[PATCH net-next] tools: ynl: add ynl_dump_empty() helper

[Jakub Kicinski <kuba@kernel.org>]

Checking if dump is empty requires a couple of casts.
Add a convenient wrapper.

Add an example use in the netdev sample, loopback is always
present so an empty dump is an error.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
---
CC: nicolas.dichtel@6wind.com
CC: sdf@google.com
---
 tools/net/ynl/lib/ynl.h        | 12 ++++++++++++
 tools/net/ynl/samples/netdev.c |  2 ++
 2 files changed, 14 insertions(+)

diff --git a/tools/net/ynl/lib/ynl.h b/tools/net/ynl/lib/ynl.h
index 9842e85a8c57..eef7c6324ed4 100644
--- a/tools/net/ynl/lib/ynl.h
+++ b/tools/net/ynl/lib/ynl.h
@@ -91,6 +91,18 @@ void ynl_sock_destroy(struct ynl_sock *ys);
 	     !ynl_dump_obj_is_last(iter);				\
 	     iter = ynl_dump_obj_next(iter))
 
+/**
+ * ynl_dump_empty() - does the dump have no entries
+ * @dump: pointer to the dump list, as returned by a dump call
+ *
+ * Check if the dump is empty, i.e. contains no objects.
+ * Dump calls return NULL on error, and terminator element if empty.
+ */
+static inline bool ynl_dump_empty(void *dump)
+{
+	return dump == (void *)YNL_LIST_END;
+}
+
 int ynl_subscribe(struct ynl_sock *ys, const char *grp_name);
 int ynl_socket_get_fd(struct ynl_sock *ys);
 int ynl_ntf_check(struct ynl_sock *ys);
diff --git a/tools/net/ynl/samples/netdev.c b/tools/net/ynl/samples/netdev.c
index 591b90e21890..3e7b29bd55d5 100644
--- a/tools/net/ynl/samples/netdev.c
+++ b/tools/net/ynl/samples/netdev.c
@@ -100,6 +100,8 @@ int main(int argc, char **argv)
 		if (!devs)
 			goto err_close;
 
+		if (ynl_dump_empty(devs))
+			fprintf(stderr, "Error: no devices reported\n");
 		ynl_dump_foreach(devs, d)
 			netdev_print_device(d, 0);
 		netdev_dev_get_list_free(devs);
-- 
2.44.0