Use isMaster.localTime instead of eval

Use isMaster.localTime instead of eval

In mongo 2.4.x (default in saucy), eval requires admin privileges. Replace by
isMaster which does not require admin privileges (but requires mongo 2.2 at
minimum).

Thanks to Gustavo for the help in implementing the fix.

https://code.launchpad.net/~sidnei/juju-core/mongo-2.4-compat/+merge/175653

(do not edit description out of merge proposal)

[sidnei.da.silva, 2013-07-18 19:15:00]

Please take a look.

[thumper, 2013-07-18 22:08:26]

https://codereview.appspot.com/11407044/diff/1/state/presence/presence.go
File state/presence/presence.go (right):

https://codereview.appspot.com/11407044/diff/1/state/presence/presence.go#new...
state/presence/presence.go:618: err := c.Database.Run("isMaster", &isMaster)
I think the command to actually get the time from the database should be
abstracted into a separate function that should first try the "isMaster" trick,
and fall back to the eval if not supported.

That command is then used inside the loop here.

I'd also love a comment as to why we are waiting 5 seconds if you can think of
one.

[sidnei.da.silva, 2013-07-18 22:32:08]

Please take a look.

[thumper, 2013-07-18 23:33:51]

LGTM

[dimitern, 2013-07-19 05:42:29]

LGTM with a minor suggestion.

https://codereview.appspot.com/11407044/diff/6001/state/presence/presence.go
File state/presence/presence.go (right):

https://codereview.appspot.com/11407044/diff/6001/state/presence/presence.go#...
state/presence/presence.go:671: return serverDelay, nil
maybe put a:

if !supportsMasterLocalTime {
    log.Warningf("state/presence: mongodb 2.2 or earlier detected, using eval
for timestamps")
}

or something similar?

[dimitern, 2013-07-19 05:42:29]

LGTM with a minor suggestion.

https://codereview.appspot.com/11407044/diff/6001/state/presence/presence.go
File state/presence/presence.go (right):

https://codereview.appspot.com/11407044/diff/6001/state/presence/presence.go#...
state/presence/presence.go:671: return serverDelay, nil
maybe put a:

if !supportsMasterLocalTime {
    log.Warningf("state/presence: mongodb 2.2 or earlier detected, using eval
for timestamps")
}

or something similar?