[erlang-questions] Systematic debugging (ssh_sample_cli)

[Brian Candler]

I have been trying to get the ssh_sample_cli application to run. This is
with R11B5 from Ubuntu Hardy.

I copied the ssh_sample_cli.erl to a local directory, and it appears to
compile and start:

  1> c(ssh_sample_cli).
  ./ssh_sample_cli.erl:128: Warning: this expression would cause a 'badarith' exception at run-time
  {ok,ssh_sample_cli}
  2> crypto:start().
  ok
  3> ssh_sample_cli:listen(2222).
  {ok,<0.46.0>}
  4>

However when I try to "ssh -p 2222 localhost help" from another terminal,
the client sees the connection drop, and the erlang shell shows

  =ERROR REPORT==== 20-Aug-2008::11:01:12 ===
  ** Generic server <0.50.0> terminating
  ** Last message in was {'EXIT',<0.47.0>,{error,eacces}}
  ** When Server state == {state,server,
                                 <0.47.0>,
                                 16,
                                 [],
                                 [{<0.51.0>,#Ref<0.0.0.213>}],
                                 0,
                                 [],
                                 [],
                                 undefined}
  ** Reason for termination ==
  ** {error,eacces}

I get a similar error if I try the ssh_sshd example from
http://www.erlang.org/doc/man/ssh_sshd.html

  Eshell V5.5.5  (abort with ^G)
  1> crypto:start().
  ok
  2> ssh_sshd:listen(9999, [{system_dir, "."}]).
  {ok,<0.41.0>}
  3>
  =ERROR REPORT==== 20-Aug-2008::11:06:32 ===
  ** Generic server <0.45.0> terminating
  ** Last message in was {'EXIT',<0.42.0>,{error,enoent}}
  ** When Server state == {state,server,
                                 <0.42.0>,
                                 16,
                                 [],
                                 [{<0.46.0>,#Ref<0.0.0.94>}],
                                 0,
                                 [{system_dir,"."}],
                                 [],
                                 undefined}
  ** Reason for termination ==
  ** {error,enoent}

Now, clearly I've missed some step. The eacces/enoent suggests to me that
it's something to do with the filesystem. (*)

But my question is really this: how should I go about debugging a problem
like this in a systematic way? That is, how do I interpret the error
messages given above, or get more detail? At the moment I cannot see how to
determine which function the error occurred in, nor the filename or line
number.

If I could see which function raised this error, and/or its arguments and in
what context it was called, it would probably be very clear what the problem
was.

Thanks,

Brian.

(*) After reading the ssh_sshd documentation more carefully, I see that
system_dir is where the ssh host key is expected to be found. I have been
able to fix the problem by "ssh-keygen -t dsa -f ssh_host_dsa_key" in the
same dir to make a new ssh host key.

But I still want to learn how to isolate problems like this systematically,
rather than by guesswork. Any pointers to documentation on how to do this
would be appreciated.