Add conninfo to pg_stat_wal_receiver

Commit b1a9bad9e7 introduced a stats view to provide insight into the
running WAL receiver, but neglected to include the connection string in
it, as reported by Michaël Paquier.  This commit fixes that omission.
(Any security-sensitive information is not disclosed).

While at it, close the mild security hole that we were exposing the
password in the connection string in shared memory.  This isn't
user-accessible, but it still looks like a good idea to avoid having the
cleartext password in memory.

Author: Michaël Paquier, Álvaro Herrera
Review by: Vik Fearing

Discussion: https://www.postgresql.org/message-id/CAB7nPqStg4M561obo7ryZ5G+fUydG4v1Ajs1xZT1ujtu+woRag@mail.gmail.com

src/include/replication/walreceiver.h

@@ -100,7 +100,8 @@ typedef struct
 	TimestampTz latestWalEndTime;
 
 	/*
-	 * connection string; is used for walreceiver to connect with the primary.
+	 * connection string; initially set to connect to the primary, and later
+	 * clobbered to hide security-sensitive fields.
 	 */
 	char		conninfo[MAXCONNINFO];
 
@@ -118,6 +119,9 @@ typedef struct
 	 */
 	bool		force_reply;
 
+	/* set true once conninfo is ready to display (obfuscated pwds etc) */
+	bool		ready_to_display;
+
 	/*
 	 * Latch used by startup process to wake up walreceiver after telling it
 	 * where to start streaming (after setting receiveStart and
@@ -133,6 +137,9 @@ extern WalRcvData *WalRcv;
 typedef void (*walrcv_connect_type) (char *conninfo);
 extern PGDLLIMPORT walrcv_connect_type walrcv_connect;
 
+typedef char *(*walrcv_get_conninfo_type) (void);
+extern PGDLLIMPORT walrcv_get_conninfo_type walrcv_get_conninfo;
+
 typedef void (*walrcv_identify_system_type) (TimeLineID *primary_tli);
 extern PGDLLIMPORT walrcv_identify_system_type walrcv_identify_system;