Using OpenSSH with bzr

One of the transports available in bzr is sftp. This is implemented using the Paramiko SSH and SFTP library. Unfortunately there are a few issues I experienced with the code:

  • Since it is an independent implementation of SSH, none of my OpenSSH settings in ~/.ssh/config were recognised. The particular options I rely on include:
    1. User: when the remote username doesn’t match my local one. One less thing to remember when connecting to a remote machine.
    2. IdentityFile: use different keys to access different machines.
    3. ProxyCommand: access work machines that are behind the firewall.
  • Paramiko does not currently support SSH compression. This is a real pain for larger trees.

The easiest way to fix all these problems would be to use OpenSSH directly, so wrote a small plugin to do so. I decided to follow the model used to do this in gnome-vfs and Bazaar 1.x: communicate with an ssh subprocess via pipes and implement the SFTP protocol internally.

Since SFTP is layered fairly cleanly on top of SSH, and the paramiko code was also quite modular, it was possible to use the paramiko SFTP implementation with openssh. The result is a small plugin that monkey-patches the existing SFTP transport:

Just copy into the ~/.bazaar/plugins directory, and use bzr as normal. The compression seems to make a noticable difference to performance, but it should be possible to improve things further with a pipelined SFTP client implementation.

Of course, the biggest performance optimisation will probably come from the smart server, when that is implemented.

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