Introduce macros for protocol characters.

This commit introduces descriptively-named macros for the
identifiers used in wire protocol messages.  These new macros are
placed in a new header file so that they can be easily used by
third-party code.

Author: Dave Cramer
Reviewed-by: Alvaro Herrera, Tatsuo Ishii, Peter Smith, Robert Haas, Tom Lane, Peter Eisentraut, Michael Paquier
Discussion: https://postgr.es/m/CADK3HHKbBmK-PKf1bPNFoMC%2BoBt%2BpD9PH8h5nvmBQskEHm-Ehw%40mail.gmail.com

src/backend/access/common/printsimple.c

@@ -20,6 +20,7 @@
 
 #include "access/printsimple.h"
 #include "catalog/pg_type.h"
+#include "libpq/protocol.h"
 #include "libpq/pqformat.h"
 #include "utils/builtins.h"
 
@@ -32,7 +33,7 @@ printsimple_startup(DestReceiver *self, int operation, TupleDesc tupdesc)
 	StringInfoData buf;
 	int			i;
 
-	pq_beginmessage(&buf, 'T'); /* RowDescription */
+	pq_beginmessage(&buf, PqMsg_RowDescription);
 	pq_sendint16(&buf, tupdesc->natts);
 
 	for (i = 0; i < tupdesc->natts; ++i)
@@ -65,7 +66,7 @@ printsimple(TupleTableSlot *slot, DestReceiver *self)
 	slot_getallattrs(slot);
 
 	/* Prepare and send message */
-	pq_beginmessage(&buf, 'D');
+	pq_beginmessage(&buf, PqMsg_DataRow);
 	pq_sendint16(&buf, tupdesc->natts);
 
 	for (i = 0; i < tupdesc->natts; ++i)

src/backend/access/transam/parallel.c

@@ -1127,7 +1127,7 @@ HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg)
 
 	switch (msgtype)
 	{
-		case 'K':				/* BackendKeyData */
+		case PqMsg_BackendKeyData:
 			{
 				int32		pid = pq_getmsgint(msg, 4);
 
@@ -1137,8 +1137,8 @@ HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg)
 				break;
 			}
 
-		case 'E':				/* ErrorResponse */
-		case 'N':				/* NoticeResponse */
+		case PqMsg_ErrorResponse:
+		case PqMsg_NoticeResponse:
 			{
 				ErrorData	edata;
 				ErrorContextCallback *save_error_context_stack;
@@ -1183,7 +1183,7 @@ HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg)
 				break;
 			}
 
-		case 'A':				/* NotifyResponse */
+		case PqMsg_NotificationResponse:
 			{
 				/* Propagate NotifyResponse. */
 				int32		pid;
@@ -1217,7 +1217,7 @@ HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg)
 				break;
 			}
 
-		case 'X':				/* Terminate, indicating clean exit */
+		case PqMsg_Terminate:
 			{
 				shm_mq_detach(pcxt->worker[i].error_mqh);
 				pcxt->worker[i].error_mqh = NULL;
@@ -1372,7 +1372,7 @@ ParallelWorkerMain(Datum main_arg)
 	 * protocol message is defined, but it won't actually be used for anything
 	 * in this case.
 	 */
-	pq_beginmessage(&msgbuf, 'K');
+	pq_beginmessage(&msgbuf, PqMsg_BackendKeyData);
 	pq_sendint32(&msgbuf, (int32) MyProcPid);
 	pq_sendint32(&msgbuf, (int32) MyCancelKey);
 	pq_endmessage(&msgbuf);
@@ -1550,7 +1550,7 @@ ParallelWorkerMain(Datum main_arg)
 	DetachSession();
 
 	/* Report success. */
-	pq_putmessage('X', NULL, 0);
+	pq_putmessage(PqMsg_Terminate, NULL, 0);
 }
 
 /*

src/backend/backup/basebackup_copy.c

@@ -152,7 +152,7 @@ bbsink_copystream_begin_backup(bbsink *sink)
 	SendTablespaceList(state->tablespaces);
 
