option to allow different users to share an mmap file and connect via unix domain sockets

[Antoine Martin]

This requires two things:

• that the user can override the default location for sockets, so they can use /tmp instead of ~/.xpra
• an option to be able to change the file mask so other users can access the mmap file

[Antoine Martin]

original patch from violetse

[Antoine Martin]

Outstanding issues and notes:

• os.chown(self.mmap_file, -1, s.st_uid) - shouldn't that be os.chown(self.mmap_file, -1, s.st_gid)? As we want the gid to match?
• agreed to rename logdir to confdir
• moved the "sockfile" setup code to the connection classes' "target" attribute
• create_unix_domain_socket is racy: we chmod by name after calling bind, this should use umask or a temporary directory. Also, this is a change from previous versions which may affect security, I can't see a problem since ".xpra" is 0700 and the socket will go there by default, but maybe I am missing something?
• the man page needs updating
• the code does not work with "--no-deamon"

Updated patch to follow.

[Antoine Martin]

updated patch with small changes

[violetse]

In patch 3 I have:

Added umask so there is no race condition in create_unix_domain_socket

Updated the man page

[Antoine Martin]

merged with only cosmetic changes in r594

You must have spotted that fchmod was unnecessary in the non-mmap_group codepath: tempfile.NamedTemporaryFile does it for us (although this isn't documented explicitly in the Python docs..)

Closing, please test and re-open if needed. Thanks!

[Antoine Martin]

FYI: this broke ssh connections, see #120 for details..

[migration script]

this ticket has been moved to: ​https://github.com/Xpra-org/xpra/issues/88