rc: Teach netwait to wait for DAD

[des]

Summary

In some configurations, especially in jails, it is possible for the
system to boot so fast that we end up launching daemons while duplicate
address detection is still ongoing. If that happens, said daemons may
fail to bind to IPv6 addresses, as they are still tentative. Teach the
netwait service to wait (up to 10 seconds, by default) for the tentative
flag to vanish.

MFC after: 1 week

[kevans]

libexec/rc/rc.d/netwait

114

Pre-existing isssue, but that seems a bit misleading? You're likely in the inner loop, in which case we'll just progress to the next $netwait_ip. It's not clear to me if the described behavior or the implemented behavior make more sense, but I guess if nobody has complained it's fine as-is.

[des]

libexec/rc/rc.d/netwait

114

yeah I can't think of a way to make it reliably dwim, except maybe rewrite in lua

[olce]

libexec/rc/rc.d/netwait

76

114

We could add trap "break 2" SIGINT before the inner loop (would work in most cases), but this doesn't solve the race that the trap is established slightly before (and still in force slightly after) the loop where the command should exclusively be applicable.

What about just dropping all trap break SIGINT in the whole function, and instead use a trap <code> SIGINT where <code> possibly prints a warning message (like at this function's end; this part may as well be factored out into a separate function) and finishes with return. If more granularity is needed (only cancelling part of the process when hitting ^C), then just split netwait_start() into as many functions that use the technique just described.

share/man/man5/rc.conf.5

4566–4567

Helpful to introduce an acronym that is used later.

[cperciva]

Can you clarify the scenario where this is needed? We're already sleeping for ${SYSCTL_N} net.inet6.ip6.dad_count as part of bringing up interfaces in netif_common.

[des]

In D51889#1205894, @cperciva wrote:

Can you clarify the scenario where this is needed? We're already sleeping for ${SYSCTL_N} net.inet6.ip6.dad_count as part of bringing up interfaces in netif_common.

netif_common is not used to configure jail addresses. They are configured by jail(8) when the jail starts up.

[olce]

More bugs.

libexec/rc/rc.d/netwait

61

Your changes also make that line wrong which, coupled with the other problem below, explains why you didn't see any failure on testing for regular cases.

[des]

rf

[olce]

Please also add the acronym in the manual page as suggested in an inline comment.