 	/* Send a CommandComplete message */
-	pq_puttextmessage('C', "SELECT");
+	pq_puttextmessage(PqMsg_CommandComplete, "SELECT");
 
 	/* Begin COPY stream. This will be used for all archives + manifest. */
 	SendCopyOutResponse();
@@ -169,7 +169,7 @@ bbsink_copystream_begin_archive(bbsink *sink, const char *archive_name)
 	StringInfoData buf;
 
 	ti = list_nth(state->tablespaces, state->tablespace_num);
-	pq_beginmessage(&buf, 'd'); /* CopyData */
+	pq_beginmessage(&buf, PqMsg_CopyData);
 	pq_sendbyte(&buf, 'n');		/* New archive */
 	pq_sendstring(&buf, archive_name);
 	pq_sendstring(&buf, ti->path == NULL ? "" : ti->path);
@@ -220,7 +220,7 @@ bbsink_copystream_archive_contents(bbsink *sink, size_t len)
 		{
 			mysink->last_progress_report_time = now;
 
-			pq_beginmessage(&buf, 'd'); /* CopyData */
+			pq_beginmessage(&buf, PqMsg_CopyData);
 			pq_sendbyte(&buf, 'p'); /* Progress report */
 			pq_sendint64(&buf, state->bytes_done);
 			pq_endmessage(&buf);
@@ -246,7 +246,7 @@ bbsink_copystream_end_archive(bbsink *sink)
 
 	mysink->bytes_done_at_last_time_check = state->bytes_done;
 	mysink->last_progress_report_time = GetCurrentTimestamp();
-	pq_beginmessage(&buf, 'd'); /* CopyData */
+	pq_beginmessage(&buf, PqMsg_CopyData);
 	pq_sendbyte(&buf, 'p');		/* Progress report */
 	pq_sendint64(&buf, state->bytes_done);
 	pq_endmessage(&buf);
@@ -261,7 +261,7 @@ bbsink_copystream_begin_manifest(bbsink *sink)
 {
 	StringInfoData buf;
 
-	pq_beginmessage(&buf, 'd'); /* CopyData */
+	pq_beginmessage(&buf, PqMsg_CopyData);
 	pq_sendbyte(&buf, 'm');		/* Manifest */
 	pq_endmessage(&buf);
 }
@@ -318,7 +318,7 @@ SendCopyOutResponse(void)
 {
 	StringInfoData buf;
 
-	pq_beginmessage(&buf, 'H');
+	pq_beginmessage(&buf, PqMsg_CopyOutResponse);
 	pq_sendbyte(&buf, 0);		/* overall format */
 	pq_sendint16(&buf, 0);		/* natts */
 	pq_endmessage(&buf);
@@ -330,7 +330,7 @@ SendCopyOutResponse(void)
 static void
 SendCopyDone(void)
 {
-	pq_putemptymessage('c');
+	pq_putemptymessage(PqMsg_CopyDone);
 }
 
 /*
@@ -368,7 +368,7 @@ SendXlogRecPtrResult(XLogRecPtr ptr, TimeLineID tli)
 	end_tup_output(tstate);
 
 	/* Send a CommandComplete message */
-	pq_puttextmessage('C', "SELECT");
+	pq_puttextmessage(PqMsg_CommandComplete, "SELECT");
 }
 
 /*

src/backend/commands/async.c

@@ -2281,7 +2281,7 @@ NotifyMyFrontEnd(const char *channel, const char *payload, int32 srcPid)
 	{
 		StringInfoData buf;
 
-		pq_beginmessage(&buf, 'A');
+		pq_beginmessage(&buf, PqMsg_NotificationResponse);
 		pq_sendint32(&buf, srcPid);
 		pq_sendstring(&buf, channel);
 		pq_sendstring(&buf, payload);

src/backend/commands/copyfromparse.c

@@ -174,7 +174,7 @@ ReceiveCopyBegin(CopyFromState cstate)
 	int16		format = (cstate->opts.binary ? 1 : 0);
 	int			i;
 
