At Thu, 14 Jan 2021 06:46:35 +0000, Li Japin <japi...@hotmail.com> wrote in > > On Jan 14, 2021, at 12:56 PM, Ashutosh Bapat > <ashutosh.bapat....@gmail.com<mailto:ashutosh.bapat....@gmail.com>> wrote: > > Hi Japin, > Thanks for the report. > > I think that comment is correct. It refers to the following code > blocks of XLogSendPhysical() > > 2744 /* > 2745 * OK to read and send the slice. > 2746 */ > 2747 resetStringInfo(&output_message); > 2748 pq_sendbyte(&output_message, 'w'); > 2749 > 2750 pq_sendint64(&output_message, startptr); /* dataStart */ > 2751 pq_sendint64(&output_message, SendRqstPtr); /* walEnd */ > 2752 pq_sendint64(&output_message, 0); /* sendtime, filled in last */ > > 2803 * Fill the send timestamp last, so that it is taken as late > as possible. > 2804 */ > 2805 resetStringInfo(&tmpbuf); > 2806 pq_sendint64(&tmpbuf, GetCurrentTimestamp()); > 2807 memcpy(&output_message.data[1 + sizeof(int64) + sizeof(int64)], > 2808 tmpbuf.data, sizeof(int64)); > 2809 > 2810 pq_putmessage_noblock('d', output_message.data, output_message.len); > > > After a quick search, I found that WalSndPrepareWrite and WalSndWriteData are > always pairs [1]. > IIUC the space of sendtime leave by WalSndPrepareWrite, it always fill out by > WalSndWriteData. > > > WalSndWriteData() also fills the timestamp there but it may not always > be used with WalSndPrepareWrite, at least theoretically. So it's the > XLogSendPhysical() that it's referring to.
The two functions are the body of two logical-decoding API functions. They are assumed to be called in that order. See OutputPluginWrite() for the restriction. The sequence of the two logica-decoding funcitons and the code block in XLogSendPhysical are parallels in (theoretically) different protocols. > Maybe a better way is to separate those two codeblocks into functions > and use it in WalSndPrepareWrite, WalSndWriteData and XLogSendPhysical > appropriately. That way we don't have to add a comment like that and > the sanity of 'w' message is preserved. > > Sure, we can write a separate function to fill out the sendtime. Any > thoughts? So I put -1 since they (physical and logical, not prepare and write) are for distinct protocols. > [1] > src/backend/replication/walsender.c:247:static void > WalSndPrepareWrite(LogicalDecodingContext *ctx, XLogRecPtr lsn, TransactionId > xid, bool last_write); > src/backend/replication/walsender.c:1015: > WalSndPrepareWrite, WalSndWriteData, > src/backend/replication/walsender.c:1176: > WalSndPrepareWrite, WalSndWriteData, > src/backend/replication/walsender.c:1233:WalSndPrepareWrite(LogicalDecodingContext > *ctx, XLogRecPtr lsn, TransactionId xid, bool last_write) > src/backend/replication/walsender.c:1255: * Actually write out data > previously prepared by WalSndPrepareWrite out to regards. -- Kyotaro Horiguchi NTT Open Source Software Center
