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

25 files changed, 305 insertions, 204 deletions

diff --git a/src/backend/access/common/printsimple.c b/src/backend/access/common/printsimple.c
index ef818228acb..675b744db20 100644
--- a/src/backend/access/common/printsimple.c
+++ b/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)
diff --git a/src/backend/access/transam/parallel.c b/src/backend/access/transam/parallel.c
index 1738aecf1f0..194a1207be6 100644
--- a/src/backend/access/transam/parallel.c
+++ b/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);
 }
 
 /*
diff --git a/src/backend/backup/basebackup_copy.c b/src/backend/backup/basebackup_copy.c
index 1db80cde1b2..fee30c21e10 100644
--- a/src/backend/backup/basebackup_copy.c
+++ b/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");
 }
 
 /*
diff --git a/src/backend/commands/async.c b/src/backend/commands/async.c
index ef909cf4e08..d148d10850a 100644
--- a/src/backend/commands/async.c
+++ b/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);
diff --git a/src/backend/commands/copyfromparse.c b/src/backend/commands/copyfromparse.c
index 232768a6e13..f553734582f 100644
--- a/src/backend/commands/copyfromparse.c
+++ b/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
diff --git a/src/backend/commands/copyto.c b/src/backend/commands/copyto.c
index 9e4b2437a57..eaa3172793a 100644
--- a/src/backend/commands/copyto.c
+++ b/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);
diff --git a/src/backend/libpq/auth-sasl.c b/src/backend/libpq/auth-sasl.c
index 684680897bc..c535bc53835 100644
--- a/src/backend/libpq/auth-sasl.c
+++ b/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)
diff --git a/src/backend/libpq/auth.c b/src/backend/libpq/auth.c
index 315a24bb3f9..0356fe3e454 100644
--- a/src/backend/libpq/auth.c
+++ b/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)
 			{
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 9c8ec779f9b..07d376d77ec 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/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)
diff --git a/src/backend/replication/walsender.c b/src/backend/replication/walsender.c
index d27ef2985d7..80374c55be5 100644
--- a/src/backend/replication/walsender.c
+++ b/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:
diff --git a/src/backend/tcop/dest.c b/src/backend/tcop/dest.c
index c0406e2ee55..06d1872b9ac 100644
--- a/src/backend/tcop/dest.c
+++ b/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);
 			}
diff --git a/src/backend/tcop/fastpath.c b/src/backend/tcop/fastpath.c
index 2f70ebd5fae..71f161dbe24 100644
--- a/src/backend/tcop/fastpath.c
+++ b/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)
 	{
diff --git a/src/backend/tcop/postgres.c b/src/backend/tcop/postgres.c
index 36cc99ec9cf..e4756f8be21 100644
--- a/src/backend/tcop/postgres.c
+++ b/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")));
diff --git a/src/backend/utils/error/elog.c b/src/backend/utils/error/elog.c
index 5898100acb6..8e1f3e85211 100644
--- a/src/backend/utils/error/elog.c
+++ b/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);
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 99bb2fdd190..84e7ad4d907 100644
--- a/src/backend/utils/misc/guc.c
+++ b/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);
diff --git a/src/include/Makefile b/src/include/Makefile
index 5d213187e24..2d5242561cd 100644
--- a/src/include/Makefile
+++ b/src/include/Makefile
@@ -40,6 +40,7 @@ install: all installdirs
 	$(INSTALL_DATA) $(srcdir)/port.h         '$(DESTDIR)$(includedir_internal)'
 	$(INSTALL_DATA) $(srcdir)/postgres_fe.h  '$(DESTDIR)$(includedir_internal)'
 	$(INSTALL_DATA) $(srcdir)/libpq/pqcomm.h '$(DESTDIR)$(includedir_internal)/libpq'
+	$(INSTALL_DATA) $(srcdir)/libpq/protocol.h '$(DESTDIR)$(includedir_internal)/libpq'
 # These headers are needed for server-side development
 	$(INSTALL_DATA) pg_config.h     '$(DESTDIR)$(includedir_server)'
 	$(INSTALL_DATA) pg_config_ext.h '$(DESTDIR)$(includedir_server)'
@@ -65,7 +66,7 @@ installdirs:
 
 uninstall:
 	rm -f $(addprefix '$(DESTDIR)$(includedir)'/, pg_config.h pg_config_ext.h pg_config_os.h pg_config_manual.h postgres_ext.h libpq/libpq-fs.h)
-	rm -f $(addprefix '$(DESTDIR)$(includedir_internal)'/, c.h port.h postgres_fe.h libpq/pqcomm.h)
+	rm -f $(addprefix '$(DESTDIR)$(includedir_internal)'/, c.h port.h postgres_fe.h libpq/pqcomm.h libpq/protocol.h)
 # heuristic...
 	rm -rf $(addprefix '$(DESTDIR)$(includedir_server)'/, $(SUBDIRS) *.h)
 
diff --git a/src/include/libpq/pqcomm.h b/src/include/libpq/pqcomm.h
index 3da00f79839..46a0946b8b2 100644
--- a/src/include/libpq/pqcomm.h
+++ b/src/include/libpq/pqcomm.h
@@ -21,6 +21,12 @@
 #include <netdb.h>
 #include <netinet/in.h>
 
