automatic DISPLAY selection over ssh

[Antoine Martin]

Split from #172.

Remote start via ssh no longer needs a display to be specified. The difficulty is twofold:

• we don't know in advance if the other end supports displayfd, and if it doesn't we need to detect that this is what happened and tell the user
• the connection string we generate for the client uses the display... and we don't have it yet. There is currently no way of knowing what display was chosen either!

[Antoine Martin]

Now that we don't support older distros, we can assume displayfd is available.
(except maybe for some older Ubuntu versions? Meh, don't care about those)

[Antoine Martin]

done in r9683 + r9684.

@afarr: this is just a FYI, but feel free to test or just close this ticket.

This should now work as long as the server has xpra installed:

xpra start --start-child=xterm ssh:hostname

Previous versions required the form:

xpra start --start-child=xterm ssh:hostname:DISPLAY

Which required choosing a display number in advance, without knowing if it would actually be free.

[J. Max Mena]

Attempted to start a r9688 Trunk Fedora 21 Server:

• Got the following traceback when starting with xpra start --start-child=xterm ssh:localhost :

[max@schlafanzug ~]$ xpra start --start-child=xterm ssh:localhost
/usr/lib64/python2.7/site-packages/gtk-2.0/gtk/__init__.py:57: GtkWarning: could not open display
  warnings.warn(str(e), _gtk.Warning)
/usr/lib64/python2.7/site-packages/xpra/client/gtk2/__init__.py:7: GtkWarning: IA__gdk_screen_get_root_window: assertion 'GDK_IS_SCREEN (screen)' failed
  from xpra.x11.gtk_x11 import gdk_display_source
/usr/lib64/python2.7/site-packages/xpra/gtk_common/gtk_util.py:181: GtkWarning: IA__gdk_cursor_new_for_display: assertion 'GDK_IS_DISPLAY (display)' failed
  default_Cursor          = gdk.Cursor(gdk.X_CURSOR)
xpra main error:
Traceback (most recent call last):
  File "/usr/lib64/python2.7/site-packages/xpra/scripts/main.py", line 114, in main
    return run_mode(script_file, err, options, args, mode, defaults)
  File "/usr/lib64/python2.7/site-packages/xpra/scripts/main.py", line 898, in run_mode
    return run_remote_server(error_cb, options, args, mode, defaults)
  File "/usr/lib64/python2.7/site-packages/xpra/scripts/main.py", line 1357, in run_remote_server
    app = make_client(error_cb, opts)
  File "/usr/lib64/python2.7/site-packages/xpra/scripts/main.py", line 1294, in make_client
    toolkit_module = __import__(client_module, globals(), locals(), ['XpraClient'])
  File "/usr/lib64/python2.7/site-packages/xpra/client/gtk2/client.py", line 24, in <module>
    from xpra.gtk_common.gtk_util import gtk_main, color_parse
  File "/usr/lib64/python2.7/site-packages/xpra/gtk_common/gtk_util.py", line 181, in <module>
    default_Cursor          = gdk.Cursor(gdk.X_CURSOR)
RuntimeError: could not create GdkCursor object

In addition, the server crashes.

---

For what it's worth: Also specifying a display number also outputs the same traceback. Not sure if it's legitimately broken or I have no idea how to start an ssh session. TCP is working, however (which is how I'm typing this comment..)

[Antoine Martin]

This means that you are attempting to start a session from a terminal which cannot access your display - this won't work, whether you specify the remote display or not: (and has little to do with this ticket!)

/usr/lib64/python2.7/site-packages/gtk-2.0/gtk/__init__.py:57: GtkWarning: could not open display

You need to run this command from a client, without starting a server in advance - it will be started for you.

[Antoine Martin]

Here's another way of triggering those same errors using an invalid display with the X11 client:

DISPLAY="" xpra attach

[J. Max Mena]

Now that I've confirmed I'm a moron:

• Started from my Fedora 20 box (same version) against Fedora 21 (same version) with

xpra start --start-child=xterm ssh:uname@testvm

Everything works as expected.

[Antoine Martin]

@maxmylyn: LOL - thanks for testing!

FWIW: I do have plans to refactor the gtk initialization code as part of #640, which would allow us to bail out earlier when the DISPLAY is not set, giving a more friendly error message for those cases.

[Antoine Martin]

@maxmylyn: FYI, as of r9721, the error message has been improved:

• gtk2:

  $ DISPLAY= python2 /usr/bin/xpra attach  --no-speaker
  /usr/lib64/python2.7/site-packages/gtk-2.0/gtk/__init__.py:57: GtkWarning: could not open display
    warnings.warn(str(e), _gtk.Warning)
  xpra initialization error: cannot access the display
  

• gtk3:

  $ DISPLAY= python3 /usr/bin/xpra attach --no-speaker
  Unable to init server: Could not connect: Connection refused
  xpra initialization error: cannot access the display
  

I could also intercept the gtk2 warnings, but I think this is clear enough as it is.

[migration script]

this ticket has been moved to: ​https://github.com/Xpra-org/xpra/issues/612