1.7.2: ssh -X unresponsive until xauth PPID killed

[Mark McConnell <markd dot mcconnell at gmail dot com>]

After installing Cygwin 1.7.2 for the first time on Windows Server 2008 SP2,
I discovered that I could not connect to a remote host using Cygwin's ssh,
if X is forwarded.

ssh -X host or ssh -Y host :  program becomes unresponsive.

I've attached the output of cygcheck -s -v -r to this message, and
/var/log/XWin.0.log.  I should mention as an aside that there are other
clues that my Cygwin installation is flawed (a local gvim gui will not
display, man dumps core).

Below, I describe two experiments that indicate that my X forwarding problem
is somehow related to xauth* and its PPID:

### 'ssh -X host gui'  displays if the PPID of xauth* is killed ###
First, I open a new mintty window within the Cygwin environment and issue
the command to start gvim on the remote server, using the forwarded X
display.  No gui is displayed.  The cursor returns below the prompt.  ^d or
^c do not terminate the process ( however, I can suspend it from that window
by typing ^z, and I can return it to the foreground with "fg <job>" ).

pt0 $ ssh -X dasftp gvim
|

I open a second mintty instance and look at the process running locally:
pt1 $ ps
      PID    PPID    PGID     WINPID  TTY  UID    STIME COMMAND
     4532       1    4532       5296  con 11137 11:40:42 /usr/bin/XWin
     3432    4532    3432       4244    ? 11137 13:15:21 /usr/bin/mintty
     3364    3432    3364       3688    0 11137 13:15:21 /usr/bin/bash
     5972    3364    5972       2436    0 11137 13:15:39 /usr/bin/ssh
     5792    5972    5972       5792    0 11137 13:15:39 /usr/bin/sh
     2632    5792    5972       2548    0 11137 13:15:39 /usr/bin/xauth

In the second terminal I kill the sh* process that is the parent of xauth*.
The remote gvim gui immediately appears.  xauth* is now running with PPID 1

pt1 $ ps | grep /usr/bin/sh | sed -e 's/^\s*//' | kill `sed -e 's/\s.*//'` ;
ps
      PID    PPID    PGID     WINPID  TTY  UID    STIME COMMAND
     4532       1    4532       5296  con 11137 11:40:42 /usr/bin/XWin
     3432    4532    3432       4244    ? 11137 13:15:21 /usr/bin/mintty
     3364    3432    3364       3688    0 11137 13:15:21 /usr/bin/bash
     5972    3364    5972       2436    0 11137 13:15:39 /usr/bin/ssh
     2632       1    5972       2548    0 11137 13:15:39 /usr/bin/xauth

After closing gvim, I take a peek at the processes left behind.  ssh* has
terminated, bash* has been idle, xauth* continues to run even if X is
closed.  Its PPID has changed to 1

pt1 $ ps
      PID    PPID    PGID     WINPID  TTY  UID    STIME COMMAND
     4532       1    4532       5296  con 11137 11:40:42 /usr/bin/XWin
     3432    4532    3432       4244    ? 11137 13:15:21 /usr/bin/mintty
I    3364    3432    3364       3688    0 11137 13:15:21 /usr/bin/bash
     2632       1    5972       2548    0 11137 13:15:39 /usr/bin/xauth

pt1 $ kill 2632
bash: kill: (2632) - Resource temporarily unavailable

Summarizing: SSH authentication succeeds but the ssh* is unresponsive when
started with X forwarding.   Killing the PPID of xauth* leaves it running in
the background.  xauth* will remain running even after the X server is
closed.  To kill the zombie xauth* I use Windows Task Manager.

## 'ssh -X host' succeeds after PPID of xauth* is killed ##
Here's a  simplified version of the experiment.

In terminal 1:
pt0 $ ssh -v -v -X dasftp 2> ssh.out

In terminal 2, I kill the PPID of xauth* (/usr/bin/sh):
pt1 $ ps | grep /usr/bin/sh | sed -e 's/^\s*//' | kill `sed -e 's/\s.*//'` ;

In terminal 1 (connected to 'host'):
Welcome to dasftp
pt3 $

I have attached ssh.out also, which is the debug output of "ssh -v -v -X
host". I can provide a much more detailed description of the steps I've
followed, if that would be helpful.

Many thanks,
Mark
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