Re: [PATCH] postgres_fdw connection caching - cause remote sessions linger till the local session exit

On Mon, Dec 14, 2020, 9:47 PM Bharath Rupireddy
<bharath.rupireddyforpostgres@gmail.com> wrote:
>
> On Mon, Dec 14, 2020 at 8:03 PM Fujii Masao <masao.fujii@oss.nttdata.com> wrote:
> > > We will not output if the invalidated entry has no active connection[2], so if we fix the connection leak issue
withthe above discussed fix i.e closing all the invalidated connections at the end of next xact, there are less chances
thatwe will output invalidated entries in the postgres_fdw_get_connections() output. Only case we may show up
invalidatedconnections(which have active connections entry->conn) in the postgres_fdw_get_connections() output is as
follows:
> > >
> > > 1) say we have few cached active connections exists in session 1
> > > 2) drop the user mapping (in another session) associated with any of the cached connections to make that entry
invalid
> > > 3) run select * from postgres_fdw_get_connections(); in session 1.  At the start of the xact, the invalidation
messagegets processed and the corresponding entry gets marked as invalid. If we allow invalid connections (that have
entry->conn)to show up in the output, then we show them in the result of the query. At the end of xact, we close these
invalidconnections, in this case, user might think that he still have invalid connections active. 
> >
> > What about the case where the transaction started at the above 1) at session 1, and postgres_fdw_get_connections()
inthe above 3) is called within that transaction at session 1? In this case, postgres_fdw_get_connections() can return
eveninvalidated connections? 
>
> In that case, since the user mapping would have been dropped in
> another session and we are in the middle of a txn in session 1, the
> entries would not get marked as invalid until the invalidation message
> gets processed by the session 1 which may happen if the session 1
> opens a sub txn, if not then for postgres_fdw_get_connections() the
> entries will still be active as they would not have been marked as
> invalid yet and postgres_fdw_get_connections() would return them in
> the output.

One more point for the above scenario: if the user mapping is dropped
in another session, then cache lookup for that entry in the
postgres_fdw_get_connections() returns a null tuple which I plan to
not throw an error, but just to skip in that case and continue. But if
the user mapping is not dropped in another session but altered, then
postgres_fdw_get_connections() still can show that in the output.

With Regards,
Bharath Rupireddy.
EnterpriseDB: http://www.enterprisedb.com