kernel protocol ignores default preference during scan

[Alexander Zubkov]

And just an idea. I've attached the patch as an example, although it
should be functional too. I mentioned that functions krt_learn_async()
and krt_learn_scan() are very similar and I thought that it may be
useful to join them into a single function. If I did not make a
mistake, all code pathes should be almost the same, with a couple of
minor exceptions:
1) old_best is set in the beginning, but was earlier only for async version
2) g = NULL after rte_free(g) in one of the branches, but it does not
affect anything and may be dropped

On Wed, Sep 21, 2022 at 6:00 PM Maria Matejka via Bird-users
<bird-users at network.cz> wrote:
>
> Hello!
>
> Thank you for finding and fixing. Will check and include.
>
> Maria
>
> On 9/21/22 17:15, Alexander Zubkov via Bird-users wrote:
> > I made a trivial patch for the case.
> >
> > On Tue, Sep 20, 2022 at 6:23 PM Alexander Zubkov <green at qrator.net> wrote:
> >>
> >> Hi,
> >>
> >> Bird from master branch ignores the default preference set in channel
> >> for a kernel protocol, like that:
> >>
> >> protocol kernel {
> >>          learn yes;
> >>          ipv4 {
> >>                  preference 200;
> >>                  import all;
> >>                  export none;
> >>          };
> >> }
> >>
> >> Version 2.0.10 seems ok. I suppose the change was introduced with this commit:
> >>
> >> https://gitlab.nic.cz/labs/bird/-/commit/eb937358c087eaeb6f209660cc7ecfe6d6eff739
> >>
> >> It adds setting of the preference to the krt_learn_async() function.
> >> But as I understand when route is learned with netlink hook it is
> >> processed like this:
> >> nl_parse_route -> krt_got_route_async -> krt_learn_async
> >> But when it is learned from scan, it is processed like this:
> >> nl_parse_route -> krt_got_route -> krt_learn_scan
> >>
> >> And setting preference was not added to krt_learn_scan() function.
> >>
> >> I also saw that in my test - when I added a new route to the kernel
> >> while bird was running, it appeared in bird with correct preference.
> >> But some time after, the scan updated the route and it was replaced
> >> with the default preference.
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