If the filesystem-backed Unix socket (G_UNIX_SOCKET_ADDRESS_PATH) does
not exist, X11 clients can also use a Linux abstract Unix socket
(G_UNIX_SOCKET_ADDRESS_ABSTRACT), or even a TCP socket.
Both of these are going to require --share=network to be enabled, so
print a warning if it isn't. We don't automatically enable
--share=network, because that elevates the privileges of apps that would
otherwise have entered a new network namespace, but users can make it
work with `flatpak run --share=network` or
`flatpak override --share=network`.
When falling back to an abstract Unix socket or to a TCP socket, we
can't remap the display number to the fixed :99.0 that we normally use,
so adjust write_xauth() to be able to avoid doing that.
Resolves: https://github.com/flatpak/flatpak/issues/4702
Signed-off-by: Simon McVittie <smcv@collabora.com>
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