+/*
+ * The definitions for the request/response codes are kept in a separate file
+ * for ease of use in third party programs.
+ */
+#include "libpq/protocol.h"
+
 typedef struct
 {
 	struct sockaddr_storage addr;
@@ -112,23 +118,6 @@ typedef uint32 PacketLen;
 #define MAX_STARTUP_PACKET_LENGTH 10000
 
 
-/* These are the authentication request codes sent by the backend. */
-
-#define AUTH_REQ_OK			0	/* User is authenticated  */
-#define AUTH_REQ_KRB4		1	/* Kerberos V4. Not supported any more. */
-#define AUTH_REQ_KRB5		2	/* Kerberos V5. Not supported any more. */
-#define AUTH_REQ_PASSWORD	3	/* Password */
-#define AUTH_REQ_CRYPT		4	/* crypt password. Not supported any more. */
-#define AUTH_REQ_MD5		5	/* md5 password */
-/* 6 is available.  It was used for SCM creds, not supported any more. */
-#define AUTH_REQ_GSS		7	/* GSSAPI without wrap() */
-#define AUTH_REQ_GSS_CONT	8	/* Continue GSS exchanges */
-#define AUTH_REQ_SSPI		9	/* SSPI negotiate without wrap() */
-#define AUTH_REQ_SASL	   10	/* Begin SASL authentication */
-#define AUTH_REQ_SASL_CONT 11	/* Continue SASL authentication */
-#define AUTH_REQ_SASL_FIN  12	/* Final SASL message */
-#define AUTH_REQ_MAX	   AUTH_REQ_SASL_FIN	/* maximum AUTH_REQ_* value */
-
 typedef uint32 AuthRequest;
 
 
diff --git a/src/include/libpq/protocol.h b/src/include/libpq/protocol.h
new file mode 100644
index 00000000000..cc46f4b586a
--- /dev/null
+++ b/src/include/libpq/protocol.h
@@ -0,0 +1,85 @@
+/*-------------------------------------------------------------------------
+ *
+ * protocol.h
+ *		Definitions of the request/response codes for the wire protocol.
+ *
+ *
+ * Portions Copyright (c) 1996-2023, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * src/include/libpq/protocol.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef PROTOCOL_H
+#define PROTOCOL_H
+
+/* These are the request codes sent by the frontend. */
+
+#define PqMsg_Bind					'B'
+#define PqMsg_Close					'C'
+#define PqMsg_Describe				'D'
+#define PqMsg_Execute				'E'
+#define PqMsg_FunctionCall			'F'
+#define PqMsg_Flush					'H'
+#define PqMsg_Parse					'P'
+#define PqMsg_Query					'Q'
+#define PqMsg_Sync					'S'
+#define PqMsg_Terminate				'X'
+#define PqMsg_CopyFail				'f'
+#define PqMsg_GSSResponse			'p'
+#define PqMsg_PasswordMessage		'p'
+#define PqMsg_SASLInitialResponse	'p'
+#define PqMsg_SASLResponse			'p'
+
+
+/* These are the response codes sent by the backend. */
+
+#define PqMsg_ParseComplete			'1'
+#define PqMsg_BindComplete			'2'
+#define PqMsg_CloseComplete			'3'
+#define PqMsg_NotificationResponse	'A'
+#define PqMsg_CommandComplete		'C'
+#define PqMsg_DataRow				'D'
+#define PqMsg_ErrorResponse			'E'
+#define PqMsg_CopyInResponse		'G'
+#define PqMsg_CopyOutResponse		'H'
+#define PqMsg_EmptyQueryResponse	'I'
+#define PqMsg_BackendKeyData		'K'
+#define PqMsg_NoticeResponse		'N'
+#define PqMsg_AuthenticationRequest 'R'
+#define PqMsg_ParameterStatus		'S'
+#define PqMsg_RowDescription		'T'
+#define PqMsg_FunctionCallResponse	'V'
+#define PqMsg_CopyBothResponse		'W'
+#define PqMsg_ReadyForQuery			'Z'
+#define PqMsg_NoData				'n'
+#define PqMsg_PortalSuspended		's'
+#define PqMsg_ParameterDescription	't'
+#define PqMsg_NegotiateProtocolVersion 'v'
+
+
+/* These are the codes sent by both the frontend and backend. */
+
+#define PqMsg_CopyDone				'c'
+#define PqMsg_CopyData				'd'
+
+
+/* These are the authentication request codes sent by the backend. */
+
+#define AUTH_REQ_OK			0	/* User is authenticated  */
+#define AUTH_REQ_KRB4		1	/* Kerberos V4. Not supported any more. */
+#define AUTH_REQ_KRB5		2	/* Kerberos V5. Not supported any more. */
+#define AUTH_REQ_PASSWORD	3	/* Password */
+#define AUTH_REQ_CRYPT		4	/* crypt password. Not supported any more. */
+#define AUTH_REQ_MD5		5	/* md5 password */
+/* 6 is available.  It was used for SCM creds, not supported any more. */
+#define AUTH_REQ_GSS		7	/* GSSAPI without wrap() */
+#define AUTH_REQ_GSS_CONT	8	/* Continue GSS exchanges */
+#define AUTH_REQ_SSPI		9	/* SSPI negotiate without wrap() */
+#define AUTH_REQ_SASL	   10	/* Begin SASL authentication */
+#define AUTH_REQ_SASL_CONT 11	/* Continue SASL authentication */
+#define AUTH_REQ_SASL_FIN  12	/* Final SASL message */
+#define AUTH_REQ_MAX	   AUTH_REQ_SASL_FIN	/* maximum AUTH_REQ_* value */
+
+#endif							/* PROTOCOL_H */
diff --git a/src/include/meson.build b/src/include/meson.build
index d7e1ecd4c96..d50897c9fda 100644
--- a/src/include/meson.build
+++ b/src/include/meson.build
@@ -94,6 +94,7 @@ install_headers(
 
