[PATCH net-next] tools: ynl-gen-c: improve support for empty nests

[Jakub Kicinski <kuba@kernel.org>]

Empty nests are the same size as a flag at the netlink level
(just a 4 byte nlattr without a payload). They are sometimes
useful in case we want to only communicate a presence of
something but may want to add more details later.
This may be the case in the upcoming io_uring ZC patches,
for example.

Improve handling of nested empty structs. We already support
empty structs since a lot of netlink replies are empty, but
for nested ones we need minor tweaks to avoid pointless empty
lines and unused variables.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
---
CC: dw@davidwei.uk
CC: donald.hunter@gmail.com
CC: nicolas.dichtel@6wind.com
CC: sdf@fomichev.me
---
 tools/net/ynl/ynl-gen-c.py | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/tools/net/ynl/ynl-gen-c.py b/tools/net/ynl/ynl-gen-c.py
index ec2288948795..9722a8f72a46 100755
--- a/tools/net/ynl/ynl-gen-c.py
+++ b/tools/net/ynl/ynl-gen-c.py
@@ -1765,7 +1765,14 @@ _C_KW = {
                   f'{struct.ptr_name}dst = yarg->data;']
     init_lines = []
 
-    _multi_parse(ri, struct, init_lines, local_vars)
+    if struct.member_list():
+        _multi_parse(ri, struct, init_lines, local_vars)
+    else:
+        # Empty nest
+        ri.cw.block_start()
+        ri.cw.p('return 0;')
+        ri.cw.block_end()
+        ri.cw.nl()
 
 
 def parse_rsp_msg(ri, deref=False):
@@ -2592,7 +2599,8 @@ _C_KW = {
                 val = attr.value
             val += 1
             cw.p(attr.enum_name + suffix)
-        cw.nl()
+        if attr_set.items():
+            cw.nl()
         cw.p(attr_set.cnt_name + ('' if max_by_define else ','))
         if not max_by_define:
             cw.p(f"{attr_set.max_name} = {max_value}")
-- 
2.47.1