tools: ynl-gen: avoid repetitive variables definitions

In the generated attribute parsing code, avoid repetitively
defining the same variables over and over again, local to
the conditional block for each attribute.

This patch consolidates the definitions of local variables
for attribute parsing, so that they are defined at the
function level, and re-used across attributes, thus making
the generated code read more natural.

If attributes defines identical local_vars, then they will
be deduplicated, attributes are assumed to only use their
local variables transiently.

The example below shows how `len` was defined repeatedly in
tools/net/ynl/generated/nl80211-user.c:

nl80211_iftype_data_attrs_parse(..) {
   [..]
   ynl_attr_for_each_nested(attr, nested) {
     unsigned int type = ynl_attr_type(attr);

     if (type == NL80211_BAND_IFTYPE_ATTR_IFTYPES) {
       unsigned int len;
       [..]
     } else if (type == NL80211_BAND_IFTYPE_ATTR_HE_CAP_MAC) {
       unsigned int len;
       [..]
     [same pattern 8 times, so 11 times in total]
     } else if (type == NL80211_BAND_IFTYPE_ATTR_EHT_CAP_PPE) {
       unsigned int len;
       [..]
     }
   }
   return 0;
}

This patch results in this diffstat for the generated code:

$ diff -Naur pre/ post/ | diffstat
  devlink-user.c      |  187 +++----------------
  dpll-user.c         |   10 -
  ethtool-user.c      |   49 +----
  fou-user.c          |    5
  handshake-user.c    |    3
  mptcp_pm-user.c     |    3
  nfsd-user.c         |   16 -
  nl80211-user.c      |  159 +---------------
  nlctrl-user.c       |   21 --
  ovpn-user.c         |    7
  ovs_datapath-user.c |    9
  ovs_flow-user.c     |   89 ---------
  ovs_vport-user.c    |    7
  rt-addr-user.c      |   14 -
  rt-link-user.c      |  183 ++----------------
  rt-neigh-user.c     |   14 -
  rt-route-user.c     |   26 --
  rt-rule-user.c      |   11 -
  tc-user.c           |  380 +++++----------------------------------
  tcp_metrics-user.c  |    7
  team-user.c         |    5
  21 files changed, 175 insertions(+), 1030 deletions(-)

The changed lines are mostly `unsigned int len;` definitions:

$ diff -Naur pre/ post/ | grep ^[-+] | grep -v '^[-+]\{3\}' |
  grep -v '^.$' | sed -e 's/\t\+/ /g' | sort | uniq -c | sort -nr
    488 - unsigned int len;
    153 + unsigned int len;
     24 - const struct nlattr *attr2;
     18 + const struct nlattr *attr2;
      1 - __u32 policy_id, attr_id;
      1 + __u32 policy_id, attr_id;
      1 - __u32 op_id;
      1 + __u32 op_id;
      1 - const struct nlattr *attr_policy_id, *attr_attr_id;
      1 + const struct nlattr *attr_policy_id, *attr_attr_id;
      1 - const struct nlattr *attr_op_id;
      1 + const struct nlattr *attr_op_id;

Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Asbjørn Sloth Tønnesen <ast@fiberby.net>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Link: https://patch.msgid.link/20250915144301.725949-6-ast@fiberby.net


Signed-off-by: Jakub Kicinski <kuba@kernel.org>