 install_headers(
   'libpq/pqcomm.h',
+  'libpq/protocol.h',
   install_dir: dir_include_internal / 'libpq',
 )
 
diff --git a/src/interfaces/libpq/fe-auth.c b/src/interfaces/libpq/fe-auth.c
index 887ca5e9e1b..912aa14821d 100644
--- a/src/interfaces/libpq/fe-auth.c
+++ b/src/interfaces/libpq/fe-auth.c
@@ -586,7 +586,7 @@ pg_SASL_init(PGconn *conn, int payloadlen)
 	/*
 	 * Build a SASLInitialResponse message, and send it.
 	 */
-	if (pqPutMsgStart('p', conn))
+	if (pqPutMsgStart(PqMsg_SASLInitialResponse, conn))
 		goto error;
 	if (pqPuts(selected_mechanism, conn))
 		goto error;
diff --git a/src/interfaces/libpq/fe-connect.c b/src/interfaces/libpq/fe-connect.c
index 837c5321aa1..bf83a9b5697 100644
--- a/src/interfaces/libpq/fe-connect.c
+++ b/src/interfaces/libpq/fe-connect.c
@@ -3591,7 +3591,9 @@ keep_going:						/* We will come back to here until there is
 				 * Anything else probably means it's not Postgres on the other
 				 * end at all.
 				 */
-				if (!(beresp == 'R' || beresp == 'v' || beresp == 'E'))
+				if (beresp != PqMsg_AuthenticationRequest &&
+					beresp != PqMsg_ErrorResponse &&
+					beresp != PqMsg_NegotiateProtocolVersion)
 				{
 					libpq_append_conn_error(conn, "expected authentication request from server, but received %c",
 											beresp);
@@ -3618,19 +3620,22 @@ keep_going:						/* We will come back to here until there is
 				 * version 14, the server also used the old protocol for
 				 * errors that happened before processing the startup packet.)
 				 */
-				if (beresp == 'R' && (msgLength < 8 || msgLength > 2000))
+				if (beresp == PqMsg_AuthenticationRequest &&
+					(msgLength < 8 || msgLength > 2000))
 				{
 					libpq_append_conn_error(conn, "received invalid authentication request");
 					goto error_return;
 				}
-				if (beresp == 'v' && (msgLength < 8 || msgLength > 2000))
+				if (beresp == PqMsg_NegotiateProtocolVersion &&
+					(msgLength < 8 || msgLength > 2000))
 				{
 					libpq_append_conn_error(conn, "received invalid protocol negotiation message");
 					goto error_return;
 				}
 
 #define MAX_ERRLEN 30000
-				if (beresp == 'E' && (msgLength < 8 || msgLength > MAX_ERRLEN))
+				if (beresp == PqMsg_ErrorResponse &&
+					(msgLength < 8 || msgLength > MAX_ERRLEN))
 				{
 					/* Handle error from a pre-3.0 server */
 					conn->inCursor = conn->inStart + 1; /* reread data */
@@ -3693,7 +3698,7 @@ keep_going:						/* We will come back to here until there is
 				}
 
 				/* Handle errors. */
-				if (beresp == 'E')
+				if (beresp == PqMsg_ErrorResponse)
 				{
 					if (pqGetErrorNotice3(conn, true))
 					{
@@ -3770,7 +3775,7 @@ keep_going:						/* We will come back to here until there is
 
 					goto error_return;
 				}
-				else if (beresp == 'v')
+				else if (beresp == PqMsg_NegotiateProtocolVersion)
 				{
 					if (pqGetNegotiateProtocolVersion3(conn))
 					{
@@ -4540,7 +4545,7 @@ sendTerminateConn(PGconn *conn)
 		 * Try to send "close connection" message to backend. Ignore any
 		 * error.
 		 */
-		pqPutMsgStart('X', conn);
+		pqPutMsgStart(PqMsg_Terminate, conn);
 		pqPutMsgEnd(conn);
 		(void) pqFlush(conn);
 	}
diff --git a/src/interfaces/libpq/fe-exec.c b/src/interfaces/libpq/fe-exec.c
index a868284ff87..974d462d4b8 100644
--- a/src/interfaces/libpq/fe-exec.c
+++ b/src/interfaces/libpq/fe-exec.c
@@ -1458,7 +1458,7 @@ PQsendQueryInternal(PGconn *conn, const char *query, bool newQuery)
 
 	/* Send the query message(s) */
 	/* construct the outgoing Query message */
-	if (pqPutMsgStart('Q', conn) < 0 ||
+	if (pqPutMsgStart(PqMsg_Query, conn) < 0 ||
 		pqPuts(query, conn) < 0 ||
 		pqPutMsgEnd(conn) < 0)
 	{
@@ -1571,7 +1571,7 @@ PQsendPrepare(PGconn *conn,
 		return 0;				/* error msg already set */
 
