is_leader requires juju 1.22.0 or better

[chuckbutler]

The ETCD charm fails with a non committal error message when probing for
is_leader on juju versions <= 1.21, 1.18 is still the default shipping
in universe, and will cause some headaches if users of older juju
versions attempt to use this charm as its leveraging the leader election
components.

There is probably a charm helper for consuming these methods that has a
sane fallback strategy, but for now this will satisfy the requirement of
blocking installation on anything < 1.22.0

Introduces a new dependency 'semver' to perform the version calculation

[mbruzek]

I believe you are calling log() incorrectly here. The message is first and the level is the second argument.

charmhelpers.core.hookenv.log(message, level=None)
    Write a message to the juju log

http://pythonhosted.org/charmhelpers/api/charmhelpers.core.hookenv.html#charmhelpers.core.hookenv.log

[mbruzek]

Hey Chuck,

Thanks for the quick turn around on this issue! I agree that this is the right way to handle this issue, if the charm makes use of a feature that is in a different version of Juju then we should exit.

[chuckbutler]

@mbruzek thanks for the review, i've updated the hookenv.log statement

[mbruzek]

Hey Chuck,
I tried to figure out when the is_leader feature landed, and as Marco pointed out it is not recommended to filter on versions rather check for that feature instead.

Could we re-factor this to call "is_leader" and if the command does not exist by exception or return code then just error out with the same message? This would prevent having to import a new package.

They do something similar here:
http://bazaar.launchpad.net/~charm-helpers/charm-helpers/devel/view/head:/charmhelpers/core/hookenv.py#L741

What do you think about that?

[marcoceppi]

I'm a +1 to testing for features instead of trying to make assumptions. You can easily modify the version method to just execute is_leader and capture OSError output. There could be situations where in later versions of juju is_leader is depreciated or removed. There is also the possibility that the juju version format changes (as it has several times already)

[chuckbutler]

@mbruzek @marcoceppi I refactored this to favor a which is_leader sniff to determine if is_leader is available. It's still a predicate check, and encapsulated with the same OSERROR diaper during execution.

LMK if there is more that needs to be done here.

[marcoceppi]

I'd remove the entire try block here, since OSError will simply raise just like any other exception. There really isn't any exception handling here. You can also simplify the if block with

return ret == 0

Finally, it appears that charmhelpers has some way of translating whether or not is-leader is available. Doing this

from charmhelpers.core.hookenv import is_leader

try:
    is_leader()
except NotImplementedError:
    print("panic message")

would achieve the same goal.

[marcoceppi]

I had some more pedantic comments, but this is better IMO as is

[marcoceppi]

LGTM +1

[mbruzek]

+1 LGTM