`std::fs` incorrect behavior when dealing with UNIX domain socket on Windows

[fanzeyi]

I tried this code:

use std::path::Path;
use std::env::args_os;

fn main() {
    let p = args_os().skip(1).next().expect("to have a path argument");
    let p: &Path = p.as_ref();

    dbg!(p.display());
    dbg!(p.exists());
    dbg!(p.metadata());
    dbg!(p.symlink_metadata());
    dbg!(p.is_file());
    dbg!(p.is_dir());
    dbg!(p.is_symlink());
}

I expected to see this happen:

The API should report the given path exists.

Instead, this happened:

When passed a path to a UNIX domain socket on Windows, I get:

[src\main.rs:22] p.display() = "<REDACTED>\\socket"
[src\main.rs:23] p.exists() = false
[src\main.rs:24] p.metadata() = Err(
    Os {
        code: 1920,
        kind: Uncategorized,
        message: "The file cannot be accessed by the system.",
    },
)
[src\main.rs:25] p.symlink_metadata() = Ok(
    Metadata {
        file_type: FileType(
            FileType {
                attributes: 1056,
                reparse_tag: 2147483683,
            },
        ),
        is_dir: false,
        is_file: true,
        permissions: Permissions(
            FilePermissions {
                attrs: 1056,
            },
        ),
        modified: Ok(
            SystemTime {
                intervals: 133232173107398736,
            },
        ),
        accessed: Ok(
            SystemTime {
                intervals: 133232173107398736,
            },
        ),
        created: Ok(
            SystemTime {
                intervals: 133172157242066860,
            },
        ),
        ..
    },
)
[src\main.rs:26] p.is_file() = false
[src\main.rs:27] p.is_dir() = false
[src\main.rs:28] p.is_symlink() = false

Meta

rustc --version --verbose:

rustc --version --verbose
rustc 1.68.0 (2c8cc3432 2023-03-06)
binary: rustc
commit-hash: 2c8cc343237b8f7d5a3c3703e3a87f2eb2c54a74
commit-date: 2023-03-06
host: x86_64-pc-windows-msvc
release: 1.68.0
LLVM version: 15.0.6

---

Analysis

I think this is simply an unintended consequence of supporting symlinks on Windows. It is interesting that symlink_metadata seems to be producing a correct result here (IMO this metadata should be returned by metadata()).

The differences between stat (metadata) and lstat (symlink_metadata) is the ReparsePoint flag.

rust/library/std/src/sys/windows/fs.rs

Lines 1152 to 1158 in 8d4adad

	pub fn stat(path: &Path) -> io::Result<FileAttr> {
	metadata(path, ReparsePoint::Follow)
	}
	pub fn lstat(path: &Path) -> io::Result<FileAttr> {
	metadata(path, ReparsePoint::Open)
	}

On Windows, UNIX domain sockets are implemented as reparse points just like symlink. However Rust is only setting the FILE_FLAG_OPEN_REPARSE_POINT flag when we are testing for symlink. Without this flag CreateFileW is going to return the access error above.

I think the issue is that, the standard library didn't consider that the possibility to have other type of files that are reparse points but not symlinks.

I'm not sure what is the best fix for this, but this is kinda surprising.

[ChrisDenton]

This is indeed a long standing issue. You also get the same thing with app aliases, etc. The fix I've been considering would be to add a fallback to stat so that if ERROR_CANT_ACCESS_FILE is encountered then it uses lstat. Though it would then need to discard the result if the reparse point tag has the name surrogate bit set (which, incidentally, is what is_symlink tests for).

If anyone is interested in trying to implement this then feel very free. I'd be happy to provide guidance if you assign me as the reviewer of your PR.

[chaitanyav]

@ChrisDenton I can give this a try. @rustbot claim