Fix several flaky tests (updated)

[Fonger]

Fix test flakes to increase CI pass rate

1. Fix flaky test case for #4014

• Ensure 2dsphere index is created before the query is executed

• Specify spherical: true to use 2dsphere index instead of 2d index

  I don't know why it passes the test without this patch sometimes. This appears to be a bug and should fail every time without this fix.

  Mocha Output

  1) document
       bug fixes
         single embedded docs with $near (gh-4014):
         Uncaught MongoError: error processing query:
           ns=mongoose_test.gh4014
           Tree: GEONEAR  field=geo maxdist=1.79769e+308 isNearSphere=0
     Sort: {}
     Proj: {}
     planner returned error: unable to find index for $geoNear query
       at queryCallback (node_modules/mongodb-core/lib/cursor.js:247:25)
       at node_modules/mongodb-core/lib/connection/pool.js:531:18
       at process._tickCallback (internal/process/next_tick.js:176:11)
  

  See build failure on travis ci - [1] [2] [3]

2. Fix flaky test for #6271 (EDIT: refactored)

• By ensuring the update request with callback is executed

  Mocha Output

  1) Query
         bug fixes
           consistently return query when callback specified (gh-6271):
        AssertionError [ERR_ASSERTION]: 1 == 2
        + expected - actual
        -1
        +2
        
        at Decorator._callFunc (node_modules/empower-core/lib/decorator.js:110:20)
        at Decorator.concreteAssert (node_modules/empower-core/lib/decorator.js:103:17)
        at Function.decoratedAssert [as equal] (node_modules/empower-core/lib/decorate.js:51:30)
        at test/query.test.js:2457:16
        at Generator.next (<anonymous>)
        at onFulfilled (node_modules/co/index.js:65:19)
        at <anonymous>
        at process._tickCallback (internal/process/next_tick.js:188:7)
  

  See build failure on travis-ci

3. Add delay for test case in #4027

• Make sure driver has enough time to give up reconnection and emit reconnectFailed event.

  Mocha Output

    1) connections:
         openUri (gh-5304)
           connection events
             reconnectFailed (gh-4027):
  
        AssertionError [ERR_ASSERTION]: 0 == 1
        + expected - actual
  
        -0
        +1
  
        at Decorator._callFunc (node_modules/empower-core/lib/decorator.js:110:20)
        at Decorator.concreteAssert (node_modules/empower-core/lib/decorator.js:103:17)
        at Function.decoratedAssert [as equal] (node_modules/empower-core/lib/decorate.js:51:30)
        at test/connection.test.js:217:20
  

Miscellaneous

1. Fix duplicate test queries introduced in #6016

• Call Query#exec() to get real Promise to prevent duplicate queries because queries are not promises and we shouldn't mix callback with then()

2. Update MongoDB configuration on Travis

• Test mongodb 3.6.6 on travis-ci (original: 3.6.4)

• Increase mocha timeout when running on travis

• Remove --nopreallocj parameter

  It has typo j and --noprealloc has been deprecated since v2.6 and it's default now.

[Fonger]

@vkarpov15 updated based on your review

I've tested several times on my local development environment and on my travis-ci.
It seems that all of the flaky tests are fixed after this pull request except for timeout error.

Maybe we can write a script to make travis-ci automatically retry the test if the failure reason is timeout?

[vkarpov15]

In general its better to use MyModel.init().then(() => {}) instead of MyModel.on('index'), because with the latter if you register the event listener after the index build is finished, you'll never get a callback.

[Fonger]

Yes it really makes sense! There are similar test codes like this elsewhere and I will try to update it in future fix to make the test more robust.

Although index takes time to build, if we use some sort of index cache in mongoose in the future, or MyModel.on('index') is not placed right after MyModel.model() in the same event loop, that's a potential issue.

[vkarpov15]

Great work, thanks! This should make Travis much more reliable.