Add missing unref implementations for replset, mongos

[zbjornson]

Fixes NODE-728 (this.s.topology.unref is not a function when calling db.unref())

[zbjornson]

@christkv have you had a chance to review this? We're using a patch in our code to add these missing methods right now.

[christkv]

I'm planning too, however it's not trivial to check if it works correctly due to the replicaset and mongos topologies so it will take a little before this potentially is in. But I hope to have it out in the next minor release end of month.

[christkv]

If your goal is to just exit when the pool of operations in progress is done then you should call close as the pools themselves will drain all operations before closing. Unref does not really work correctly with topologies other than a single instance.

[zbjornson]

The goal is to let the process exit but still let new mongo operations be performed (e.g. there may be an HTTP server socket keeping the process alive that needs to perform queries, so we can't call close on the db). We've been using a patch I made to unref a replset in production for about a month and it's working perfectly. Could you elaborate on how it doesn't work correctly?

[christkv]

The problem is caused by the monitoring process for replicasets, they will keep opening new sockets potentially and that means there will be sockets that are not unreffed alive and thus the process will never exit.

[zbjornson]

Ah. The same dynamic connection behavior exists for single servers too, right? Guess that particular situation hasn't occurred for us or we haven't noticed.

Maybe this could be amped up, so that once unref is called, all new connections are also unref'ed?

[christkv]

Yes it would have to be a new state in the server.js and the driver needs to shut down monitoring. Then move to destroyed state once the connection pool is empty. I'll have a look at this next week and see if it can make the next 2.2.22 release.