-	pq_beginmessage(&buf, 'G');
+	pq_beginmessage(&buf, PqMsg_CopyInResponse);
 	pq_sendbyte(&buf, format);	/* overall format */
 	pq_sendint16(&buf, natts);
 	for (i = 0; i < natts; i++)
@@ -279,13 +279,13 @@ CopyGetData(CopyFromState cstate, void *databuf, int minread, int maxread)
 					/* Validate message type and set packet size limit */
 					switch (mtype)
 					{
-						case 'd':	/* CopyData */
+						case PqMsg_CopyData:
 							maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 							break;
-						case 'c':	/* CopyDone */
-						case 'f':	/* CopyFail */
-						case 'H':	/* Flush */
-						case 'S':	/* Sync */
+						case PqMsg_CopyDone:
+						case PqMsg_CopyFail:
+						case PqMsg_Flush:
+						case PqMsg_Sync:
 							maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 							break;
 						default:
@@ -305,20 +305,20 @@ CopyGetData(CopyFromState cstate, void *databuf, int minread, int maxread)
 					/* ... and process it */
 					switch (mtype)
 					{
-						case 'd':	/* CopyData */
+						case PqMsg_CopyData:
 							break;
-						case 'c':	/* CopyDone */
+						case PqMsg_CopyDone:
 							/* COPY IN correctly terminated by frontend */
 							cstate->raw_reached_eof = true;
 							return bytesread;
-						case 'f':	/* CopyFail */
+						case PqMsg_CopyFail:
 							ereport(ERROR,
 									(errcode(ERRCODE_QUERY_CANCELED),
 									 errmsg("COPY from stdin failed: %s",
 											pq_getmsgstring(cstate->fe_msgbuf))));
 							break;
-						case 'H':	/* Flush */
-						case 'S':	/* Sync */
+						case PqMsg_Flush:
+						case PqMsg_Sync:
 
 							/*
 							 * Ignore Flush/Sync for the convenience of client

src/backend/commands/copyto.c

@@ -144,7 +144,7 @@ SendCopyBegin(CopyToState cstate)
 	int16		format = (cstate->opts.binary ? 1 : 0);
 	int			i;
 
-	pq_beginmessage(&buf, 'H');
+	pq_beginmessage(&buf, PqMsg_CopyOutResponse);
 	pq_sendbyte(&buf, format);	/* overall format */
 	pq_sendint16(&buf, natts);
 	for (i = 0; i < natts; i++)
@@ -159,7 +159,7 @@ SendCopyEnd(CopyToState cstate)
 	/* Shouldn't have any unsent data */
 	Assert(cstate->fe_msgbuf->len == 0);
 	/* Send Copy Done message */
-	pq_putemptymessage('c');
+	pq_putemptymessage(PqMsg_CopyDone);
 }
 
 /*----------
@@ -247,7 +247,7 @@ CopySendEndOfRow(CopyToState cstate)
 				CopySendChar(cstate, '\n');
 
 			/* Dump the accumulated row as one CopyData message */
-			(void) pq_putmessage('d', fe_msgbuf->data, fe_msgbuf->len);
+			(void) pq_putmessage(PqMsg_CopyData, fe_msgbuf->data, fe_msgbuf->len);
 			break;
 		case COPY_CALLBACK:
 			cstate->data_dest_cb(fe_msgbuf->data, fe_msgbuf->len);

src/backend/libpq/auth-sasl.c

@@ -87,7 +87,7 @@ CheckSASLAuth(const pg_be_sasl_mech *mech, Port *port, char *shadow_pass,
 	{
 		pq_startmsgread();
 		mtype = pq_getbyte();
-		if (mtype != 'p')
+		if (mtype != PqMsg_SASLResponse)
 		{
 			/* Only log error if client didn't disconnect. */
 			if (mtype != EOF)

src/backend/libpq/auth.c

@@ -665,7 +665,7 @@ sendAuthRequest(Port *port, AuthRequest areq, const char *extradata, int extrale
 
