fchmodatFallback

fn fchmodatFallback(
    dir_fd: posix.fd_t,
    path: [*:0]const u8,
    mode: posix.mode_t,
) Dir.SetFilePermissionsError!void {
    comptime assert(native_os == .linux);

    // Fallback to changing permissions using procfs:
    //
    // 1. Open `path` as a `PATH` descriptor.
    // 2. Stat the fd and check if it isn't a symbolic link.
    // 3. Generate the procfs reference to the fd via `/proc/self/fd/{fd}`.
    // 4. Pass the procfs path to `chmod` with the `mode`.
    const path_fd: posix.fd_t = fd: {
        const syscall: Syscall = try .start();
        while (true) {
            const rc = posix.system.openat(dir_fd, path, .{
                .PATH = true,
                .NOFOLLOW = true,
                .CLOEXEC = true,
            }, @as(posix.mode_t, 0));
            switch (posix.errno(rc)) {
                .SUCCESS => {
                    syscall.finish();
                    break :fd @intCast(rc);
                },
                .INTR => {
                    try syscall.checkCancel();
                    continue;
                },
                else => |e| {
                    syscall.finish();
                    switch (e) {
                        .FAULT => |err| return errnoBug(err),
                        .INVAL => |err| return errnoBug(err),
                        .ACCES => return error.AccessDenied,
                        .PERM => return error.PermissionDenied,
                        .LOOP => return error.SymLinkLoop,
                        .MFILE => return error.ProcessFdQuotaExceeded,
                        .NAMETOOLONG => return error.NameTooLong,
                        .NFILE => return error.SystemFdQuotaExceeded,
                        .NOENT => return error.FileNotFound,
                        .NOMEM => return error.SystemResources,
                        else => |err| return posix.unexpectedErrno(err),
                    }
                },
            }
        }
    };
    defer closeFd(path_fd);

    const path_mode = mode: {
        const sys = if (statx_use_c) std.c else std.os.linux;
        const syscall: Syscall = try .start();
        while (true) {
            var statx = std.mem.zeroes(std.os.linux.Statx);
            switch (sys.errno(sys.statx(path_fd, "", posix.AT.EMPTY_PATH, .{ .TYPE = true }, &statx))) {
                .SUCCESS => {
                    syscall.finish();
                    if (!statx.mask.TYPE) return error.Unexpected;
                    break :mode statx.mode;
                },
                .INTR => {
                    try syscall.checkCancel();
                    continue;
                },
                else => |e| {
                    syscall.finish();
                    switch (e) {
                        .ACCES => return error.AccessDenied,
                        .LOOP => return error.SymLinkLoop,
                        .NOMEM => return error.SystemResources,
                        else => |err| return posix.unexpectedErrno(err),
                    }
                },
            }
        }
    };

    // Even though we only wanted TYPE, the kernel can still fill in the additional bits.
    if ((path_mode & posix.S.IFMT) == posix.S.IFLNK)
        return error.OperationUnsupported;

    var procfs_buf: ["/proc/self/fd/-2147483648\x00".len]u8 = undefined;
    const proc_path = std.mem.printSentinel(&procfs_buf, "/proc/self/fd/{d}", .{path_fd}, 0) catch unreachable;
    const syscall: Syscall = try .start();
    while (true) {
        switch (posix.errno(posix.system.chmod(proc_path, mode))) {
            .SUCCESS => return syscall.finish(),
            .INTR => {
                try syscall.checkCancel();
                continue;
            },
            else => |e| {
                syscall.finish();
                switch (e) {
                    .NOENT => return error.OperationUnsupported, // procfs not mounted.
                    .BADF => |err| return errnoBug(err),
                    .FAULT => |err| return errnoBug(err),
                    .INVAL => |err| return errnoBug(err),
                    .ACCES => return error.AccessDenied,
                    .IO => return error.InputOutput,
                    .LOOP => return error.SymLinkLoop,
                    .NOMEM => return error.SystemResources,
                    .NOTDIR => return error.FileNotFound,
                    .PERM => return error.PermissionDenied,
                    .ROFS => return error.ReadOnlyFileSystem,
                    else => |err| return posix.unexpectedErrno(err),
                }
            },
        }
    }
}