Robert Haas <robertmh...@gmail.com> 于2024年8月13日周二 00:28写道：
> On Thu, Aug 8, 2024 at 10:27 PM Xiaoran Wang <fanfuxiao...@gmail.com> > wrote: > > > Add function 'pq_leave_shm_mq' to allow the process to go > > > back to the previous pq environment. > > > > >. In the code as it currently exists, a parallel worker never has a > > >. connected client, and it talks to a shm_mq instead. So there's no > need > > >. for this. If a backend needs to communicate with both a connected > > > client and also a shm_mq, it probably should not use pqmq but > rather > > > decide explicitly which messages should be sent to the client and > > > which to the shm_mq. Otherwise, it seems hard to avoid possible > loss > > > of protocol sync. > > > > As described above, session B will send a signal to session A, then > > session A handle the signal and send the message into the shm_mq. > > The message is sent by pq protocol. So session A will firstly call > > 'pq_redirect_to_shm_mq' and then call 'pq_leave_shm_mq' to > > continue to do its work. > > In this kind of use case, there is really no reason to use the libpq > protocol at all. You would be better off just using a shm_mq directly, > and then you don't need this patch. See tqueue.c for an example of > such a coding pattern. > Thanks for your reply and suggestion, I will look into that. > > Using pqmq is very error-prone here. In particular, if a backend > unexpectedly hits an ERROR while the direct is in place, the error > will be sent to the other session rather than to the connected client. > This breaks wire protocol synchronization. > Yes, I found this problem too. Between the 'pq_beginmessage' and 'pq_endmessage', any log should not be emitted to the client as it will be sent to the shm_mq instead of client. Such as I sometimes set client_min_messages='debug1' in psql, then it will go totally wrong. It maybe better to firstly write the 'msg' into a StringInfoData, then send the 'msg' by libpq. I agree that it is not good way to communicate between tow sessions. > -- > Robert Haas > EDB: http://www.enterprisedb.com > -- Best regards ! Xiaoran Wang
