Symptom

Querying a viewful timeseries collection, on a sharded cluster, while a concurrent rename is targeting its main namespace may return an empty list of documents when the queried namespace has always had documents.

const collT = db.createCollection("collT", {timeseries:{timeField:"t"}});
const collO = db.createCollection("collO");

// Inserting documents to both collections so that none of them are empty.
collO.insert([{x:1},{x:2},{x:3}]);
collT.insert(timeseriesDocs);

// Run rename 'collO' to 'collT' in a parallell thread:
const waitForParallelShell = startParallelShell(function() {
    db.adminCommand({renameCollection:"collO", to: "collT", dropTarget: true});
});


// The 'docs' variable can be empty even if neither of the two collections is empty.
const docs = collT.find().toArray();

This is a transitory behavior, meaning that the following queries will get correct results.

Sequence of events that led to this behavior

1. [Query] The mongos targets first the DBPrimary to resolve the view.
2. [Query] The mongos receives the resolved namespace (which is db.system.buckets.coll).
3. [Rename] At this moment, 'collO' gets renamed to 'collT'. This operation also drops the buckets collection.
4. [Query] The mongos will fetch the routing information for the buckets namespace, which will be UNSHARDED since the collection has been dropped.
5. [Query] The mongos will propagate the find command to the DBPrimary shard, which will return an empty list of documents since the buckets collection doesn't exist.

Summary

This bug is a consequence of the extra DBPrimary round trip done when querying viewful timeseries. Hence, it's been there since v5.0 when timeseries were introduced.

Proposal solution

A possible solution is to use the same method that write commands use to target a viewful timeseries collection. This involves checking whether routing information for the underlying bucket collection exists. If it does, the namespace and command are automatically translated on the mongos, without requiring a round-trip to the DBPrimary.