reactivemongo.api.Cursor#collect maxDocs not respected

[João Ferreira]

Hi

After upgrading from reactivemongo 0.11.14 to 0.12.5 I noticed that the parameter maxDocs in reactivemongo.api.Cursor#collect method is not respected, because more documents that the requested can be returned.

I have simple reproduction test case in this branch https://github.com/jtjeferreira/ReactiveMongo/tree/maxDocs-0.12, where I increased the size of the inserted documents, modified the test "get 10 first docs", and commented the other tests so it is easier to reproduce.

"get 10000 first docs" in { implicit ee: EE =>
  c.find(matchAll("cursorspec1")).cursor().collect[List](10000).
    map(_.size) aka "result size" must beEqualTo(10000).await(1, timeout)
}

fails with 

[error] x get 10000 first docs
[error]  '12637' is not equal to '10000' (Cursor1Spec.scala:122)

and these are the requests made to the DB

2017-10-04T19:25:24.996+0000 I COMMAND  [conn1196] query specs2-test-reactivemongo.cursorspec1066531655 query: { $query: { cursorspec1: { $exists: false } }, $readPreference: { mode: "primary" } } planSummary: COLLSCAN cursorid:598621171031 ntoreturn:10000 ntoskip:0 keysExamined:0 docsExamined:6320 numYields:49 nreturned:6319 reslen:16775855 locks:{ Global: { acquireCount: { r: 100 } }, Database: { acquireCount: { r: 50 } }, Collection: { acquireCount: { r: 50 } } } 9ms
2017-10-04T19:25:25.557+0000 I COMMAND  [conn1196] getmore specs2-test-reactivemongo.cursorspec1066531655 query: { cursorspec1: { $exists: false } } planSummary: COLLSCAN cursorid:598621171031 ntoreturn:10000 keysExamined:0 docsExamined:6318 numYields:49 nreturned:6318 reslen:16776947 locks:{ Global: { acquireCount: { r: 100 } }, Database: { acquireCount: { r: 50 } }, Collection: { acquireCount: { r: 50 } } } 5ms

I have another branch with the same test case against the 0.11.x branch in https://github.com/jtjeferreira/ReactiveMongo/tree/maxDocs-0.11 where the test passes successfully, and the requests made against the DB are

017-10-04T19:43:10.839+0000 I COMMAND  [conn25] query specs2-test-reactivemongo.cursorspec690285554 query: { $query: { cursorspec1: { $exists: false } }, $readPreference: { mode: "primary" } } planSummary: COLLSCAN cursorid:24609772243 ntoreturn:10000 ntoskip:0 keysExamined:0 docsExamined:6320 numYields:49 nreturned:6319 reslen:16775855 locks:{ Global: { acquireCount: { r: 100 } }, Database: { acquireCount: { r: 50 } }, Collection: { acquireCount: { r: 50 } } } 17ms
2017-10-04T19:43:11.328+0000 I COMMAND  [conn25] getmore specs2-test-reactivemongo.cursorspec690285554 query: { cursorspec1: { $exists: false } } planSummary: COLLSCAN cursorid:24609772243 ntoreturn:3681 keysExamined:0 docsExamined:3680 numYields:28 nreturned:3681 reslen:9773075 locks:{ Global: { acquireCount: { r: 58 } }, Database: { acquireCount: { r: 29 } }, Collection: { acquireCount: { r: 29 } } } 3ms

The difference between the two seems the ntoreturn value in the 2nd request (10000 vs 3681) 

I will now try to fix the issue, but if you have any starting pointers it would be helpful

Thanks again for this great project

Cheers

João

[João Ferreira]

Created PR with fix https://github.com/ReactiveMongo/ReactiveMongo/pull/707

[Cédric Chantepie]

Will have a look