fileDowngradeLock

fn fileDowngradeLock(userdata: ?*anyopaque, file: File) File.DowngradeLockError!void {
    const ev: *Evented = @ptrCast(@alignCast(userdata));
    var sync: CancelRegion.Sync = try .init(ev);
    defer sync.deinit(ev);
    ev.flock(&sync, file.handle, .shared, .nonblocking) catch |err| switch (err) {
        error.WouldBlock => return errnoBug(.AGAIN), // File was not locked in exclusive mode.
        error.SystemResources => return errnoBug(.NOLCK), // Lock already obtained.
        error.FileLocksUnsupported => return errnoBug(.OPNOTSUPP), // Lock already obtained.
        else => |e| return e,
    };
}