Hi,
we have some code* that accepts a list of MongoDB servers (3.6.x), databases, and collections and proceeds to download these. We did this a while ago and it worked just fine. When we tried this a few weeks ago, downloading larger collections often ended with CursorNotFound during iteration. We did not time out, it could happen as soon as a few seconds in. We tried making batch sizes smaller, to no avail (same for no_cursor_timeout and a few other timeout settings).

At first we thought we faced this server issue SERVER-34810, but updating all our MongoDBs to 3.6.6 didn't resolved this. What worked was downgrading PyMongo to 3.5.1. We then proceeded with a git bisect, which revealed this commit to be the offending one.

https://github.com/mongodb/mongo-python-driver/commit/c1ec855060c0d79613f10e984086c6d400ccc312

Sadly, it's quite a large commit, so I'm not sure where to start.

We were not able to replicate this in a dummy environment. My guess is that since we're accessing ~15 collections in ~4 different mongo servers, the driver closes a cursor it believes was an old one (for a given server), but it's actually a current one. If you could help me instrument this, I could give you more diagnostics.

• the code pretty much just creates MongoClient, does a `.find()` and iterates over this cursor and serialises each document into a file