If darcs asks darcs transfer-mode for a missing file (say _darcs/format on an
old-fashioned repository), darcs transfer-mode will die and this sort of nasty
thing will happen:

Fri Aug 15 08:41:39 BST 2008: with ssh code.haskell.org: get format
Fri Aug 15 08:41:39 BST 2008: Error reading file remotely:
format: openBinaryFile: does not exist (No such file or directory)
Fri Aug 15 08:41:39 BST 2008: Failed to start ssh connection:
    Error reading file remotely:
format: openBinaryFile: does not exist (No such file or directory)
Fri Aug 15 08:41:39 BST 2008: Severing ssh failed connection to
code.haskell.org:empty/_darcs/format

Ugh! and then we go back to using scp :-(

The workaround is very simple: touch _darcs/format on the remote repository.

Note: as this only affects old-fashioned repositories, a simple solution would
be to put this advice into the upgrade section of the manual

Ok, I just thought of a solution on IRC:

darcs-transfer mode should have a list of files that it knows have the right not
to exist so that it can peacefully accept their non-existence

traps to watch out for are backwards compatibility with older darcs clients,
maybe by returning an empty file?

This was fixed while email to bugs wasn't working.

David

Just noting the resolution: it turns out I had misdiagnosed this bug.  It wasn't
darcs transfer-mode that was dying, but the client end that was prematurely
severing the connection.  This has since been fixed.