 	/* construct the Parse message */
-	if (pqPutMsgStart('P', conn) < 0 ||
+	if (pqPutMsgStart(PqMsg_Parse, conn) < 0 ||
 		pqPuts(stmtName, conn) < 0 ||
 		pqPuts(query, conn) < 0)
 		goto sendFailed;
@@ -1599,7 +1599,7 @@ PQsendPrepare(PGconn *conn,
 	/* Add a Sync, unless in pipeline mode. */
 	if (conn->pipelineStatus == PQ_PIPELINE_OFF)
 	{
-		if (pqPutMsgStart('S', conn) < 0 ||
+		if (pqPutMsgStart(PqMsg_Sync, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			goto sendFailed;
 	}
@@ -1784,7 +1784,7 @@ PQsendQueryGuts(PGconn *conn,
 	if (command)
 	{
 		/* construct the Parse message */
-		if (pqPutMsgStart('P', conn) < 0 ||
+		if (pqPutMsgStart(PqMsg_Parse, conn) < 0 ||
 			pqPuts(stmtName, conn) < 0 ||
 			pqPuts(command, conn) < 0)
 			goto sendFailed;
@@ -1808,7 +1808,7 @@ PQsendQueryGuts(PGconn *conn,
 	}
 
 	/* Construct the Bind message */
-	if (pqPutMsgStart('B', conn) < 0 ||
+	if (pqPutMsgStart(PqMsg_Bind, conn) < 0 ||
 		pqPuts("", conn) < 0 ||
 		pqPuts(stmtName, conn) < 0)
 		goto sendFailed;
@@ -1874,14 +1874,14 @@ PQsendQueryGuts(PGconn *conn,
 		goto sendFailed;
 
 	/* construct the Describe Portal message */
-	if (pqPutMsgStart('D', conn) < 0 ||
+	if (pqPutMsgStart(PqMsg_Describe, conn) < 0 ||
 		pqPutc('P', conn) < 0 ||
 		pqPuts("", conn) < 0 ||
 		pqPutMsgEnd(conn) < 0)
 		goto sendFailed;
 
 	/* construct the Execute message */
-	if (pqPutMsgStart('E', conn) < 0 ||
+	if (pqPutMsgStart(PqMsg_Execute, conn) < 0 ||
 		pqPuts("", conn) < 0 ||
 		pqPutInt(0, 4, conn) < 0 ||
 		pqPutMsgEnd(conn) < 0)
@@ -1890,7 +1890,7 @@ PQsendQueryGuts(PGconn *conn,
 	/* construct the Sync message if not in pipeline mode */
 	if (conn->pipelineStatus == PQ_PIPELINE_OFF)
 	{
-		if (pqPutMsgStart('S', conn) < 0 ||
+		if (pqPutMsgStart(PqMsg_Sync, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			goto sendFailed;
 	}
@@ -2422,7 +2422,7 @@ PQdescribePrepared(PGconn *conn, const char *stmt)
 {
 	if (!PQexecStart(conn))
 		return NULL;
-	if (!PQsendTypedCommand(conn, 'D', 'S', stmt))
+	if (!PQsendTypedCommand(conn, PqMsg_Describe, 'S', stmt))
 		return NULL;
 	return PQexecFinish(conn);
 }
@@ -2441,7 +2441,7 @@ PQdescribePortal(PGconn *conn, const char *portal)
 {
 	if (!PQexecStart(conn))
 		return NULL;
-	if (!PQsendTypedCommand(conn, 'D', 'P', portal))
+	if (!PQsendTypedCommand(conn, PqMsg_Describe, 'P', portal))
 		return NULL;
 	return PQexecFinish(conn);
 }
@@ -2456,7 +2456,7 @@ PQdescribePortal(PGconn *conn, const char *portal)
 int
 PQsendDescribePrepared(PGconn *conn, const char *stmt)
 {
-	return PQsendTypedCommand(conn, 'D', 'S', stmt);
+	return PQsendTypedCommand(conn, PqMsg_Describe, 'S', stmt);
 }
 
 /*
@@ -2469,7 +2469,7 @@ PQsendDescribePrepared(PGconn *conn, const char *stmt)
 int
 PQsendDescribePortal(PGconn *conn, const char *portal)
 {
-	return PQsendTypedCommand(conn, 'D', 'P', portal);
+	return PQsendTypedCommand(conn, PqMsg_Describe, 'P', portal);
 }
 
 /*
@@ -2488,7 +2488,7 @@ PQclosePrepared(PGconn *conn, const char *stmt)
 {
 	if (!PQexecStart(conn))
 		return NULL;
-	if (!PQsendTypedCommand(conn, 'C', 'S', stmt))
+	if (!PQsendTypedCommand(conn, PqMsg_Close, 'S', stmt))
 		return NULL;
 	return PQexecFinish(conn);
 }
@@ -2506,7 +2506,7 @@ PQclosePortal(PGconn *conn, const char *portal)
 {
 	if (!PQexecStart(conn))
 		return NULL;
-	if (!PQsendTypedCommand(conn, 'C', 'P', portal))
+	if (!PQsendTypedCommand(conn, PqMsg_Close, 'P', portal))
 		return NULL;
 	return PQexecFinish(conn);
 }
@@ -2521,7 +2521,7 @@ PQclosePortal(PGconn *conn, const char *portal)
 int
 PQsendClosePrepared(PGconn *conn, const char *stmt)
 {
-	return PQsendTypedCommand(conn, 'C', 'S', stmt);
+	return PQsendTypedCommand(conn, PqMsg_Close, 'S', stmt);
 }
 
