Expose PQbackendPID() through walreceiver API

This will be used by a subsequent patch.

Reviewed-by: Masahiko Sawada <masahiko.sawada@2ndquadrant.com>
Discussion: https://www.postgresql.org/message-id/CA%2Bfd4k4dM0iEPLxyVyme2RAFsn8SUgrNtBJOu81YqTY4V%2BnqZA%40mail.gmail.com

src/backend/replication/libpqwalreceiver/libpqwalreceiver.c

@@ -74,6 +74,7 @@ static char *libpqrcv_create_slot(WalReceiverConn *conn,
 								  bool temporary,
 								  CRSSnapshotAction snapshot_action,
 								  XLogRecPtr *lsn);
+static pid_t libpqrcv_get_backend_pid(WalReceiverConn *conn);
 static WalRcvExecResult *libpqrcv_exec(WalReceiverConn *conn,
 									   const char *query,
 									   const int nRetTypes,
@@ -93,6 +94,7 @@ static WalReceiverFunctionsType PQWalReceiverFunctions = {
 	libpqrcv_receive,
 	libpqrcv_send,
 	libpqrcv_create_slot,
+	libpqrcv_get_backend_pid,
 	libpqrcv_exec,
 	libpqrcv_disconnect
 };
@@ -858,6 +860,15 @@ libpqrcv_create_slot(WalReceiverConn *conn, const char *slotname,
 	return snapshot;
 }
 
+/*
+ * Return PID of remote backend process.
+ */
+static pid_t
+libpqrcv_get_backend_pid(WalReceiverConn *conn)
+{
+	return PQbackendPID(conn->streamConn);
+}
+
 /*
  * Convert tuple query result to tuplestore.
  */

src/include/replication/walreceiver.h

@@ -226,6 +226,7 @@ typedef char *(*walrcv_create_slot_fn) (WalReceiverConn *conn,
 										const char *slotname, bool temporary,
 										CRSSnapshotAction snapshot_action,
 										XLogRecPtr *lsn);
+typedef pid_t (*walrcv_get_backend_pid_fn) (WalReceiverConn *conn);
 typedef WalRcvExecResult *(*walrcv_exec_fn) (WalReceiverConn *conn,
 											 const char *query,
 											 const int nRetTypes,
@@ -246,6 +247,7 @@ typedef struct WalReceiverFunctionsType
 	walrcv_receive_fn walrcv_receive;
 	walrcv_send_fn walrcv_send;
 	walrcv_create_slot_fn walrcv_create_slot;
+	walrcv_get_backend_pid_fn walrcv_get_backend_pid;
 	walrcv_exec_fn walrcv_exec;
 	walrcv_disconnect_fn walrcv_disconnect;
 } WalReceiverFunctionsType;
@@ -276,6 +278,8 @@ extern PGDLLIMPORT WalReceiverFunctionsType *WalReceiverFunctions;
 	WalReceiverFunctions->walrcv_send(conn, buffer, nbytes)
 #define walrcv_create_slot(conn, slotname, temporary, snapshot_action, lsn) \
 	WalReceiverFunctions->walrcv_create_slot(conn, slotname, temporary, snapshot_action, lsn)
+#define walrcv_get_backend_pid(conn) \
+	WalReceiverFunctions->walrcv_get_backend_pid(conn)
 #define walrcv_exec(conn, exec, nRetTypes, retTypes) \
 	WalReceiverFunctions->walrcv_exec(conn, exec, nRetTypes, retTypes)
 #define walrcv_disconnect(conn) \