 	CHECK_FOR_INTERRUPTS();
 
-	pq_beginmessage(&buf, 'R');
+	pq_beginmessage(&buf, PqMsg_AuthenticationRequest);
 	pq_sendint32(&buf, (int32) areq);
 	if (extralen > 0)
 		pq_sendbytes(&buf, extradata, extralen);
@@ -698,7 +698,7 @@ recv_password_packet(Port *port)
 
 	/* Expect 'p' message type */
 	mtype = pq_getbyte();
-	if (mtype != 'p')
+	if (mtype != PqMsg_PasswordMessage)
 	{
 		/*
 		 * If the client just disconnects without offering a password, don't
@@ -961,7 +961,7 @@ pg_GSS_recvauth(Port *port)
 		CHECK_FOR_INTERRUPTS();
 
 		mtype = pq_getbyte();
-		if (mtype != 'p')
+		if (mtype != PqMsg_GSSResponse)
 		{
 			/* Only log error if client didn't disconnect. */
 			if (mtype != EOF)
@@ -1232,7 +1232,7 @@ pg_SSPI_recvauth(Port *port)
 	{
 		pq_startmsgread();
 		mtype = pq_getbyte();
-		if (mtype != 'p')
+		if (mtype != PqMsg_GSSResponse)
 		{
 			if (sspictx != NULL)
 			{

src/backend/postmaster/postmaster.c

@@ -2357,7 +2357,7 @@ SendNegotiateProtocolVersion(List *unrecognized_protocol_options)
 	StringInfoData buf;
 	ListCell   *lc;
 
-	pq_beginmessage(&buf, 'v'); /* NegotiateProtocolVersion */
+	pq_beginmessage(&buf, PqMsg_NegotiateProtocolVersion);
 	pq_sendint32(&buf, PG_PROTOCOL_LATEST);
 	pq_sendint32(&buf, list_length(unrecognized_protocol_options));
 	foreach(lc, unrecognized_protocol_options)

src/backend/replication/walsender.c

@@ -603,7 +603,7 @@ SendTimeLineHistory(TimeLineHistoryCmd *cmd)
 	dest->rStartup(dest, CMD_SELECT, tupdesc);
 
 	/* Send a DataRow message */
-	pq_beginmessage(&buf, 'D');
+	pq_beginmessage(&buf, PqMsg_DataRow);
 	pq_sendint16(&buf, 2);		/* # of columns */
 	len = strlen(histfname);
 	pq_sendint32(&buf, len);	/* col1 len */
@@ -801,7 +801,7 @@ StartReplication(StartReplicationCmd *cmd)
 		WalSndSetState(WALSNDSTATE_CATCHUP);
 
 		/* Send a CopyBothResponse message, and start streaming */
-		pq_beginmessage(&buf, 'W');
+		pq_beginmessage(&buf, PqMsg_CopyBothResponse);
 		pq_sendbyte(&buf, 0);
 		pq_sendint16(&buf, 0);
 		pq_endmessage(&buf);
@@ -1294,7 +1294,7 @@ StartLogicalReplication(StartReplicationCmd *cmd)
 	WalSndSetState(WALSNDSTATE_CATCHUP);
 
