tools/xenstored: Limit the number of requests a connection can delay

Currently, only liveupdate request can be delayed. The request can only
be performed by a privileged connection (e.g. dom0). So it is fine to
have no limits.

In a follow-up patch we will want to delay request for unprivileged
connection as well. So it is best to apply a limit.

For now and for simplicity, only a single request can be delayed
for a given unprivileged connection.

Take the opportunity to tweak the prototype and provide a way to
bypass the quota check. This would be useful when the function
is called from the restore code.

Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Luca Fancellu <luca.fancellu@arm.com>
Reviewed-by: Juergen Gross <jgross@suse.com>

tools/xenstore/xenstored_control.c

@@ -737,7 +737,7 @@ static const char *lu_start(const void *ctx, struct connection *conn,
 	lu_status->timeout = to;
 	lu_status->started_at = time(NULL);
 
-	errno = delay_request(conn, conn->in, do_lu_start, NULL);
+	errno = delay_request(conn, conn->in, do_lu_start, NULL, false);
 
 	return NULL;
 }

tools/xenstore/xenstored_core.c

@@ -279,10 +279,19 @@ static void call_delayed(struct delayed_request *req)
 }
 
 int delay_request(struct connection *conn, struct buffered_data *in,
-		  bool (*func)(struct delayed_request *), void *data)
+		  bool (*func)(struct delayed_request *), void *data,
+		  bool no_quota_check)
 {
 	struct delayed_request *req;
 
+	/*
+	 * Only allow one request can be delayed for an unprivileged
+	 * connection.
+	 */
+	if (!no_quota_check && domain_is_unprivileged(conn) &&
+	    !list_empty(&conn->delayed))
+		return ENOSPC;
+
 	req = talloc(in, struct delayed_request);
 	if (!req)
 		return ENOMEM;

tools/xenstore/xenstored_core.h

@@ -213,7 +213,8 @@ char *get_parent(const void *ctx, const char *node);
 
 /* Delay a request. */
 int delay_request(struct connection *conn, struct buffered_data *in,
-		  bool (*func)(struct delayed_request *), void *data);
+		  bool (*func)(struct delayed_request *), void *data,
+		  bool no_quota_check);
 
 /* Tracing infrastructure. */
 void trace_create(const void *data, const char *type);