[Issue #323] stable remote agent API

Author: gsmolk

src/pg_probackup.h

@@ -307,7 +307,10 @@ typedef enum ShowFormat
 #define FILE_NOT_FOUND		(-2) /* file disappeared during backup */
 #define BLOCKNUM_INVALID	(-1)
 #define PROGRAM_VERSION	"2.4.9"
+
+/* update when remote agent API or behaviour changes */
 #define AGENT_PROTOCOL_VERSION 20409
+#define AGENT_PROTOCOL_VERSION_STR "2.4.9"
 
 /* update only when changing storage format */
 #define STORAGE_FORMAT_VERSION "2.4.4"

src/utils/remote.c

@@ -247,8 +247,9 @@ bool launch_agent(void)
 				(agent_version / 100) % 100,
 				agent_version % 100);
 
-		elog(ERROR, "Remote agent version %s does not match local program version %s",
-			agent_version_str, PROGRAM_VERSION);
+		elog(ERROR, "Remote agent protocol version %s does not match local program protocol version %s, "
+					"consider to upgrade pg_probackup binary",
+			agent_version_str, AGENT_PROTOCOL_VERSION_STR);
 	}
 
 	return true;