BUG #14891: Old cancel request presented by pgbouncer honored after skipping a query.

The following bug has been logged on the website:

Bug reference: 14891
Logged by: Skarsol
Email address: postgresql(at)skarsol(dot)com
PostgreSQL version: 9.6.3
Operating system: Linux 4.4.8-hardened-r1 #4 SMP Mon Jun 12
Description:

This might be a symptom of the issue discussed in the ToDo "Changes to make
cancellations more reliable and more secure" but as it is related to the
pgbouncer bug I've opened at
https://github.com/pgbouncer/pgbouncer/issues/245 I figured I'd post it over
here just to make sure.

As the last step of this bug, pgbouncer 1.7.2 presents a cancel request to
postgres 9.6.3. This request targets pid 29330 which is connected to
pgbouncer on port 33024. That pid then accepts a new query, returns a result
set, accepts another new query, and then cancels that one out.

Expected behavior would have been for either no cancel (as that pid was
between queries at the time) or to cancel the first query. Cancelling the
2nd query is just weird (to me).

I have no idea how much of this is related to whatever pgbouncer is doing to
delay the cancel in the first place before presenting it to postgres.

I'm aware that we're 2 minor versions behind, but I don't see anything that
seems relevant to this in the changelogs.

Image of the relevant wireshark display at
https://user-images.githubusercontent.com/1915152/32578433-d5d4a71c-c4a2-11e7-9d25-f59d5afbb06b.jpg