'readLink' fails on non-symlink file

[Daimanta]

Summary

When running the std.os.readLink function on a non-link path on Linux, an unreachable place of code is reached.

Setup

1. WSL or Linux environment
2. Zig 0.8.1

Reproduction

1. Choose a non-link file in linux.

var buffer: [2 << 20]u8 = undefined;
const result = try std.fs.cwd().readLink(link, buffer[0..]);

result:

/home/foobar/zig/lib/std/os.zig:2559:19: 0x20a017 in std.os.readlinkatZ (readlink)
        EINVAL => unreachable,    

Analysis

The system call likely returns EINVAL if a non-link file is link resolved. The code at readlinkatZ doesn't expect this value as the switch case in unreachable.

Possible solution

Add a 'NotALink' error and return it when receiving EINVAL from the system.