Log remote NOTICE, WARNING, and similar messages using ereport().

Previously, NOTICE, WARNING, and similar messages received from remote
servers over replication, postgres_fdw, or dblink connections were printed
directly to stderr on the local server (e.g., the subscriber). As a result,
these messages lacked log prefixes (e.g., timestamp), making them harder
to trace and correlate with other log entries.

This commit addresses the issue by introducing a custom notice receiver
for replication, postgres_fdw, and dblink connections. These messages
are now logged via ereport(), ensuring they appear in the logs with proper
formatting and context, which improves clarity and aids in debugging.

Author: Vignesh C <vignesh21@gmail.com>
Reviewed-by: Álvaro Herrera <alvherre@kurilemu.de>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/CALDaNm2xsHpWRtLm-VL_HJCsaE3+1Y_n-jDEAr3-suxVqc3xoQ@mail.gmail.com

contrib/dblink/dblink.c

@@ -240,6 +240,10 @@ dblink_get_conn(char *conname_or_str,
 					 errmsg("could not establish connection"),
 					 errdetail_internal("%s", msg)));
 		}
+
+		PQsetNoticeReceiver(conn, libpqsrv_notice_receiver,
+							gettext_noop("received message via remote connection"));
+
 		dblink_security_check(conn, NULL, connstr);
 		if (PQclientEncoding(conn) != GetDatabaseEncoding())
 			PQsetClientEncoding(conn, GetDatabaseEncodingName());
@@ -338,6 +342,9 @@ dblink_connect(PG_FUNCTION_ARGS)
 				 errdetail_internal("%s", msg)));
 	}
 
+	PQsetNoticeReceiver(conn, libpqsrv_notice_receiver,
+						gettext_noop("received message via remote connection"));
+
 	/* check password actually used if not superuser */
 	dblink_security_check(conn, connname, connstr);
 

contrib/postgres_fdw/connection.c

@@ -625,6 +625,9 @@ connect_pg_server(ForeignServer *server, UserMapping *user)
 							server->servername),
 					 errdetail_internal("%s", pchomp(PQerrorMessage(conn)))));
 
+		PQsetNoticeReceiver(conn, libpqsrv_notice_receiver,
+							gettext_noop("received message via remote connection"));
+
 		/* Perform post-connection security checks. */
 		pgfdw_security_check(keywords, values, user, conn);
 

src/backend/replication/libpqwalreceiver/libpqwalreceiver.c

@@ -232,6 +232,9 @@ libpqrcv_connect(const char *conninfo, bool replication, bool logical,
 				 errhint("Target server's authentication method must be changed, or set password_required=false in the subscription parameters.")));
 	}
 
+	PQsetNoticeReceiver(conn->streamConn, libpqsrv_notice_receiver,
+						gettext_noop("received message via replication"));
+
 	/*
 	 * Set always-secure search path for the cases where the connection is
 	 * used to run SQL queries, so malicious users can't get control.

src/include/libpq/libpq-be-fe-helpers.h

@@ -454,4 +454,34 @@ exit:	;
 	return error;
 }
 
+/*
+ * libpqsrv_notice_receiver
+ *
+ * Custom notice receiver for libpq connections.
+ *
+ * This function is intended to be set via PQsetNoticeReceiver() so that
+ * NOTICE, WARNING, and similar messages from the connection are reported via
+ * ereport(), instead of being printed to stderr.
+ */
+static inline void
+libpqsrv_notice_receiver(void *arg, const PGresult *res)
+{
+	char	   *message;
+	int			len;
+	char	   *prefix = (char *) arg;
+
+	/*
+	 * Trim the trailing newline from the message text returned from
+	 * PQresultErrorMessage(), as it always includes one, to produce cleaner
+	 * log output.
+	 */
+	message = PQresultErrorMessage(res);
+	len = strlen(message);
+	if (len > 0 && message[len - 1] == '\n')
+		len--;
+
+	ereport(LOG,
+			errmsg_internal("%s: %.*s", _(prefix), len, message));
+}
+
 #endif							/* LIBPQ_BE_FE_HELPERS_H */