I'm unsure if this is a bug or simply a change, but it needs to be documented or fixed as certain tests drivers are running have started failing.

The gist of this is that when 3.0 exhausted the documents from a query, it didn't return a cursorId. This has changed in 3.1.x where a cursor will be returned even when there are no more documents to iterate. This prompts a driver to issue a kill cursor.

I grabbed the wire bits to make this easier to understand.

MessageLength       95
RequestID           XX
ResponseTo          0
OpCode              OP_QUERY(2004)
Flags               0
FullCollectionName  test.foo
NumberToSkip        0
NumberToReturn      2
Query               { $query: { _id: { $gt: 1 } } }

When running against server 3.0.5, the OP_REPLY is the following:

RequestID       XX
MessageLength   78
ResponseTo      XX
OpCode          OP_REPLY(1)
ResponseFlags   AwaitCapable(8)
CursorID        0
StartingFrom    0
NumberReturned  2
Documents       [{ _id: 2, x: 22 },{ _id: 3, x: 33 }]

However, when running against 3.1.7, the OP_REPLY is the following:

RequestID       XX
MessageLength   78
ResponseTo      XX
OpCode          OP_REPLY(1)
ResponseFlags   AwaitCapable(8)
CursorID        14624793327
StartingFrom    0
NumberReturned  2
Documents       [{ _id: 2, x: 22 },{ _id: 3, x: 33 }]

Notice that the CursorID is non-zero in the 3.1 code.

When running this from the shell, the shell does not issue a kill cursor, so if you are looking at the wire from the shell, that won't be there. I'm unsure if that's a bug in the shell or not.