Debugging nova — a small illustration with pdb

The other day, I was testing non-ascii characters with nova commands. I started with adding an ssh key to nova with a non-ascii name, not realizing key-pair names won’t support them. This is how we ended up figuring it after a small debugging session with Flavio.

First, let’s try to add an ssh keypair & attempt to provide a non-ascii name:

$ nova keypair-add --pub_key ~/.ssh/ äçë
ERROR: Keypair data is invalid (HTTP 400) (Request-ID: req-5a1a2683-d658-417b-a805-91346d5202dc)

Note – supplying --debug provides more info on the stdout. However, we’re interested to find out the root cause.

Let’s find out why the key-pair data is invalid by stepping through code using pdb. To start, stop the nova-api service, and ensure none of its processes are running :

$ service openstack-nova-api stop
$ ps aux | grep -i nova-api

Ensure to have the config attribute osapi_compute_workers set to 1 — I had it set to 5 while testing rhbz-81349 — so they won’t run in parallel, in /etc/nova/nova.conf :

$ grep osapi_compute_workers \
  /etc/nova/nova.conf | grep -v ^$ | grep -v ^# \

Let’s set the pdb trace — import pdb; pdb.set_trace() — in the source code to break into the debugger — /usr/lib/python2.6/site-packages/nova/compute/

Code context — I’m setting the breakpoint just after the self-explanatory function _validate_keypair_name :

class KeypairAPI(base.Base):
    """Sub-set of the Compute Manager API for managing key pairs."""
    def __init__(self, **kwargs):
        super(KeypairAPI, self).__init__(**kwargs)

    def _validate_keypair_name(self, context, user_id, key_name):

        import pdb; pdb.set_trace()       

        safechars = "_- " + string.digits + string.ascii_letters
        clean_value = "".join(x for x in key_name if x in safechars)

Now, invoke the nova-api service manually:

$ /usr/bin/nova-api --config-file \
  /usr/share/nova/nova-dist.conf --config-file \
  /etc/nova/nova.conf --logfile /var/log/nova/api.log

From a different terminal, again, try to create a keypair with non-ascii characters — this time, it’ll let us break into the debugger

$ nova keypair-add --pub-key ~/.ssh/ äçë

Now, from shell from where we invoked the nova-api server daemon, a debugger Pdb would be waiting, to step through the code, do:

  • type n — which continues execution until the next line in the current function is reached
  • print the value safechars — indicates what values are considered safe — which is what we’re trying to find out
  • quit the debugger
2013-03-26 19:56:26 71243 DEBUG nova.utils [-] Reloading cached file /etc/nova/policy.json read_cached_file /usr/lib/python2.6/site-packages/nova/uti1
2013-03-26 19:56:27 INFO nova.api.openstack.wsgi [req-9ef1fd31-0cec-4465-894a-f1ae0fa9a77e 320ce46de7e24a75a7ff8906d7355ff7 57ff99aae24b4035b52177a72s
> /usr/lib/python2.6/site-packages/nova/compute/
-> safechars = "_- " + string.digits + string.ascii_letters
(Pdb)  n
> /usr/lib/python2.6/site-packages/nova/compute/
-> clean_value = "".join(x for x in key_name if x in safechars)
(Pdb)  safechars
 '_- 0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ' 
(Pdb)  quit

So, the above indicates non-ascii characters are not allowed for key-pair names.

Clean up

Remove the debugging fragment from the source file. Start the nova-api server daemon gracefully —

 $ service openstack-nova-api start 


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3 responses to “Debugging nova — a small illustration with pdb

  1. Pingback: OpenStack Community Weekly Newsletter (Mar 8-22) » The OpenStack Blog

  2. Good to see you digging into the code to test such features. You have interesting writeups about your tryst with openstack. Looking fwd to reading more of them :)

  3. Kashyap, thank you so much for the article really helped me out to troubleshoot 404 on nova command!

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