proxy start should not set encodings

[Antoine Martin]

As per #2180, the server start command includes the list of encodings that the client supports.
We should leave this empty unless the client specified them explicitly on the command line (distinguishing the two is the hard part) otherwise the server may start with a more restricted set of encodings and subsequent clients won't be able to use all encodings.

[Antoine Martin]

Easy to reproduce:

• proxy:

  sudo python3 /usr/bin/xpra proxy --bind-tcp=0.0.0.0:10000 \
      --auth=allow --no-daemon -d proxy,exec --bind=/run/xpra/system --socket-permissions=666
  

• start via proxy:

  xpra start --start=xterm --start-via-proxy=true
  

The resulting server process:

/usr/bin/python3 /usr/bin/xpra start \ --chdir=/home/antoine/projects/Xpra/trunk/src --attach=no \
    --encodings=h264,vp9,vp8,mpeg4,mpeg4+mp4,h264+mp4,vp8+webm,vp9+webm,png,png/P,png/L,webp,rgb,rgb24,rgb32,jpeg,h265,mpeg1,mpeg2 \
    --video-encoders=all --csc-modules=all --video-decoders=all \
    --compressors=lz4, lzo, zlib, brotli \
    --packet-encoders=rencode, bencode, yaml --start=xterm \
    --env=XPRA_PROXY_START_UUID=a62c7b8cfd884dbdb86ad17d6b966387 \
    --daemon=yes --systemd-run=no --uid=1000 --gid=1000 --displayfd=13

[Antoine Martin]

The start-new-session dict sent to the proxy contains:

{
'mode': 'start', 'start-env': ["#avoid .. GTK_CSD=0'], 'encoding': 'auto', \
'encodings': ['h264', 'vp9', 'vp8', 'mpeg4', 'mpeg4+mp4', 'h264+mp4', 'vp8+webm', 'vp9+webm', 'png', 'png/P', 'png/L', 'webp', 'rgb', 'rgb24', 'rgb32', 'jpeg', 'h265', 'mpeg1', 'mpeg2'], \
'min-quality': 30, 'min-speed': 30, 'compression_level': 1, 'video-scaling': 'auto', \
'title': '@title@ on @client-machine@', 'clipboard': 'yes', 'clipboard-direction': 'both', \
'input-method': 'none', 'microphone': 'off', 'speaker': 'on', 'fake-xinerama': True, \
'resize_display': True, 'keyboard-sync': True, 'cursors': True, 'bell': True, 'notifications': True, \
'xsettings': True, 'system-tray': True, 'windows': True, 'webcam': 'auto', 'html': 'auto', \
'av-sync': True, 'global-menus': True, 'forward-xdg-open': True, 'modal-windows': True, \
'bandwidth-detection': True, 'ssh-upgrade': True, 'printing': 'no', 'file-transfer': 'auto', \
'open-command': '/usr/bin/xdg-open', 'open-files': 'auto', 'open-url': 'auto', 'start-new-commands': True, \
'mmap': 'yes', 'mmap-group': 'auto', 'mdns': True, 'bind': ['auto'], 'rfb-upgrade': 5, \
'bandwidth-limit': 'auto', 'start': ['xterm'], 'chdir': '/home/antoine/projects/Xpra/trunk/src'
}
Clearly lots of things we don't need to be sending.

[Antoine Martin]

Mostly done in r22729 (client-side fix) by only sending options when their values differ from the default config.

We still need to do something similar proxy-side:

/usr/bin/python3 /usr/bin/xpra start --chdir=/home/antoine/projects/Xpra/trunk/src \
    --attach=no --encodings=all --video-encoders=all --csc-modules=all --video-decoders=all \
    --compressors=lz4, lzo, zlib, brotli --packet-encoders=rencode, bencode, yaml \
    --start=xterm --env=XPRA_PROXY_START_UUID=8523b7547812495ebeea0c632b3d9923 \
    --daemon=yes --systemd-run=no --uid=1000 --gid=1000 --displayfd=13