Temporary directory but _not_ deleted on panic?

help
1

This is kind of a weird ask, but I'm wondering if there exists a tempfile-like crate that creates temporary directories that are automatically deleted if you exit a scope normally, but not if you panic. This would allow me to test filesystem operations and have each test's scratch directory automatically stick around for inspection when the test fails.

2

You might be able to newtype this pretty easily -- hold an Option<TypicalTempDir>, check for panics on drop, take and forget the field if panicking.

3 Likes
3
2 Likes
4

Another option would be to wrap the TempDir in a ManuallyDrop, which will prevent it from being automatically dropped, and then call ManuallyDrop::into_inner at the end of the scope to actually drop it when you want it to.

2 Likes
5

the scopeguard crate is for this exact purpose. there's guard, guard_on_success, guard_on_unwind to guard a value, and there's also defer, defer_on_success, defer_on_unwind that simply defers some code if you don't have a value to guard.

// guards/wraps a `tempdir`, the wrapper implements `Deref`
// if the custom `drop_fn` is skipped, the wrapped value is dropped normally
let td = guard_on_unwind(tempdir(), |td| {
    mem::forget(td);
});
println!("path: {}", td.path().display());

that's roughly how ScopeGuard is implemented, with some added details like the Strategy generic parameter.

6 Likes
6

Thanks! I'll need to experiment a little but that looks like exactly what I need.

7

I wound up building a RAII "test scratch directory" object by hand, without using either tempdir or scope_guard, because of various other requirements. I'm going to document its Drop implementation here just because it was surprising to me that std::thread::panicking lives in std::thread rather than std::panic. I mean, I guess it does report a property of the current thread, but that feels like an exposed implementation detail.

impl Drop for TestScratchDir {
    fn drop(&mut self) {
        if !std::thread::panicking()
            && std::env::var_os("TEST_KEEP_SCRATCH").is_none()
        {
            let _ = fs::remove_dir_all(&self.path);
        }
    }
}