 /*
@@ -2534,7 +2534,7 @@ PQsendClosePrepared(PGconn *conn, const char *stmt)
 int
 PQsendClosePortal(PGconn *conn, const char *portal)
 {
-	return PQsendTypedCommand(conn, 'C', 'P', portal);
+	return PQsendTypedCommand(conn, PqMsg_Close, 'P', portal);
 }
 
 /*
@@ -2542,8 +2542,8 @@ PQsendClosePortal(PGconn *conn, const char *portal)
  *	 Common code to send a Describe or Close command
  *
  * Available options for "command" are
- *	 'C' for Close; or
- *	 'D' for Describe.
+ *	 PqMsg_Close for Close; or
+ *	 PqMsg_Describe for Describe.
  *
  * Available options for "type" are
  *	 'S' to run a command on a prepared statement; or
@@ -2577,17 +2577,17 @@ PQsendTypedCommand(PGconn *conn, char command, char type, const char *target)
 	/* construct the Sync message */
 	if (conn->pipelineStatus == PQ_PIPELINE_OFF)
 	{
-		if (pqPutMsgStart('S', conn) < 0 ||
+		if (pqPutMsgStart(PqMsg_Sync, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			goto sendFailed;
 	}
 
 	/* remember if we are doing a Close or a Describe */
-	if (command == 'C')
+	if (command == PqMsg_Close)
 	{
 		entry->queryclass = PGQUERY_CLOSE;
 	}
-	else if (command == 'D')
+	else if (command == PqMsg_Describe)
 	{
 		entry->queryclass = PGQUERY_DESCRIBE;
 	}
@@ -2696,7 +2696,7 @@ PQputCopyData(PGconn *conn, const char *buffer, int nbytes)
 				return pqIsnonblocking(conn) ? 0 : -1;
 		}
 		/* Send the data (too simple to delegate to fe-protocol files) */
-		if (pqPutMsgStart('d', conn) < 0 ||
+		if (pqPutMsgStart(PqMsg_CopyData, conn) < 0 ||
 			pqPutnchar(buffer, nbytes, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			return -1;
@@ -2731,7 +2731,7 @@ PQputCopyEnd(PGconn *conn, const char *errormsg)
 	if (errormsg)
 	{
 		/* Send COPY FAIL */
-		if (pqPutMsgStart('f', conn) < 0 ||
+		if (pqPutMsgStart(PqMsg_CopyFail, conn) < 0 ||
 			pqPuts(errormsg, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			return -1;
@@ -2739,7 +2739,7 @@ PQputCopyEnd(PGconn *conn, const char *errormsg)
 	else
 	{
 		/* Send COPY DONE */
-		if (pqPutMsgStart('c', conn) < 0 ||
+		if (pqPutMsgStart(PqMsg_CopyDone, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			return -1;
 	}
@@ -2751,7 +2751,7 @@ PQputCopyEnd(PGconn *conn, const char *errormsg)
 	if (conn->cmd_queue_head &&
 		conn->cmd_queue_head->queryclass != PGQUERY_SIMPLE)
 	{
-		if (pqPutMsgStart('S', conn) < 0 ||
+		if (pqPutMsgStart(PqMsg_Sync, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			return -1;
 	}
@@ -3263,7 +3263,7 @@ PQpipelineSync(PGconn *conn)
 	entry->query = NULL;
 
 	/* construct the Sync message */
-	if (pqPutMsgStart('S', conn) < 0 ||
+	if (pqPutMsgStart(PqMsg_Sync, conn) < 0 ||
 		pqPutMsgEnd(conn) < 0)
 		goto sendFailed;
 
@@ -3311,7 +3311,7 @@ PQsendFlushRequest(PGconn *conn)
 		return 0;
 	}
 
-	if (pqPutMsgStart('H', conn) < 0 ||
+	if (pqPutMsgStart(PqMsg_Flush, conn) < 0 ||
 		pqPutMsgEnd(conn) < 0)
 	{
 		return 0;
diff --git a/src/interfaces/libpq/fe-protocol3.c b/src/interfaces/libpq/fe-protocol3.c
index 7bc6355d17f..5613c56b141 100644
--- a/src/interfaces/libpq/fe-protocol3.c
+++ b/src/interfaces/libpq/fe-protocol3.c
@@ -34,8 +34,13 @@
  * than a couple of kilobytes).
  */
 #define VALID_LONG_MESSAGE_TYPE(id) \
-	((id) == 'T' || (id) == 'D' || (id) == 'd' || (id) == 'V' || \
-	 (id) == 'E' || (id) == 'N' || (id) == 'A')
+	((id) == PqMsg_CopyData || \
+	 (id) == PqMsg_DataRow || \
+	 (id) == PqMsg_ErrorResponse || \
+	 (id) == PqMsg_FunctionCallResponse || \
+	 (id) == PqMsg_NoticeResponse || \
+	 (id) == PqMsg_NotificationResponse || \
+	 (id) == PqMsg_RowDescription)
 
