Use PqMsg_* macros in more places.

Commit f4b54e1, which introduced macros for protocol characters,
missed updating a few places.  It also did not introduce macros for
messages sent from parallel workers to their leader processes.
This commit adds a new section in protocol.h for those.

Author: Aleksander Alekseev
Discussion: https://postgr.es/m/CAJ7c6TNTd09AZq8tGaHS3LDyH_CCnpv0oOz2wN1dGe8zekxrdQ%40mail.gmail.com
Backpatch-through: 17

Author: nathan-bossart

src/backend/access/common/printtup.c

@@ -17,6 +17,7 @@
 
 #include "access/printtup.h"
 #include "libpq/pqformat.h"
+#include "libpq/protocol.h"
 #include "tcop/pquery.h"
 #include "utils/lsyscache.h"
 #include "utils/memdebug.h"
@@ -170,7 +171,7 @@ SendRowDescriptionMessage(StringInfo buf, TupleDesc typeinfo,
 	ListCell   *tlist_item = list_head(targetlist);
 
 	/* tuple descriptor message type */
-	pq_beginmessage_reuse(buf, 'T');
+	pq_beginmessage_reuse(buf, PqMsg_RowDescription);
 	/* # of attrs in tuples */
 	pq_sendint16(buf, natts);
 
@@ -322,7 +323,7 @@ printtup(TupleTableSlot *slot, DestReceiver *self)
 	/*
 	 * Prepare a DataRow message (note buffer is in per-query context)
 	 */
-	pq_beginmessage_reuse(buf, 'D');
+	pq_beginmessage_reuse(buf, PqMsg_DataRow);
 
 	pq_sendint16(buf, natts);
 

src/backend/commands/explain.c

@@ -21,6 +21,7 @@
 #include "foreign/fdwapi.h"
 #include "jit/jit.h"
 #include "libpq/pqformat.h"
+#include "libpq/protocol.h"
 #include "nodes/extensible.h"
 #include "nodes/makefuncs.h"
 #include "nodes/nodeFuncs.h"
@@ -5497,7 +5498,7 @@ serializeAnalyzeReceive(TupleTableSlot *slot, DestReceiver *self)
 	 * Note that we fill a StringInfo buffer the same as printtup() does, so
 	 * as to capture the costs of manipulating the strings accurately.
 	 */
-	pq_beginmessage_reuse(buf, 'D');
+	pq_beginmessage_reuse(buf, PqMsg_DataRow);
 
 	pq_sendint16(buf, natts);
 

src/backend/replication/walsender.c

@@ -695,7 +695,7 @@ UploadManifest(void)
 	ib = CreateIncrementalBackupInfo(mcxt);
 
 	/* Send a CopyInResponse message */
-	pq_beginmessage(&buf, 'G');
+	pq_beginmessage(&buf, PqMsg_CopyInResponse);
 	pq_sendbyte(&buf, 0);
 	pq_sendint16(&buf, 0);
 	pq_endmessage_reuse(&buf);

src/backend/tcop/postgres.c

@@ -2651,8 +2651,7 @@ exec_describe_statement_message(const char *stmt_name)
 	/*
 	 * First describe the parameters...
 	 */
-	pq_beginmessage_reuse(&row_description_buf, 't');	/* parameter description
-														 * message type */
+	pq_beginmessage_reuse(&row_description_buf, PqMsg_ParameterDescription);
 	pq_sendint16(&row_description_buf, psrc->num_params);
 
 	for (int i = 0; i < psrc->num_params; i++)

src/backend/utils/activity/backend_progress.c

@@ -92,7 +92,7 @@ void
 pgstat_progress_parallel_incr_param(int index, int64 incr)
 {
 	/*
-	 * Parallel workers notify a leader through a 'P' protocol message to
+	 * Parallel workers notify a leader through a PqMsg_Progress message to
 	 * update progress, passing the progress index and incremented value.
 	 * Leaders can just call pgstat_progress_incr_param directly.
 	 */
@@ -102,7 +102,7 @@ pgstat_progress_parallel_incr_param(int index, int64 incr)
 
 		initStringInfo(&progress_message);
 
-		pq_beginmessage(&progress_message, 'P');
+		pq_beginmessage(&progress_message, PqMsg_Progress);
 		pq_sendint32(&progress_message, index);
 		pq_sendint64(&progress_message, incr);
 		pq_endmessage(&progress_message);

src/include/libpq/protocol.h

@@ -65,6 +65,10 @@
 #define PqMsg_CopyData				'd'
 
 
+/* These are the codes sent by parallel workers to leader processes. */
+#define PqMsg_Progress              'P'
+
+
 /* These are the authentication request codes sent by the backend. */
 
 #define AUTH_REQ_OK			0	/* User is authenticated  */