[PATCH net-next v2 2/2] tools: ynl: rework the string representation of NlError

[Jakub Kicinski <kuba@kernel.org>]

In early days of YNL development dumping the NlMsg on errors
was quite useful, as the library itself could have been buggy.
These days increasingly the NlMsg is just taking up screen space
and means nothing to a typical user. Try to format the errors
more in line with how YNL C formats its errors strings.

Before:
  $ ynl --family ethtool  --do channels-set  --json '{}'
  Netlink error: Invalid argument
  nl_len = 44 (28) nl_flags = 0x300 nl_type = 2
	error: -22
	extack: {'miss-type': 'header'}

  $ ynl --family ethtool  --do channels-set  --json '{..., "tx-count": 999}'
  Netlink error: Invalid argument
  nl_len = 88 (72) nl_flags = 0x300 nl_type = 2
	error: -22
	extack: {'msg': 'requested channel count exceeds maximum', 'bad-attr': '.tx-count'}

After:
  $ ynl --family ethtool  --do channels-set  --json '{}'
  Netlink error: Invalid argument {'miss-type': 'header'}

  $ ynl --family ethtool  --do channels-set  --json '{..., "tx-count": 999}'
  Netlink error: requested channel count exceeds maximum: Invalid argument {'bad-attr': '.tx-count'}

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
---
v2:
 - remove an unnecessary condition
v1: https://lore.kernel.org/20251024215713.1250688-1-kuba@kernel.org

CC: donald.hunter@gmail.com
CC: ast@fiberby.net
---
 tools/net/ynl/pyynl/lib/ynl.py | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/tools/net/ynl/pyynl/lib/ynl.py b/tools/net/ynl/pyynl/lib/ynl.py
index bdcc4f031d39..225baad3c8f8 100644
--- a/tools/net/ynl/pyynl/lib/ynl.py
+++ b/tools/net/ynl/pyynl/lib/ynl.py
@@ -105,7 +105,16 @@ from .nlspec import SpecFamily
         self.error = -nl_msg.error
 
     def __str__(self):
-        return f"Netlink error: {os.strerror(self.error)}\n{self.nl_msg}"
+        msg = "Netlink error: "
+
+        extack = self.nl_msg.extack.copy() if self.nl_msg.extack else {}
+        if 'msg' in extack:
+            msg += extack['msg'] + ': '
+            del extack['msg']
+        msg += os.strerror(self.error)
+        if extack:
+            msg += ' ' + str(extack)
+        return msg
 
 
 class ConfigError(Exception):
-- 
2.51.0