Refactor code in ProcessRepliesIfAny() to match upstream.

The reason we needed the FIXME pq_getmessage() call, marked with the
FIXME comment, was that we were missing the pq_getmessage() call from
ProcessStandbyMessage(), that the corresponding upstream version, at the
point that we're caught up in the merge, had. I believe the reason it was
missing from ProcessStandbyMessage() was that we had earlier backported
upstream commit cd19848bd55. That commit removed the pq_getmessage() call
from ProcessStandbyMessage(), and added one in ProcessRepliesIfAny(),
instead.

Clarify this by changing the code to match upstream commit cd19848bd55.
(Except that we don't have pq_startmsgread() yet, that will arrive when
we merge the rest of commit cd19848bd55.)

src/backend/replication/walsender.c

@@ -740,6 +740,16 @@ ProcessRepliesIfAny(void)
 			break;
 		}
 
+		/* Read the message contents */
+		resetStringInfo(&reply_message);
+		if (pq_getmessage(&reply_message, 0))
+		{
+			ereport(COMMERROR,
+					(errcode(ERRCODE_PROTOCOL_VIOLATION),
+					 errmsg("unexpected EOF on standby connection")));
+			proc_exit(0);
+		}
+
 		/*
 		 * If we already received a CopyDone from the frontend, the frontend
 		 * should not send us anything until we've closed our end of the COPY.
@@ -760,23 +770,6 @@ ProcessRepliesIfAny(void)
 				 * 'd' means a standby reply wrapped in a CopyData packet.
 				 */
 			case 'd':
-				/*
-				 * GPDB_93_MERGE_FIXME: invoking pq_getmessage() seems to be
-				 * necessary here.  It avoids failures during walreceiver reply
-				 * processing in ProcessStandbyMessage().  If pq_getmessage()
-				 * is not called, reply_message is not populated with what's
-				 * received on the libpq connection and we get "no data left in
-				 * message" error.
-				 */
-				/* consume the CopyData message */
-				resetStringInfo(&reply_message);
-				if (pq_getmessage(&reply_message, 0))
-				{
-					ereport(COMMERROR,
-							(errcode(ERRCODE_PROTOCOL_VIOLATION),
-							 errmsg("unexpected EOF on standby connection")));
-					proc_exit(0);
-				}
 				ProcessStandbyMessage();
 				received = true;
 				break;
@@ -792,16 +785,6 @@ ProcessRepliesIfAny(void)
 					streamingDoneSending = true;
 				}
 
-				/* consume the CopyData message */
-				resetStringInfo(&reply_message);
-				if (pq_getmessage(&reply_message, 0))
-				{
-					ereport(COMMERROR,
-							(errcode(ERRCODE_PROTOCOL_VIOLATION),
-							 errmsg("unexpected EOF on standby connection")));
-					proc_exit(0);
-				}
-
 				streamingDoneReceiving = true;
 				received = true;
 				break;