On 20 Jun, 10:22 pm, m...@catseye.org wrote:
We're writing a Twisted 14.0.0 application (on Python 2.7.7, Mac OS
10.9.3) that uses Conch as an SSH client; this is working fine.
However, we have the requirement that in an advanced mode of operation
for power users that the application take advantage of OpenSSH
connection multiplexing over an already-established-by-the-user OpenSSH
ControlMaster session (via an OpenSSH ControlPath socket) instead of
using Conch.
OpenSSH requires its new session command and forwarded file descriptors
to be sent over the socket in a very particular way: the command must
be sent first, followed by message with a '\0' byte with each forwarded
file descriptor. OpenSSH ignores the '\0' for each file descriptor,
extracting the file descriptors themselves from the message's ancillary
data.
The following will not work because none of the calls to write() send
their data until control is returned to the reactor, while
sendFileDescriptor() queues up the descriptors such that they get sent
them with the very next data that is sent -- which wind up being the
first three bytes of the command rather than the three '\0' bytes.
class OpenSSHMuxProtocol( protocol.Protocol ):
# built via reactor.connectUNIX()
def sendCommand( self, command ):
# Does not work:
self.transport.write( command )
self.transport.sendFileDescriptor( sys.stdin.fileno() )
self.transport.write( '\0' ) # payload for the stdin file
descriptor
self.transport.sendFileDescriptor( sys.stdout.fileno() )
self.transport.write( '\0' ) # payload for the stdout file
descriptor
self.transport.sendFileDescriptor( sys.stdout.fileno() )
self.transport.write( '\0' ) # payload for the stderr file
descriptor
# ^^^ Does not work
But this next solution /does/ work:
from socket import SOL_SOCKET
from twisted.python.sendmsg import SCM_RIGHTS, send1msg
class OpenSSHMuxProtocol( protocol.Protocol ):
# built via reactor.connectUNIX()
def sendCommand( self, command ):
self.transport.writeSomeData( command ) # data is sent over the
socket immediately
send1msg( self.transport.socket.fileno(), "\0", 0,
[ ( SOL_SOCKET, SCM_RIGHTS, pack( 'i', sys.stdin.fileno() )
) ] )
send1msg( self.transport.socket.fileno(), "\0", 0,
[ ( SOL_SOCKET, SCM_RIGHTS, pack( 'i', sys.stdout.fileno()
) ) ] )
send1msg( self.transport.socket.fileno(), "\0", 0,
[ ( SOL_SOCKET, SCM_RIGHTS, pack( 'i', sys.stderr.fileno()
) ) ] )
My questions are:
Is it bad to bypass the reactor and send data directly/immediately this
way using writeSomeData() and send1msg()? Note that sendCommand()
actually gets
Yes. `writeSomeData` is not a method on any transport interface. It is
an implementation detail of particular transports.
called in response to a OpenSSHMuxProtocol.dataReceived() event. If
bypassing the reactor this way is bad, how bad is it and what are the
consequences or effects?
This use is untested. There's no reason to expect it will continue to
work with future Twisted releases (or, really, that it fully works now;
since `writeSomeData` bypasses the transport's buffering layer, it seems
like you're risking an out-of-order or partial send; probably these will
only arise under load so you may not have observed them in your
testing).
Is there a better way to get a working solution? I think I'd need some
way to guarantee that the write of the command was actually sent to the
OpenSSH server before the file descriptors are forwarded -- for
example, if a Deferred was used whose first callback wrote the command
and whose second callback forwarded the descriptors, would a call to
the reactor to actually sent the command be guaranteed between the two
callbacks?
The proper way to do this would be for OpenSSH to acknowledge the
operation. At this point you would know it's safe to proceed to the
next operation. Since you didn't mention anything about
acknowledgements, I'm guessing there are none.
Since you're already relying on `self.transport.socket.fileno()` and
`send1msg` (basically, bypassing the transport abstraction and just
doing socket operations yourself) one improvement you could make would
be just to rely on that for the whole thing. Don't use `writeSomeData`.
Use `socket.send(command)`. At least this way you're only relying on
being able to treat a transport like a UNIX socket - not on the
particulars of the transport's buffering implementation.
A different approach you could take would be to implement this
connection sharing feature for Conch. I can pretty much guarantee it's
possible to implement since an older version of Conch actually did
implement it. :) The implementation was removed because it was fragile,
complicated, and poorly tested. It would be great to re-introduce the
functionality with a higher quality implementation.
Jean-Paul
_______________________________________________
Twisted-Python mailing list
Twisted-Python@twistedmatrix.com
http://twistedmatrix.com/cgi-bin/mailman/listinfo/twisted-python
