As per XDG Base Directory Specification: Files in this directory MAY be subjected to periodic clean-up. To ensure that your files are not removed, they should have their access time timestamp modified at least once every 6 hours of monotonic time or the 'sticky' bit should be set on the file.
Let's use this to also make dead socket probing faster: if the socket is likely to be live then it will have a recent timestamp.
@maxmylyn: just a FYI, feel free to close.
this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/1889