PERL-799: parse Logical session timeout from isMaster

[TBSliver]

Includes a skipped test spec that I am unsure if applies to this driver, with the PossiblePrimary option.

[xdg]

I think this logic has turned out (a) more complex than it needs to be and (b) possibly incorrect as a result.

For one, by not checking all server descriptions for a minimum (or null), the value becomes a one-way ratchet.

I also find having both "defined_ls_timeout_minutes" and "logical_session_timeout_minutes" to add a lot of complexity.

I suggest having a single "logical_session_timeout_minutes" attribute of type Num and assign it -1 (as a signaling value) iff the ismaster response has no logicalSessionTimeoutMinutes value.

I also suggest adding a "is_data_bearing" method on Server that returns true for Standalone, RSPrimary, RSSecondary and Mongos types.

Then, the "update topology logical session timeout minutes" routine becomes relatively straightforward and easy to understand:

@data_bearing_servers = grep { $_->is_data_bearing } @all_servers;
$new_ls_timeout = @data_bearing_servers
    ? ( min map { $_->logical_session_timeout_minutes } @data_bearing_servers  )
    : -1;

(It might be slightly more complicated, but I think that's enough to give you the conceptual gist of it.)

Topology would also have to use "-1" as a signal value.

[xdg]

Updated my comment with a better example.

[TBSliver]

Right have updated the logic - I think I may have misunderstood part of the spec originally.

Note that instead of using a -1 for a signalling value, I've used undef - as this is the value given in the spec, and used in the spec json files, so saves reworking all of those.

I've also added an is_data_bearing boolean to the server as suggested :D

[coveralls]

Coverage decreased (-1.09%) to 82.741% when pulling e2f10f8 on shadow-dot-cat:TBSliver/PERL-799 into e477f53 on mongodb:master.

[xdg]

Looks good with one minor request for a comment

[xdg]

I think $timeout would only be undef here if there were no data-bearing servers, right? If that's right, please add a comment to that effect for clarity.

[xdg]

Awesome! Rebased, squashed and merged!