selftests/tc-testing: Test htb_dequeue_tree with deactivation and row emptying

Ensure that any deactivation and row emptying that occurs
during htb_dequeue_tree does not cause a kernel panic.
This scenario originally triggered a kernel BUG_ON, and
we are checking for a graceful fail now.

Signed-off-by: William Liu <will@willsroot.io>
Signed-off-by: Savino Dicanosa <savy@syst3mfailure.io>
Link: https://patch.msgid.link/20250717022912.221426-1-will@willsroot.io
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: BitsByWill

tools/testing/selftests/tc-testing/tc-tests/infra/qdiscs.json

@@ -127,6 +127,32 @@
             "$IP addr del 10.10.10.10/24 dev $DUMMY"
         ]
     },
+    {
+        "id": "5456",
+        "name": "Test htb_dequeue_tree with deactivation and row emptying",
+        "category": [
+            "qdisc",
+            "htb"
+        ],
+        "plugins": {
+            "requires": "nsPlugin"
+        },
+        "setup": [
+            "$IP link set dev $DUMMY up || true",
+            "$IP addr add 10.10.11.10/24 dev $DUMMY || true",
+            "$TC qdisc add dev $DUMMY root handle 1: htb default 1",
+            "$TC class add dev $DUMMY parent 1: classid 1:1 htb rate 64bit ",
+            "$TC qdisc add dev $DUMMY parent 1:1 handle 2: netem",
+            "$TC qdisc add dev $DUMMY parent 2:1 handle 3: blackhole"
+        ],
+        "cmdUnderTest": "ping -c1 -W0.01 -I $DUMMY 10.10.11.11",
+        "expExitCode": "1",
+        "verifyCmd": "$TC -j qdisc show dev $DUMMY",
+        "matchJSON": [],
+        "teardown": [
+            "$TC qdisc del dev $DUMMY root"
+        ]
+    },
     {
         "id": "c024",
         "name": "Test TBF with SKBPRIO - catch qlen corner cases",