[Bug 2964] New: OpenSSH for Windows often hangs if no data sent over the connection

[bugzilla-daemon at bugzilla.mindrot.org]

https://bugzilla.mindrot.org/show_bug.cgi?id=2964

            Bug ID: 2964
           Summary: OpenSSH for Windows often hangs if no data sent over
                    the connection
           Product: Portable OpenSSH
           Version: 7.9p1
          Hardware: Other
                OS: Windows 10
            Status: NEW
          Severity: normal
          Priority: P5
         Component: ssh
          Assignee: unassigned-bugs at mindrot.org
          Reporter: leese at google.com

This was reproduced on both 7.7p1 and 7.9p1, while ssh-ing from a
Windows machine to a Linux machine. For a simple repro I'm using the
bash commands `echo ""` (should output a single newline) and `echo -n
""` (should output nothing and immediately exit successfully; I've also
used /bin/true which is equivalent.)

> ssh user at host -- echo ""

Reliably returns right away as expected.

> ssh user at host -tt -- echo ""

Reliably returns right away as expected.

> ssh user at host -- echo -n ""

Seems to hang, but if you press a key it returns.

> ssh user at host -tt -- echo -n ""

Sometimes it works, sometimes it hangs. When it hangs it will be
unresponsive to input, including ^C. You need to kill it in task
manager. A wireshark trace shows that the server sent the TCP FIN
packet, but the client is still holding open the connection.

> ssh user at host -tt -v -- echo -n ""

Turning on verbose output seems to make it work reliably.

Not sure to what extent network latency affects this but my ping time
is 11ms.

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