[Twisted-Python] Effects of calling transport.writeSomeData() ?
Mark Montague
mark at catseye.org
Fri Jun 20 16:22:57 MDT 2014
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 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?
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?
Any information and/or advice is appreciated!
--
Mark Montague
mark at catseye.org
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