gives me more flexibility in implementing this idea.