As a result of PYTHON-799, cursors are added to a queue upon closure and killCursors on their IDs are issued periodically in the background. In cases where an uniterated cursor is queued for closure shortly before termination (for instance, in a signal handler), the process can exit before the next execution of the periodic cleanup thread, leaving open cursors on the server. This can have negative consequences in e.g. a sharded cluster, where open cursors can block chunk migrations. The latter problem is worsened in cases where customers have increased the server-side cursorTimeoutMillis for operational reasons.

A minimal reproduction script is attached, which quite reliably leaves a cursor open on the server after termination as shown below. Tests were conducted with pymongo 3.4.0 on a 3.4.3 replica set and sharded cluster.

> db.serverStatus().metrics.cursor
{
	"timedOut" : NumberLong(0),
	"open" : {
		"noTimeout" : NumberLong(0),
		"pinned" : NumberLong(0),
		"total" : NumberLong(1)
	}
}

Sleeping for a few seconds after closing the cursor avoids this issue, as does adding one final execution of the cleanup function after the join in the periodic executor's exit handler. Registering a custom CursorManager to close the cursors synchronously also prevents orphaned cursors, although I believe that this approach would again be susceptible to PYTHON-799.

Thanks!
Bernard