std::fs::create_dir_all fails on path ending with . if it does not already exist

[Raniz85]

I tried this code:

std::fs::create_dir_all("/tmp/foo/.").unwrap();

I expected to see this happen: /tmp/foo is created

Instead, this happened:

Err(Os { code: 2, kind: NotFound, message: "No such file or directory" })

If the path already exists, the call succeeds without errors.
See this play for a demo:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=b93fae57ba5820c2bfeb70e7808a0ab9

Meta

Checked on both stable and nightly on play.rust-lang.org

Backtrace

thread 'main' (14) panicked at src/main.rs:4:62:
called `Result::unwrap()` on an `Err` value: Os { code: 2, kind: NotFound, message: "No such file or directory" }
stack backtrace:
   0: __rustc::rust_begin_unwind
             at /rustc/2e2b193f8ada105f27608b7be81c293e0d7292cb/library/std/src/panicking.rs:676:5
   1: core::panicking::panic_fmt
             at /rustc/2e2b193f8ada105f27608b7be81c293e0d7292cb/library/core/src/panicking.rs:80:14
   2: core::result::unwrap_failed
             at /rustc/2e2b193f8ada105f27608b7be81c293e0d7292cb/library/core/src/result.rs:1870:5
   3: <core::result::Result<(), core::io::error::Error>>::unwrap
             at ./.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/result.rs:1232:23
   4: playground::main
             at ./src/main.rs:4:62
   5: <fn() as core::ops::function::FnOnce<()>>::call_once
             at ./.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/function.rs:250:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

[Raniz85]

I'd be willing to try and fix this myself if it should be fixed.

[ChrisDenton]

I do think this is a bug, though I can see why this happens. Note that it doesn't affect Windows and possibly there are other platforms it doesn't affect but I've not surveyed them all.

In any case, it should be made to work as consistently as possible across platforms and this is surprising behaviour.

[maxdexh]

FWIW coreutils mkdir works the same way NVM this is create_dir_all, yeah i think this should work (mkdir -p a/. works)

[ChrisDenton]

Yeah for create_dir_all we are meant to only error if it does not exist and it's not possible to create the path.

[Raniz85]

I'll have a go at fixing it tomorrow (CEST)

[asder8215]

I can put up a fix with this pretty quick. I kinda feel responsible for this because I changed up the implementation of create_dir_all entirely last year.

I also have plans to change this function a bit more to do no heap allocation once this gets merged in the future.

[asder8215]

So a part of the issue with this has less to do with std::fs::create_dir_all and more to do with what Ancestors is doing.

From this loop:

rust/library/std/src/fs.rs

Lines 3748 to 3766 in a69a632

	for ancestor in ancestors {
	// for relative paths like "foo/bar", the parent of
	// "foo" will be "" which there's no need to invoke
	// a mkdir syscall on
	if ancestor.is_empty() || ancestor.parent().is_none() {
	break;
	}
	match self.inner.mkdir(ancestor) {
	Ok(()) => break,
	Err(e) if e.kind() == io::ErrorKind::NotFound => uncreated_dirs += 1,
	// we check if the err is AlreadyExists for two reasons
	// - in case the path exists as a *file*
	// - and to avoid calls to .is_dir() in case of other errs
	// (i.e. PermissionDenied)
	Err(e) if e.kind() == io::ErrorKind::AlreadyExists && ancestor.is_dir() => break,
	Err(e) => return Err(e),
	}
	}

When we have something like "/tmp/foo/.", the first path we observe is "tmp/foo/." (as Ancestors doesn't normalize the first path). This is going to end up triggering the NotFound case for Rust Playground since it seems like it views that "/tmp/foo" is missing and it cannot produce "/tmp/foo/." without "/tmp/foo" existing. The thing is the parent of "/tmp/foo/." is "/tmp" because the Ancestors (or more specifically Components) iterator normalizes the cur dir components away if it's not the starting component.

So when we go down to the for loop later to create missing directory:

rust/library/std/src/fs.rs

Lines 3772 to 3778 in a69a632

	for uncreated_dir in uncreated_dirs_vec.iter().rev() {
	if let Err(e) = self.inner.mkdir(uncreated_dir) {
	if e.kind() != io::ErrorKind::AlreadyExists || !uncreated_dir.is_dir() {
	return Err(e);
	}
	}
	}

The missing directory will be "/tmp/foo/.", which mkdir earlier triggered NotFound on this, so it will trigger that again in this for loop.

Right now, the easy fix is to just normalize the provided path by turning it into a Components iterator, using Components::as_path (which normalizes trailing cur dir and seps bytes at the end) and taking that Path as the Ancestors iterator.