std.os.fsync, File.sync hit unreachable EINVAL for special files

[nissaofthesea]

Zig Version

0.11.0

Steps to Reproduce and Observed Behavior

On x86_64 Linux:

const std = @import("std");

pub fn main() !void {
    const arbitrary_user_input = "/dev/null";
    const file = try std.fs.cwd().openFile(arbitrary_user_input, .{ .mode = .write_only });
    defer file.close();
    try file.writeAll("poetry machine");
    try std.os.fsync(file.handle); // fsync returns EINVAL and std encounters unreachable
}

Expected Behavior

(Some discussion about this in general at #10776)

Currently, std.os.fsync enters unreachable code if fsync returns EINVAL. This means for files like /dev/null, it panics.

It seems reasonable to me that an arbitrary file may be opened (eg. from command line arguments), and that this file could be special. It also seems reasonable that the program may try to sync writes to ensure that data is correctly written to the file.
However, these are currently incompatible without somehow checking that the file is special, and I don't know of a function to check this in std (please correct me if there is one).
I think that returning the error would be more elegant than having this explicit check before the call.

[GalaxySnail]

Related:

• handle impossible errors from the kernel with error.Unexpected instead of unreachable #6389
• Should EINVAL always be unreachable? #6925
• Linux system calls may return unreachable or unexpected error codes #10776
• linux.getdents64 may return EINVAL #11178
• INVAL/OBJECT_NAME_INVALID in fs APIs incorrectly marked as unreachable #15607
• Use of unreachable in stdlib allows a poorly behaving child process to trigger UB in a parent process.  #15664
• panic in std.os.poll, INVAL is not unreachable #17285