 
 static void handleSyncLoss(PGconn *conn, char id, int msgLength);
@@ -140,12 +145,12 @@ pqParseInput3(PGconn *conn)
 		 * from config file due to SIGHUP), but otherwise we hold off until
 		 * BUSY state.
 		 */
-		if (id == 'A')
+		if (id == PqMsg_NotificationResponse)
 		{
 			if (getNotify(conn))
 				return;
 		}
-		else if (id == 'N')
+		else if (id == PqMsg_NoticeResponse)
 		{
 			if (pqGetErrorNotice3(conn, false))
 				return;
@@ -165,12 +170,12 @@ pqParseInput3(PGconn *conn)
 			 * it is about to close the connection, so we don't want to just
 			 * discard it...)
 			 */
-			if (id == 'E')
+			if (id == PqMsg_ErrorResponse)
 			{
 				if (pqGetErrorNotice3(conn, false /* treat as notice */ ))
 					return;
 			}
-			else if (id == 'S')
+			else if (id == PqMsg_ParameterStatus)
 			{
 				if (getParameterStatus(conn))
 					return;
@@ -192,7 +197,7 @@ pqParseInput3(PGconn *conn)
 			 */
 			switch (id)
 			{
-				case 'C':		/* command complete */
+				case PqMsg_CommandComplete:
 					if (pqGets(&conn->workBuffer, conn))
 						return;
 					if (!pgHavePendingResult(conn))
@@ -210,13 +215,12 @@ pqParseInput3(PGconn *conn)
 								CMDSTATUS_LEN);
 					conn->asyncStatus = PGASYNC_READY;
 					break;
-				case 'E':		/* error return */
+				case PqMsg_ErrorResponse:
 					if (pqGetErrorNotice3(conn, true))
 						return;
 					conn->asyncStatus = PGASYNC_READY;
 					break;
-				case 'Z':		/* sync response, backend is ready for new
-								 * query */
+				case PqMsg_ReadyForQuery:
 					if (getReadyForQuery(conn))
 						return;
 					if (conn->pipelineStatus != PQ_PIPELINE_OFF)
@@ -246,7 +250,7 @@ pqParseInput3(PGconn *conn)
 						conn->asyncStatus = PGASYNC_IDLE;
 					}
 					break;
-				case 'I':		/* empty query */
+				case PqMsg_EmptyQueryResponse:
 					if (!pgHavePendingResult(conn))
 					{
 						conn->result = PQmakeEmptyPGresult(conn,
@@ -259,7 +263,7 @@ pqParseInput3(PGconn *conn)
 					}
 					conn->asyncStatus = PGASYNC_READY;
 					break;
-				case '1':		/* Parse Complete */
+				case PqMsg_ParseComplete:
 					/* If we're doing PQprepare, we're done; else ignore */
 					if (conn->cmd_queue_head &&
 						conn->cmd_queue_head->queryclass == PGQUERY_PREPARE)
@@ -277,10 +281,10 @@ pqParseInput3(PGconn *conn)
 						conn->asyncStatus = PGASYNC_READY;
 					}
 					break;
-				case '2':		/* Bind Complete */
+				case PqMsg_BindComplete:
 					/* Nothing to do for this message type */
 					break;
-				case '3':		/* Close Complete */
+				case PqMsg_CloseComplete:
 					/* If we're doing PQsendClose, we're done; else ignore */
 					if (conn->cmd_queue_head &&
 						conn->cmd_queue_head->queryclass == PGQUERY_CLOSE)
@@ -298,11 +302,11 @@ pqParseInput3(PGconn *conn)
 						conn->asyncStatus = PGASYNC_READY;
 					}
 					break;
-				case 'S':		/* parameter status */
+				case PqMsg_ParameterStatus:
 					if (getParameterStatus(conn))
 						return;
 					break;
-				case 'K':		/* secret key data from the backend */
+				case PqMsg_BackendKeyData:
 
 					/*
 					 * This is expected only during backend startup, but it's
@@ -314,7 +318,7 @@ pqParseInput3(PGconn *conn)
 					if (pqGetInt(&(conn->be_key), 4, conn))
 						return;
 					break;
-				case 'T':		/* Row Description */
+				case PqMsg_RowDescription:
 					if (conn->error_result ||
 						(conn->result != NULL &&
 						 conn->result->resultStatus == PGRES_FATAL_ERROR))
@@ -346,7 +350,7 @@ pqParseInput3(PGconn *conn)
 						return;
 					}
 					break;
-				case 'n':		/* No Data */
+				case PqMsg_NoData:
 
