walsender termination error messages worse in v10

Hi,

commit 7c4f52409a8c7d85ed169bbbc1f6092274d03920
Author: Peter Eisentraut <peter_e@gmx.net>
Date:   2017-03-23 08:36:36 -0400

    Logical replication support for initial data copy

made walreceiver emit worse messages in v10 than before when the master
gets shut down.  Before 10 you'll get something like:

2017-06-02 11:46:07.173 PDT [16143][] LOG:  replication terminated by primary server
2017-06-02 11:46:07.173 PDT [16143][] DETAIL:  End of WAL reached on timeline 1 at 0/1B7FB8C8.
2017-06-02 11:46:07.173 PDT [16143][] FATAL:  could not send end-of-streaming message to primary: no COPY in progress

the last bit is a bit superflous, but it still kinda makes sense.  Now
you get:
2017-06-02 11:47:09.362 PDT [17061][] FATAL:  unexpected result after CommandComplete: server closed the connection unexpectedly
                This probably means the server terminated abnormally
                before or while processing the request.

the reason is that

static int
libpqrcv_receive(char **buffer, pgsocket *wait_fd)
{

previously did:
    if (rawlen == -1)           /* end-of-streaming or error */
    {
        PGresult   *res;

        res = PQgetResult(streamConn);
        if (PQresultStatus(res) == PGRES_COMMAND_OK ||
            PQresultStatus(res) == PGRES_COPY_IN)
        {
            PQclear(res);
            return -1;
        }
        else
        {
            PQclear(res);
            ereport(ERROR,
                    (errmsg("could not receive data from WAL stream: %s",
                            PQerrorMessage(streamConn))));
        }
    }

and now does

    if (rawlen == -1)           /* end-of-streaming or error */
    {
        PGresult   *res;

        res = PQgetResult(conn->streamConn);
        if (PQresultStatus(res) == PGRES_COMMAND_OK)
        {
            PQclear(res);

            /* Verify that there are no more results */
            res = PQgetResult(conn->streamConn);
            if (res != NULL)
                ereport(ERROR,
                        (errmsg("unexpected result after CommandComplete: %s",
                                PQerrorMessage(conn->streamConn))));
            return -1;
        }
        else if (PQresultStatus(res) == PGRES_COPY_IN)
        {
            PQclear(res);
            return -1;
        }
        else
        {
            PQclear(res);
            ereport(ERROR,
                    (errmsg("could not receive data from WAL stream: %s",
                            pchomp(PQerrorMessage(conn->streamConn)))));
        }
    }

note the new /* Verify that there are no more results */ bit.

I don't understand why the new block is there, nor does the commit
message explain it.


Greetings,

Andres Freund