Re: application_name backend (not) reporting back to the client : pgbouncer, PgSQL 16.1, pgbouncer 1.21.0

Achilleas Mantzios <a(dot)mantzios(at)cloud(dot)gatewaynet(dot)com> writes:
> the trick is to have "application_name=" with no value.  This causes the
> server to not set application_name initially.

No, that sets it to an empty string, while application_name='' sets
it to two single quotes, according to the testing I did. In either
case, that value will be reported to the client as the active value
during connection start. Then the subsequent SET causes a new
report, but (since v14) only if the value being set is different.

> Please check the scenario

> Client C1 connects to S1, sets application_name. BEGINS, COMMITS, the
> server gets freed to server the next client.

> Client C2 connects to the same S1, sets application_name, and gets no
> report back. So it stays with application_name empty. Then this breaks
> the application.

> How could pgbouncer prevent this from happening ?

pgbouncer would have to regurgitate the latest ParameterStatus
messages to the new client (during its faked-up initial connection
handshake) to ensure everything's synchronized correctly.
If for some reason it's not doing that, that would be a pretty
serious bug if you ask me, since clients might be relying on
hearing the truth about settings like client_encoding. Since
the whole point of pgbouncer is that the backend doesn't know
a new client has been slotted in, it's not clear to me how or
why the backend should solve this.

regards, tom lane