[HACKERS] 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
therequest.
 

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