channel: retry ssh_channel_open_session with SSH_AGAIN

Setting a low SSH options timeout value can lead to

LibsshChannelException: Failed to open_session: [-2]

Attempt to retry when this occurs.

Author: justin-stephenson

src/pylibsshext/channel.pyx

@@ -63,8 +63,9 @@ cdef class Channel:
 
         if self._libssh_channel is NULL:
             raise MemoryError
-        rc = libssh.ssh_channel_open_session(self._libssh_channel)
-
+        rc = libssh.SSH_AGAIN
+        while rc == libssh.SSH_AGAIN:
+            rc = libssh.ssh_channel_open_session(self._libssh_channel)
         if rc != libssh.SSH_OK:
             libssh.ssh_channel_free(self._libssh_channel)
             self._libssh_channel = NULL
@@ -164,7 +165,9 @@ cdef class Channel:
         if channel is NULL:
             raise MemoryError
 
-        rc = libssh.ssh_channel_open_session(channel)
+        rc = libssh.SSH_AGAIN
+        while rc == libssh.SSH_AGAIN:
+            rc = libssh.ssh_channel_open_session(channel)
         if rc != libssh.SSH_OK:
             libssh.ssh_channel_free(channel)
             raise LibsshChannelException("Failed to open_session: [{0}]".format(rc))