gnrc/rpl: check old rank before removing parents

[elenaf9]

Contribution description

gnrc_rpl_dodag_remove_all_parents sets the node's rank to GNRC_RPL_INFINITE_RANK, so checking the (old) rank should be done before that function call.

Testing procedure

See Testing procedure in #21586.

Issues/PRs references

Came up while testing #21586.

[riot-ci]

Murdock results

✔️ PASSED

2d4542e gnrc/rpl: check old rank before removing parents

Success	Failures	Total	Runtime
10503	0	10504	19m:34s

Artifacts

• Documentation preview

[Teufelchen1]

LGTM. You tested it, right? (and it's not just an unchecked finding while reading code)

[elenaf9]

LGTM. You tested it, right? (and it's not just an unchecked finding while reading code)

Yes :) I tested initially with #21586, and just now again also on the current master.

Without this fix, there is some weird ping pong behavior where a parent, after local_repair was triggered, becomes a child of their own child. Both nodes, former parent and child, believe to be each other's children, resulting in a lot of DIOs being sent back and forth that each time decrease the rank, slowly converging towards the INFINITE_RANK.
With this PR's fix, both parent and child immediately set their rank to INFINITE_RANK without the above behavior.

I assume that the reason is that without the fix the if-branch is skipped and the trickle timer not reset, resulting in the child not being informed of the poisoned (i.e. infinite) parent rank.