In SERVER-32169 we noticed that it's easy to miss bugs in drivers' killCursors implementations. The killCursors command can fail but still report "ok: 1", listing the cursor ids it failed to kill in the "cursorsAlive" array:

> db.runCommand({killCursors: "collection", cursors: [NumberLong(0)]})
{
  ok: 1,
  cursorsNotFound: [NumberLong(0)],
  cursorsKilled: [],
  cursorsAlive: []
}

If your driver already includes tests that prove it really can kill a cursor, then you don't need to do anything. For example, PyMongo can't always kill a cursor with MongoDB 3.7.x due to SERVER-32169, and this was discovered because three of PyMongo's tests fail with MongoDB 3.7.x. If you driver has tests which fail due to SERVER-32169 then you may be able to validate this ticket already. Otherwise, here's a suggested test.

Write a test in each driver that checks we really kill a cursor:

• insert 1000 documents into a collection
• do a "find" on the collection and iterate the cursor past the first document
• store the cursor id in a variable
• assert the cursor id is nonzero
• kill the cursor (explicitly, or by letting it go out of scope, depending on the driver)
• assert that the driver receives a server reply to "killCursors" with "ok: 1", an empty "cursorsNotFound" array, an empty "cursorsAlive" array, an empty "cursorsUnknown" array, and a "cursorsKilled" array with one element equal to the cursor id

Drivers can use Command Monitoring, if they've implemented it, to check the server reply.