pgwire: multiple active result sets (portals) not supported

[jordanlewis]

CockroachDB does not currently support multiple active pgwire portals on the same session. This means that you can't perform the following sequence of events:

1. open portal for query a, which will eventually return 1000 rows
2. ask for 500 rows of query a
3. open portal for query b
4. ask for 500 rows of query b
5. ask for 500 rows of query a

This sequence will fail at step 3, unless the user closes the portal for query a first, in which case this sequence will fail at step 5.

Jira issue: CRDB-5559
Epic: CRDB-17622

[rafiss]

The pgjcbc test org.postgresql.test.jdbc2.CursorFetchTest.testGetRow used to fail because of this lack of support, but that test started passing. I don't know why. #41366

[imalsogreg]

We (myself and @robo-corg) hit an edge-case related to this issue. When we use the rust sqlx postgres bindings, queries run inside of transactions trigger the "multiple portals" issue. This happens even when we run just a single query, which we think creates just one portal.

Here's a wireshark/tcpdump of a session with sqlx that exhibits this behavior. We can attach the .pcap file if it's helpful.