nftables monitor json mode is broken

[Florian Westphal <fw@strlen.de>]

Hi,

as subject says, 'nft monitor -j' is broken.
Example:

./run-tests.sh -j testcases/object.t
monitor: running tests from file object.t
monitor output differs!
--- /tmp/tmp.emU4zIN8UT/tmp.C4TeyO6xYk  2025-08-27 19:05:08.039619097 +0200
+++ /tmp/tmp.emU4zIN8UT/tmp.jBOL3aIrp5  2025-08-27 19:05:09.062551248 +0200
@@ -1 +1 @@
-{"delete": {"quota": {"family": "ip", "name": "q", "table": "t", "handle": 0, "bytes": 26214400, "used": 0, "inv": false}}}
+{"delete": {"quota": {"family": "ip", "name": "q", "table": "t", "handle": 0, "bytes": 0, "used": 0, "inv": false}}}
monitor output differs!
--- /tmp/tmp.emU4zIN8UT/tmp.C4TeyO6xYk  2025-08-27 19:05:10.095619097 +0200
+++ /tmp/tmp.emU4zIN8UT/tmp.Guz55knY19  2025-08-27 19:05:11.117393075 +0200
@@ -1 +1 @@
-{"delete": {"limit": {"family": "ip", "name": "l", "table": "t", "handle": 0, "rate": 1, "per": "second", "burst": 5}}}
+{"delete": {"limit": {"family": "ip", "name": "l", "table": "t", "handle": 0, "rate": 0, "per": "error"}}}

I did notice this weeks ago but thought it was a problem on my end
and then didn't have time to investigate closer.

But its in fact broken on kernel side, since

netfilter: nf_tables: Reintroduce shortened deletion notifications

In short, unlike the normal output, json output wants to dump
everything, but the notifications no longer include the extra data, just
the bare minimum to identify the object being deleted.

As noone has complained so far I am inclinded to delete the
tests and rip out json support from monitor mode, it seems noone
uses it or even runs the tests for it.

Alternatives i see are:
1. implement a cache and query it
2. rework the json mode to be forgiving as to what is set
   and what isn't in the object.

Object here also means any object reported in any delete kind,
not just NFT_MSG_DELOBJ.  This applies to set elements etc. too,
json expects the full info, but the kernel notifications no longer
provide this.

Alternative options?

Thanks.