 	/* Send a CopyBothResponse message, and start streaming */
-	pq_beginmessage(&buf, 'W');
+	pq_beginmessage(&buf, PqMsg_CopyBothResponse);
 	pq_sendbyte(&buf, 0);
 	pq_sendint16(&buf, 0);
 	pq_endmessage(&buf);
@@ -1923,11 +1923,11 @@ ProcessRepliesIfAny(void)
 		/* Validate message type and set packet size limit */
 		switch (firstchar)
 		{
-			case 'd':
+			case PqMsg_CopyData:
 				maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 				break;
-			case 'c':
-			case 'X':
+			case PqMsg_CopyDone:
+			case PqMsg_Terminate:
 				maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 				break;
 			default:
@@ -1955,7 +1955,7 @@ ProcessRepliesIfAny(void)
 				/*
 				 * 'd' means a standby reply wrapped in a CopyData packet.
 				 */
-			case 'd':
+			case PqMsg_CopyData:
 				ProcessStandbyMessage();
 				received = true;
 				break;
@@ -1964,7 +1964,7 @@ ProcessRepliesIfAny(void)
 				 * CopyDone means the standby requested to finish streaming.
 				 * Reply with CopyDone, if we had not sent that already.
 				 */
-			case 'c':
+			case PqMsg_CopyDone:
 				if (!streamingDoneSending)
 				{
 					pq_putmessage_noblock('c', NULL, 0);
@@ -1978,7 +1978,7 @@ ProcessRepliesIfAny(void)
 				/*
 				 * 'X' means that the standby is closing down the socket.
 				 */
-			case 'X':
+			case PqMsg_Terminate:
 				proc_exit(0);
 
 			default:

src/backend/tcop/dest.c

@@ -176,7 +176,7 @@ EndCommand(const QueryCompletion *qc, CommandDest dest, bool force_undecorated_o
 
 			len = BuildQueryCompletionString(completionTag, qc,
 											 force_undecorated_output);
-			pq_putmessage('C', completionTag, len + 1);
+			pq_putmessage(PqMsg_Close, completionTag, len + 1);
 
 		case DestNone:
 		case DestDebug:
@@ -200,7 +200,7 @@ EndCommand(const QueryCompletion *qc, CommandDest dest, bool force_undecorated_o
 void
 EndReplicationCommand(const char *commandTag)
 {
-	pq_putmessage('C', commandTag, strlen(commandTag) + 1);
+	pq_putmessage(PqMsg_Close, commandTag, strlen(commandTag) + 1);
 }
 
 /* ----------------
@@ -220,7 +220,7 @@ NullCommand(CommandDest dest)
 		case DestRemoteSimple:
 
 			/* Tell the FE that we saw an empty query string */
-			pq_putemptymessage('I');
+			pq_putemptymessage(PqMsg_EmptyQueryResponse);
 			break;
 
 		case DestNone:
@@ -258,7 +258,7 @@ ReadyForQuery(CommandDest dest)
 			{
 				StringInfoData buf;
 
-				pq_beginmessage(&buf, 'Z');
+				pq_beginmessage(&buf, PqMsg_ReadyForQuery);
 				pq_sendbyte(&buf, TransactionBlockStatusCode());
 				pq_endmessage(&buf);
 			}

src/backend/tcop/fastpath.c

@@ -69,7 +69,7 @@ SendFunctionResult(Datum retval, bool isnull, Oid rettype, int16 format)
 {
 	StringInfoData buf;
 
-	pq_beginmessage(&buf, 'V');
+	pq_beginmessage(&buf, PqMsg_FunctionCallResponse);
 
 	if (isnull)
 	{

src/backend/tcop/postgres.c

@@ -402,37 +402,37 @@ SocketBackend(StringInfo inBuf)
 	 */
 	switch (qtype)
 	{
-		case 'Q':				/* simple query */
+		case PqMsg_Query:
 			maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 			doing_extended_query_message = false;
 			break;
 
-		case 'F':				/* fastpath function call */
+		case PqMsg_FunctionCall:
 			maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 			doing_extended_query_message = false;
 			break;
 
-		case 'X':				/* terminate */
+		case PqMsg_Terminate:
 			maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 			doing_extended_query_message = false;
 			ignore_till_sync = false;
 			break;
 
-		case 'B':				/* bind */
-		case 'P':				/* parse */
+		case PqMsg_Bind:
+		case PqMsg_Parse:
 			maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 			doing_extended_query_message = true;
 			break;
 
-		case 'C':				/* close */
-		case 'D':				/* describe */
-		case 'E':				/* execute */
-		case 'H':				/* flush */
+		case PqMsg_Close:
+		case PqMsg_Describe:
+		case PqMsg_Execute:
+		case PqMsg_Flush:
 			maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 			doing_extended_query_message = true;
 			break;
 
