It's not safe to return the pinned connection to the pool from within
Cursor.del because the Pool's lock may be held by a python thread
while the cyclic garbage collector runs. Instead we send the cursor
cleanup request to the client's background thread. The thread will
send killCursors on the pinned socket and then return the socket to
the pool.
Also fixed a similar bug when garbage collecting a pinned session.
Skip test for symbol type which pymongo converts to string.
Fix {} comparison with RawBSONDocument in command events.
Add support for $$type assertions.
Nicer message in check_events.
Support errorContains with empty string.
Move custom data files to custom/.
Add retryReads URI option that defaults to True.
Supported read operations will be retried once after transient
network, election, and shutdown errors on MongoDB 3.6+.
Supported operations are:
listCollections, listIndexes, and listDatabases
distinct
count, estimated_document_count, count_documents
aggregate (not including $out)
find (only for the initial find command, getMore commands are not
retried).
ChangeStreams: watch (initial aggregate command).
GridFS read APIs.
Test changes:
Add retryable reads spec test runner.
Disable retryable reads in network error tests.