 					/*
 					 * NoData indicates that we will not be seeing a
@@ -374,11 +378,11 @@ pqParseInput3(PGconn *conn)
 						conn->asyncStatus = PGASYNC_READY;
 					}
 					break;
-				case 't':		/* Parameter Description */
+				case PqMsg_ParameterDescription:
 					if (getParamDescriptions(conn, msgLength))
 						return;
 					break;
-				case 'D':		/* Data Row */
+				case PqMsg_DataRow:
 					if (conn->result != NULL &&
 						conn->result->resultStatus == PGRES_TUPLES_OK)
 					{
@@ -405,24 +409,24 @@ pqParseInput3(PGconn *conn)
 						conn->inCursor += msgLength;
 					}
 					break;
-				case 'G':		/* Start Copy In */
+				case PqMsg_CopyInResponse:
 					if (getCopyStart(conn, PGRES_COPY_IN))
 						return;
 					conn->asyncStatus = PGASYNC_COPY_IN;
 					break;
-				case 'H':		/* Start Copy Out */
+				case PqMsg_CopyOutResponse:
 					if (getCopyStart(conn, PGRES_COPY_OUT))
 						return;
 					conn->asyncStatus = PGASYNC_COPY_OUT;
 					conn->copy_already_done = 0;
 					break;
-				case 'W':		/* Start Copy Both */
+				case PqMsg_CopyBothResponse:
 					if (getCopyStart(conn, PGRES_COPY_BOTH))
 						return;
 					conn->asyncStatus = PGASYNC_COPY_BOTH;
 					conn->copy_already_done = 0;
 					break;
-				case 'd':		/* Copy Data */
+				case PqMsg_CopyData:
 
 					/*
 					 * If we see Copy Data, just silently drop it.  This would
@@ -431,7 +435,7 @@ pqParseInput3(PGconn *conn)
 					 */
 					conn->inCursor += msgLength;
 					break;
-				case 'c':		/* Copy Done */
+				case PqMsg_CopyDone:
 
 					/*
 					 * If we see Copy Done, just silently drop it.  This is
@@ -1692,21 +1696,21 @@ getCopyDataMessage(PGconn *conn)
 		 */
 		switch (id)
 		{
-			case 'A':			/* NOTIFY */
+			case PqMsg_NotificationResponse:
 				if (getNotify(conn))
 					return 0;
 				break;
-			case 'N':			/* NOTICE */
+			case PqMsg_NoticeResponse:
 				if (pqGetErrorNotice3(conn, false))
 					return 0;
 				break;
-			case 'S':			/* ParameterStatus */
+			case PqMsg_ParameterStatus:
 				if (getParameterStatus(conn))
 					return 0;
 				break;
-			case 'd':			/* Copy Data, pass it back to caller */
+			case PqMsg_CopyData:
 				return msgLength;
-			case 'c':
+			case PqMsg_CopyDone:
 
 				/*
 				 * If this is a CopyDone message, exit COPY_OUT mode and let
@@ -1929,7 +1933,7 @@ pqEndcopy3(PGconn *conn)
 	if (conn->asyncStatus == PGASYNC_COPY_IN ||
 		conn->asyncStatus == PGASYNC_COPY_BOTH)
 	{
-		if (pqPutMsgStart('c', conn) < 0 ||
+		if (pqPutMsgStart(PqMsg_CopyDone, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			return 1;
 
@@ -1940,7 +1944,7 @@ pqEndcopy3(PGconn *conn)
 		if (conn->cmd_queue_head &&
 			conn->cmd_queue_head->queryclass != PGQUERY_SIMPLE)
 		{
-			if (pqPutMsgStart('S', conn) < 0 ||
+			if (pqPutMsgStart(PqMsg_Sync, conn) < 0 ||
 				pqPutMsgEnd(conn) < 0)
 				return 1;
 		}
@@ -2023,7 +2027,7 @@ pqFunctionCall3(PGconn *conn, Oid fnid,
 
 	/* PQfn already validated connection state */
 
-	if (pqPutMsgStart('F', conn) < 0 || /* function call msg */
+	if (pqPutMsgStart(PqMsg_FunctionCall, conn) < 0 ||
 		pqPutInt(fnid, 4, conn) < 0 ||	/* function id */
 		pqPutInt(1, 2, conn) < 0 || /* # of format codes */
 		pqPutInt(1, 2, conn) < 0 || /* format code: BINARY */
diff --git a/src/interfaces/libpq/fe-trace.c b/src/interfaces/libpq/fe-trace.c
index 402784f40e3..b18e3deab6a 100644
--- a/src/interfaces/libpq/fe-trace.c
+++ b/src/interfaces/libpq/fe-trace.c
@@ -562,110 +562,120 @@ pqTraceOutputMessage(PGconn *conn, const char *message, bool toServer)
 