-		case 'S':				/* sync */
+		case PqMsg_Sync:
 			maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 			/* stop any active skip-till-Sync */
 			ignore_till_sync = false;
@@ -440,13 +440,13 @@ SocketBackend(StringInfo inBuf)
 			doing_extended_query_message = false;
 			break;
 
-		case 'd':				/* copy data */
+		case PqMsg_CopyData:
 			maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 			doing_extended_query_message = false;
 			break;
 
-		case 'c':				/* copy done */
-		case 'f':				/* copy fail */
+		case PqMsg_CopyDone:
+		case PqMsg_CopyFail:
 			maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 			doing_extended_query_message = false;
 			break;
@@ -1589,7 +1589,7 @@ exec_parse_message(const char *query_string,	/* string to execute */
 	 * Send ParseComplete.
 	 */
 	if (whereToSendOutput == DestRemote)
-		pq_putemptymessage('1');
+		pq_putemptymessage(PqMsg_ParseComplete);
 
 	/*
 	 * Emit duration logging if appropriate.
@@ -2047,7 +2047,7 @@ exec_bind_message(StringInfo input_message)
 	 * Send BindComplete.
 	 */
 	if (whereToSendOutput == DestRemote)
-		pq_putemptymessage('2');
+		pq_putemptymessage(PqMsg_BindComplete);
 
 	/*
 	 * Emit duration logging if appropriate.
@@ -2290,7 +2290,7 @@ exec_execute_message(const char *portal_name, long max_rows)
 	{
 		/* Portal run not complete, so send PortalSuspended */
 		if (whereToSendOutput == DestRemote)
-			pq_putemptymessage('s');
+			pq_putemptymessage(PqMsg_PortalSuspended);
 
 		/*
 		 * Set XACT_FLAGS_PIPELINING whenever we suspend an Execute message,
@@ -2683,7 +2683,7 @@ exec_describe_statement_message(const char *stmt_name)
 								  NULL);
 	}
 	else
-		pq_putemptymessage('n');	/* NoData */
+		pq_putemptymessage(PqMsg_NoData);
 }
 
 /*
@@ -2736,7 +2736,7 @@ exec_describe_portal_message(const char *portal_name)
 								  FetchPortalTargetList(portal),
 								  portal->formats);
 	else
-		pq_putemptymessage('n');	/* NoData */
+		pq_putemptymessage(PqMsg_NoData);
 }
 
 
@@ -4239,7 +4239,7 @@ PostgresMain(const char *dbname, const char *username)
 	{
 		StringInfoData buf;
 
-		pq_beginmessage(&buf, 'K');
+		pq_beginmessage(&buf, PqMsg_BackendKeyData);
 		pq_sendint32(&buf, (int32) MyProcPid);
 		pq_sendint32(&buf, (int32) MyCancelKey);
 		pq_endmessage(&buf);
@@ -4618,7 +4618,7 @@ PostgresMain(const char *dbname, const char *username)
 
 		switch (firstchar)
 		{
-			case 'Q':			/* simple query */
+			case PqMsg_Query:
 				{
 					const char *query_string;
 
@@ -4642,7 +4642,7 @@ PostgresMain(const char *dbname, const char *username)
 				}
 				break;
 
-			case 'P':			/* parse */
+			case PqMsg_Parse:
 				{
 					const char *stmt_name;
 					const char *query_string;
@@ -4672,7 +4672,7 @@ PostgresMain(const char *dbname, const char *username)
 				}
 				break;
 
-			case 'B':			/* bind */
+			case PqMsg_Bind:
 				forbidden_in_wal_sender(firstchar);
 
 				/* Set statement_timestamp() */
@@ -4687,7 +4687,7 @@ PostgresMain(const char *dbname, const char *username)
 				/* exec_bind_message does valgrind_report_error_query */
 				break;
 
