Re: Question about WalSndWriteData

On 21.03.2018 04:50, Peter Eisentraut wrote:
> On 3/16/18 12:08, Konstantin Knizhnik wrote:
>> pq_putmessage_noblock copies data from ctx->out buffer to libpq buffers.
>> After it we write timestamp to ctx->out buffer.
>> And comments says that we should do it "as late as possible".
>> But this timestamp is not included in the copy data packet which is
>> already copied to libpq connection buffer.
> There is a pq_flush_if_writable() right after this that will write out
> the rest of ctx->out.
>
Sorry, But PQ_flush_if_writable calls socket_flush_if_writable (or
mq_flush_if_writable).
This function flushes pqlib connection buffer, i.e. PqSendBuffer.
This buffer has no relation to ctx->out_buffer, where timestamp is written.

The obvious fix is to move assignment of timestamp prior to
pq_putmessage_noblock:

WalSndWriteData(LogicalDecodingContext *ctx, XLogRecPtr lsn,
TransactionId xid,
                bool last_write)
{
    TimestampTz    now;

    /*
     * Fill the send timestamp last, so that it is taken as late as
possible.
     * This is somewhat ugly, but the protocol is set as it's already
used for
     * several releases by streaming physical replication.
     */
    resetStringInfo(&tmpbuf);
    now = GetCurrentTimestamp();
    pq_sendint64(&tmpbuf, now);
    memcpy(&ctx->out->data[1 + sizeof(int64) + sizeof(int64)],
           tmpbuf.data, sizeof(int64));

    /* output previously gathered data in a CopyData packet */
    pq_putmessage_noblock('d', ctx->out->data, ctx->out->len);

--
Konstantin Knizhnik
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