How Darcs interacts with SSH

TODO: this page is very out of date (darcs transfer-mode could be described here in more detail, for example), but there are things in here which may be useful for long term history-of-darcs knowledge (for example, understanding our choices behind some SSH-related stuff).

Notes from the Darcs 1 era


1.0.9rc2 works fine with PuTTY on Windows XP provided that you use public key authentication via pageant.

command uses 1.0.9rc2 putty pageant 1.0.9rc putty w/out pageant
get scp,sftp works
push scp,ssh works
pull scp works
put sftp,ssh works
send scp works works


Note: the following results are for 1.0.9rc2, openssh 4.5 on MacOS X (Intel)

command uses –ssh-cm –no-ssh-cm sshagent –no-ssh-cm, sshagent
get scp,sftp works works works works
push scp,ssh works works works works
pull scp works works works works
put sftp,ssh works works works works
send scp works NO! [1] works must fix

[1] –no-ssh-cm not supported (must fix)

General notes

Running just the SSH tests

cd tests

See also the SSH tester script in

  • Seems to be scp (or sftp) related in that if I replace it with just a generic program like ping, then everything goes fine


Putty has this bad habit of using stdin/stdout for asking for passwords. It’s not a problem if you’re using pageant. Note that Pagaent does not appear to be available outside of windows, so no using darcs/PuTTY under Unix! (then again, you have OpenSSH anyway, so why worry?)

What happens without pageant (Windows):

  • under 1.0.9rc2 - psftp (get) will continously ask you for your password and just die. This is because we are using stdin for batch-controlling psftp (this was added in 1.0.9rc2). This works fine when you are using PuTTY+pageant (no passwords to read), or if you are using OpenSSH (reads directly from the tty, not from stdin… cool!), but it breaks psftp sans pageant because it will try to read bits of batch file for the password
  • under 1.0.9rc2 - if you patch darcs to use the -b argument instead of feeding the batchfile in, darcs+PuTTY (sans pageant) just hangs
  • under 1.0.5 - you get the same hanging behaviour (whew! this means the hanging is not a regression that I introduced). No ideas yet what causes the hanging

PuTTY under Unix:

  • Interestingly, pscp continuously asks for passwords (pscp is called before psftp in darcs get) and then dies. Pscp is much better behaved in Windows, possibly because of how we have redirection set up, but then again psftp seems to get us on both platforms.

Weird hanging

A user has reported subprocess hanging with 1.0.9rc2. He says it affects large repositories only. The user is on MacIntel, and says that it’s highly reproducible. I wonder if it’s just macs, or if there is a more general problem.

2010-06: After much investigation, we eventually discovered (around 2007-10) that this was due to an OpenSSH bug. See issue437 for more details.


We still have some unpleasant issues with darcs:

  1. creating the darcs-ssh in weird places (temp dir creation seems a little too loose to me; surely we can improve on it)
  2. not exiting the control master cleanly in all cases (if you Ctrl-C out of darcs, for example, you might have a loose cm process lying around)
  3. not cleaning up the darcs-ssh directory on exit (even a clean exit)

Is creating the directory in TEMPDIR neccesarily the best idea?