posixFchmodat

fn posixFchmodat(
    t: *Threaded,
    dir_fd: posix.fd_t,
    path: [*:0]const u8,
    mode: posix.mode_t,
    flags: u32,
) Dir.SetFilePermissionsError!void {
    // No special handling for linux is needed if we can use the libc fallback
    // or `flags` is empty. Glibc only added the fallback in 2.32.
    if (have_fchmodat_flags or flags == 0) {
        const syscall: Syscall = try .start();
        while (true) {
            const rc = if (have_fchmodat_flags or builtin.link_libc)
                posix.system.fchmodat(dir_fd, path, mode, flags)
            else
                posix.system.fchmodat(dir_fd, path, mode);
            switch (posix.errno(rc)) {
                .SUCCESS => return syscall.finish(),
                .INTR => {
                    try syscall.checkCancel();
                    continue;
                },
                else => |e| {
                    syscall.finish();
                    switch (e) {
                        .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,
                        .MFILE => return error.ProcessFdQuotaExceeded,
                        .NAMETOOLONG => return error.NameTooLong,
                        .NFILE => return error.SystemFdQuotaExceeded,
                        .NOENT => return error.FileNotFound,
                        .NOTDIR => return error.FileNotFound,
                        .NOMEM => return error.SystemResources,
                        .OPNOTSUPP => return error.OperationUnsupported,
                        .PERM => return error.PermissionDenied,
                        .ROFS => return error.ReadOnlyFileSystem,
                        else => |err| return posix.unexpectedErrno(err),
                    }
                },
            }
        }
    }

    if (@atomicLoad(UseFchmodat2, &t.use_fchmodat2, .monotonic) == .disabled)
        return fchmodatFallback(dir_fd, path, mode);

    comptime assert(native_os == .linux);

    const syscall: Syscall = try .start();
    while (true) {
        switch (std.os.linux.errno(std.os.linux.fchmodat2(dir_fd, path, mode, flags))) {
            .SUCCESS => return syscall.finish(),
            .INTR => {
                try syscall.checkCancel();
                continue;
            },
            else => |e| {
                syscall.finish();
                switch (e) {
                    .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,
                    .NOENT => return error.FileNotFound,
                    .NOMEM => return error.SystemResources,
                    .NOTDIR => return error.FileNotFound,
                    .OPNOTSUPP => return error.OperationUnsupported,
                    .PERM => return error.PermissionDenied,
                    .ROFS => return error.ReadOnlyFileSystem,
                    .NOSYS => {
                        @atomicStore(UseFchmodat2, &t.use_fchmodat2, .disabled, .monotonic);
                        return fchmodatFallback(dir_fd, path, mode);
                    },
                    else => |err| return posix.unexpectedErrno(err),
                }
            },
        }
    }
}