-			case 'E':			/* execute */
+			case PqMsg_Execute:
 				{
 					const char *portal_name;
 					int			max_rows;
@@ -4707,7 +4707,7 @@ PostgresMain(const char *dbname, const char *username)
 				}
 				break;
 
-			case 'F':			/* fastpath function call */
+			case PqMsg_FunctionCall:
 				forbidden_in_wal_sender(firstchar);
 
 				/* Set statement_timestamp() */
@@ -4742,7 +4742,7 @@ PostgresMain(const char *dbname, const char *username)
 				send_ready_for_query = true;
 				break;
 
-			case 'C':			/* close */
+			case PqMsg_Close:
 				{
 					int			close_type;
 					const char *close_target;
@@ -4782,13 +4782,13 @@ PostgresMain(const char *dbname, const char *username)
 					}
 
 					if (whereToSendOutput == DestRemote)
-						pq_putemptymessage('3');	/* CloseComplete */
+						pq_putemptymessage(PqMsg_CloseComplete);
 
 					valgrind_report_error_query("CLOSE message");
 				}
 				break;
 
-			case 'D':			/* describe */
+			case PqMsg_Describe:
 				{
 					int			describe_type;
 					const char *describe_target;
@@ -4822,13 +4822,13 @@ PostgresMain(const char *dbname, const char *username)
 				}
 				break;
 
-			case 'H':			/* flush */
+			case PqMsg_Flush:
 				pq_getmsgend(&input_message);
 				if (whereToSendOutput == DestRemote)
 					pq_flush();
 				break;
 
-			case 'S':			/* sync */
+			case PqMsg_Sync:
 				pq_getmsgend(&input_message);
 				finish_xact_command();
 				valgrind_report_error_query("SYNC message");
@@ -4847,7 +4847,7 @@ PostgresMain(const char *dbname, const char *username)
 
 				/* FALLTHROUGH */
 
-			case 'X':
+			case PqMsg_Terminate:
 
 				/*
 				 * Reset whereToSendOutput to prevent ereport from attempting
@@ -4865,9 +4865,9 @@ PostgresMain(const char *dbname, const char *username)
 				 */
 				proc_exit(0);
 
-			case 'd':			/* copy data */
-			case 'c':			/* copy done */
-			case 'f':			/* copy fail */
+			case PqMsg_CopyData:
+			case PqMsg_CopyDone:
+			case PqMsg_CopyFail:
 
 				/*
 				 * Accept but ignore these messages, per protocol spec; we
@@ -4897,7 +4897,7 @@ forbidden_in_wal_sender(char firstchar)
 {
 	if (am_walsender)
 	{
-		if (firstchar == 'F')
+		if (firstchar == PqMsg_FunctionCall)
 			ereport(ERROR,
 					(errcode(ERRCODE_PROTOCOL_VIOLATION),
 					 errmsg("fastpath function calls not supported in a replication connection")));

src/backend/utils/error/elog.c

@@ -3465,7 +3465,10 @@ send_message_to_frontend(ErrorData *edata)
 		char		tbuf[12];
 
 		/* 'N' (Notice) is for nonfatal conditions, 'E' is for errors */
-		pq_beginmessage(&msgbuf, (edata->elevel < ERROR) ? 'N' : 'E');
+		if (edata->elevel < ERROR)
+			pq_beginmessage(&msgbuf, PqMsg_NoticeResponse);
+		else
+			pq_beginmessage(&msgbuf, PqMsg_ErrorResponse);
 
 		sev = error_severity(edata->elevel);
 		pq_sendbyte(&msgbuf, PG_DIAG_SEVERITY);

src/backend/utils/misc/guc.c

@@ -2593,7 +2593,7 @@ ReportGUCOption(struct config_generic *record)
 	{
 		StringInfoData msgbuf;
 
-		pq_beginmessage(&msgbuf, 'S');
+		pq_beginmessage(&msgbuf, PqMsg_ParameterStatus);
 		pq_sendstring(&msgbuf, record->name);
 		pq_sendstring(&msgbuf, val);
 		pq_endmessage(&msgbuf);