Add walreceiver API to get remote server version

Add a separate walreceiver API function walrcv_server_version() to get
the version of the remote server, instead of doing it as part of
walrcv_identify_system().  This allows the server version to be
available even for uses that don't call IDENTIFY_SYSTEM, and it seems
cleaner anyway.

This is for an upcoming patch, not currently used.
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://www.postgresql.org/message-id/20190115071359.GF1433@paquier.xyz

src/backend/replication/libpqwalreceiver/libpqwalreceiver.c

@@ -56,8 +56,8 @@ static char *libpqrcv_get_conninfo(WalReceiverConn *conn);
 static void libpqrcv_get_senderinfo(WalReceiverConn *conn,
 						char **sender_host, int *sender_port);
 static char *libpqrcv_identify_system(WalReceiverConn *conn,
-						 TimeLineID *primary_tli,
-						 int *server_version);
+						 TimeLineID *primary_tli);
+static int libpqrcv_server_version(WalReceiverConn *conn);
 static void libpqrcv_readtimelinehistoryfile(WalReceiverConn *conn,
 								 TimeLineID tli, char **filename,
 								 char **content, int *len);
@@ -86,6 +86,7 @@ static WalReceiverFunctionsType PQWalReceiverFunctions = {
 	libpqrcv_get_conninfo,
 	libpqrcv_get_senderinfo,
 	libpqrcv_identify_system,
+	libpqrcv_server_version,
 	libpqrcv_readtimelinehistoryfile,
 	libpqrcv_startstreaming,
 	libpqrcv_endstreaming,
@@ -309,8 +310,7 @@ libpqrcv_get_senderinfo(WalReceiverConn *conn, char **sender_host,
  * timeline ID of the primary.
  */
 static char *
-libpqrcv_identify_system(WalReceiverConn *conn, TimeLineID *primary_tli,
-						 int *server_version)
+libpqrcv_identify_system(WalReceiverConn *conn, TimeLineID *primary_tli)
 {
 	PGresult   *res;
 	char	   *primary_sysid;
@@ -343,11 +343,18 @@ libpqrcv_identify_system(WalReceiverConn *conn, TimeLineID *primary_tli,
 	*primary_tli = pg_strtoint32(PQgetvalue(res, 0, 1));
 	PQclear(res);
 
-	*server_version = PQserverVersion(conn->streamConn);
-
 	return primary_sysid;
 }
 
+/*
+ * Thin wrapper around libpq to obtain server version.
+ */
+static int
+libpqrcv_server_version(WalReceiverConn *conn)
+{
+	return PQserverVersion(conn->streamConn);
+}
+
 /*
  * Start streaming WAL data from given streaming options.
  *

src/backend/replication/logical/worker.c

@@ -1674,7 +1674,6 @@ ApplyWorkerMain(Datum main_arg)
 		RepOriginId originid;
 		TimeLineID	startpointTLI;
 		char	   *err;
-		int			server_version;
 
 		myslotname = MySubscription->slotname;
 
@@ -1708,8 +1707,7 @@ ApplyWorkerMain(Datum main_arg)
 		 * We don't really use the output identify_system for anything but it
 		 * does some initializations on the upstream so let's still call it.
 		 */
-		(void) walrcv_identify_system(wrconn, &startpointTLI,
-									  &server_version);
+		(void) walrcv_identify_system(wrconn, &startpointTLI);
 
 	}
 

src/backend/replication/walreceiver.c

@@ -330,7 +330,6 @@ WalReceiverMain(void)
 	{
 		char	   *primary_sysid;
 		char		standby_sysid[32];
-		int			server_version;
 		WalRcvStreamOptions options;
 
 		/*
@@ -338,8 +337,7 @@ WalReceiverMain(void)
 		 * IDENTIFY_SYSTEM replication command.
 		 */
 		EnableWalRcvImmediateExit();
-		primary_sysid = walrcv_identify_system(wrconn, &primaryTLI,
-											   &server_version);
+		primary_sysid = walrcv_identify_system(wrconn, &primaryTLI);
 
 		snprintf(standby_sysid, sizeof(standby_sysid), UINT64_FORMAT,
 				 GetSystemIdentifier());

src/include/replication/walreceiver.h

@@ -209,8 +209,8 @@ typedef void (*walrcv_get_senderinfo_fn) (WalReceiverConn *conn,
 										  char **sender_host,
 										  int *sender_port);
 typedef char *(*walrcv_identify_system_fn) (WalReceiverConn *conn,
-											TimeLineID *primary_tli,
-											int *server_version);
+											TimeLineID *primary_tli);
+typedef int (*walrcv_server_version_fn) (WalReceiverConn *conn);
 typedef void (*walrcv_readtimelinehistoryfile_fn) (WalReceiverConn *conn,
 												   TimeLineID tli,
 												   char **filename,
@@ -240,6 +240,7 @@ typedef struct WalReceiverFunctionsType
 	walrcv_get_conninfo_fn walrcv_get_conninfo;
 	walrcv_get_senderinfo_fn walrcv_get_senderinfo;
 	walrcv_identify_system_fn walrcv_identify_system;
+	walrcv_server_version_fn walrcv_server_version;
 	walrcv_readtimelinehistoryfile_fn walrcv_readtimelinehistoryfile;
 	walrcv_startstreaming_fn walrcv_startstreaming;
 	walrcv_endstreaming_fn walrcv_endstreaming;
@@ -260,8 +261,10 @@ extern PGDLLIMPORT WalReceiverFunctionsType *WalReceiverFunctions;
 	WalReceiverFunctions->walrcv_get_conninfo(conn)
 #define walrcv_get_senderinfo(conn, sender_host, sender_port) \
 	WalReceiverFunctions->walrcv_get_senderinfo(conn, sender_host, sender_port)
-#define walrcv_identify_system(conn, primary_tli, server_version) \
-	WalReceiverFunctions->walrcv_identify_system(conn, primary_tli, server_version)
+#define walrcv_identify_system(conn, primary_tli) \
+	WalReceiverFunctions->walrcv_identify_system(conn, primary_tli)
+#define walrcv_server_version(conn) \
+	WalReceiverFunctions->walrcv_server_version(conn)
 #define walrcv_readtimelinehistoryfile(conn, tli, filename, content, size) \
 	WalReceiverFunctions->walrcv_readtimelinehistoryfile(conn, tli, filename, content, size)
 #define walrcv_startstreaming(conn, options) \