ZAPI-660 Requests to /vms hang forever which have a "fields" param and match no VMs

[arekinath]

On any "list VMs" request that asks for a "fields" limit, if the query returns no results, we currently hang forever.

I first observed this when asking for an offset >= # of VMs. In this case, the req.vms array is empty. Currently we call Object.keys() on req.vms[0] in this case, which spits an error (since vms[0] is undefined). What we want to do is just return [].

Example of the error message in the vmapi logs:

[2015-06-11T01:19:01.505Z]  INFO: vmapi/api/33158 on 836d01b7-ff64-4717-958d-2bbd0cae17c6:  (req_id=d7a46a50-0fd7-11e5-b667-e3e634d86ba9)
    TypeError: Object.keys called on non-object
        at Function.keys (native)
        at Server.renderVms (/opt/smartdc/vmapi/lib/endpoints/vms.js:66:27)
        at next (/opt/smartdc/vmapi/node_modules/restify/lib/server.js:731:30)
        at f (/opt/smartdc/vmapi/node_modules/restify/node_modules/once/once.js:16:25)
        at /opt/smartdc/vmapi/lib/endpoints/vms.js:219:20
        at EventEmitter.<anonymous> (/opt/smartdc/vmapi/lib/apis/moray.js:522:20)
        at EventEmitter.g (events.js:180:16)
        at EventEmitter.emit (events.js:92:17)
        at EventEmitter.<anonymous> (/opt/smartdc/vmapi/node_modules/moray/lib/objects.js:197:13)
        at EventEmitter.g (events.js:180:16)
    --
    url: /vms?state=active&fields=uuid%2Cstate%2Cdestroyed%2Calias%2Cowner_uuid%2Cnics%2Cserver_uuid&offset=799&limit=799
    --
    params: {
      "state": "active",
      "fields": "uuid,state,destroyed,alias,owner_uuid,nics,server_uuid",
      "offset": "799",
      "limit": "799"
    }

I've attempted to add some tests in vms.test.js to cover this case, but I can't quite figure out how the ordering of tests is meant to work in there (they seem to just be set as keys on the exports object, which shouldn't have any ordering guarantee? but they assume ordering by re-using variables like vmLocation between tests...). Hopefully they work.

This has also been reported on IRC by ep1a in #smartos, on a request with no limit/offset parameters:

$ sdc-vmapi /vms?fields=nics&query=(alias%3Ddoesnotexist)
(hangs forever)

[arekinath]

Merged as cff6bd4