#3755's hand-off listens on a loopback port, and every `flatpak run` gets
its own network namespace, so inside the sandbox the second launch could
never reach the first and opened a separate window. When FLATPAK_ID and
XDG_RUNTIME_DIR are set the primary now listens on
$XDG_RUNTIME_DIR/app/<app id>/instance.sock (0600), a directory every
sandbox of the app shares, and records "socket" instead of "port" in
instance.json. The lock, token, client cap, deadlines and reply are
unchanged; a crashed primary's socket file is unlinked under the lock,
and shutdown removes the socket.
Diego Chagas
f2a487b088