 	switch (id)
 	{
-		case '1':
+		case PqMsg_ParseComplete:
 			fprintf(conn->Pfdebug, "ParseComplete");
 			/* No message content */
 			break;
-		case '2':
+		case PqMsg_BindComplete:
 			fprintf(conn->Pfdebug, "BindComplete");
 			/* No message content */
 			break;
-		case '3':
+		case PqMsg_CloseComplete:
 			fprintf(conn->Pfdebug, "CloseComplete");
 			/* No message content */
 			break;
-		case 'A':				/* Notification Response */
+		case PqMsg_NotificationResponse:
 			pqTraceOutputA(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'B':				/* Bind */
+		case PqMsg_Bind:
 			pqTraceOutputB(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'c':
+		case PqMsg_CopyDone:
 			fprintf(conn->Pfdebug, "CopyDone");
 			/* No message content */
 			break;
-		case 'C':				/* Close(F) or Command Complete(B) */
+		case PqMsg_CommandComplete:
+			/* Close(F) and CommandComplete(B) use the same identifier. */
+			Assert(PqMsg_Close == PqMsg_CommandComplete);
 			pqTraceOutputC(conn->Pfdebug, toServer, message, &logCursor);
 			break;
-		case 'd':				/* Copy Data */
+		case PqMsg_CopyData:
 			/* Drop COPY data to reduce the overhead of logging. */
 			break;
-		case 'D':				/* Describe(F) or Data Row(B) */
+		case PqMsg_Describe:
+			/* Describe(F) and DataRow(B) use the same identifier. */
+			Assert(PqMsg_Describe == PqMsg_DataRow);
 			pqTraceOutputD(conn->Pfdebug, toServer, message, &logCursor);
 			break;
-		case 'E':				/* Execute(F) or Error Response(B) */
+		case PqMsg_Execute:
+			/* Execute(F) and ErrorResponse(B) use the same identifier. */
+			Assert(PqMsg_Execute == PqMsg_ErrorResponse);
 			pqTraceOutputE(conn->Pfdebug, toServer, message, &logCursor,
 						   regress);
 			break;
-		case 'f':				/* Copy Fail */
+		case PqMsg_CopyFail:
 			pqTraceOutputf(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'F':				/* Function Call */
+		case PqMsg_FunctionCall:
 			pqTraceOutputF(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'G':				/* Start Copy In */
+		case PqMsg_CopyInResponse:
 			pqTraceOutputG(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'H':				/* Flush(F) or Start Copy Out(B) */
+		case PqMsg_Flush:
+			/* Flush(F) and CopyOutResponse(B) use the same identifier */
+			Assert(PqMsg_CopyOutResponse == PqMsg_Flush);
 			if (!toServer)
 				pqTraceOutputH(conn->Pfdebug, message, &logCursor);
 			else
 				fprintf(conn->Pfdebug, "Flush");	/* no message content */
 			break;
-		case 'I':
+		case PqMsg_EmptyQueryResponse:
 			fprintf(conn->Pfdebug, "EmptyQueryResponse");
 			/* No message content */
 			break;
-		case 'K':				/* secret key data from the backend */
+		case PqMsg_BackendKeyData:
 			pqTraceOutputK(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'n':
+		case PqMsg_NoData:
 			fprintf(conn->Pfdebug, "NoData");
 			/* No message content */
 			break;
-		case 'N':
+		case PqMsg_NoticeResponse:
 			pqTraceOutputNR(conn->Pfdebug, "NoticeResponse", message,
 							&logCursor, regress);
 			break;
-		case 'P':				/* Parse */
+		case PqMsg_Parse:
 			pqTraceOutputP(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'Q':				/* Query */
+		case PqMsg_Query:
 			pqTraceOutputQ(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'R':				/* Authentication */
+		case PqMsg_AuthenticationRequest:
 			pqTraceOutputR(conn->Pfdebug, message, &logCursor);
 			break;
-		case 's':
+		case PqMsg_PortalSuspended:
 			fprintf(conn->Pfdebug, "PortalSuspended");
 			/* No message content */
 			break;
-		case 'S':				/* Parameter Status(B) or Sync(F) */
+		case PqMsg_Sync:
+			/* Parameter Status(B) and Sync(F) use the same identifier */
+			Assert(PqMsg_ParameterStatus == PqMsg_Sync);
 			if (!toServer)
 				pqTraceOutputS(conn->Pfdebug, message, &logCursor);
 			else
 				fprintf(conn->Pfdebug, "Sync"); /* no message content */
 			break;
-		case 't':				/* Parameter Description */
+		case PqMsg_ParameterDescription:
 			pqTraceOutputt(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'T':				/* Row Description */
+		case PqMsg_RowDescription:
 			pqTraceOutputT(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'v':				/* Negotiate Protocol Version */
+		case PqMsg_NegotiateProtocolVersion:
 			pqTraceOutputv(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'V':				/* Function Call response */
+		case PqMsg_FunctionCallResponse:
 			pqTraceOutputV(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'W':				/* Start Copy Both */
+		case PqMsg_CopyBothResponse:
 			pqTraceOutputW(conn->Pfdebug, message, &logCursor, length);
 			break;
-		case 'X':
+		case PqMsg_Terminate:
 			fprintf(conn->Pfdebug, "Terminate");
 			/* No message content */
 			break;
-		case 'Z':				/* Ready For Query */
+		case PqMsg_ReadyForQuery:
 			pqTraceOutputZ(conn->Pfdebug, message, &logCursor);
 			break;
 		default:
diff --git a/src/tools/msvc/Install.pm b/src/tools/msvc/Install.pm
index 05548d7c0aa..b6dd2c3bbac 100644
--- a/src/tools/msvc/Install.pm
+++ b/src/tools/msvc/Install.pm
@@ -614,6 +614,8 @@ sub CopyIncludeFiles
 		'src/include/', 'c.h', 'port.h', 'postgres_fe.h');
 	lcopy('src/include/libpq/pqcomm.h', $target . '/include/internal/libpq/')
 	  || croak 'Could not copy pqcomm.h';
+	lcopy('src/include/libpq/protocol.h', $target . '/include/internal/libpq/')
+	  || croak 'Could not copy protocol.h';
 
 	CopyFiles(
 		'Server headers',