bin/57958: dhcpcd sometimes configures the same two DHCPv6 addresses on different interfaces

[campbell+netbsd%mumble.net@localhost]

>Number:         57958
>Category:       bin
>Synopsis:       dhcpcd sometimes configures the same two DHCPv6 addresses on different interfaces
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    bin-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri Feb 23 23:00:00 +0000 2024
>Originator:     Taylor R Campbell
>Release:        netbsd-10
>Organization:
The You Say Network, I Say Notwork Foundatoin
>Environment:
NetBSD nanocons.local 10.0_RC3 NetBSD 10.0_RC3 (GENERIC64) #15: Wed Jan 17 05:31:14 UTC 2024  root@manticore.local:/usr/obj/10/evbarm64/sys/arch/evbarm/compile/GENERIC64 evbarm
>Description:
I have a LAN configured on a subnet, say 2001:db8::/64, with SLAAC and DHCPv6.  I have a NetBSD device with two interfaces, ure0 and urtwn0, configured with dhcpcd, using the only two changes from 10.0_RC3 dhcpcd.conf:

-option rapid_commit
+#option rapid_commit
...
+nooption dhcp6_reconfigure_accept

The DHCPv6 server hands out two different addresses, say 2001:db8::123 and 2001:db8::456, for the two different interfaces.  I'm not sure how it distinguishes which one should get which interface.

Somehow, dhcpcd sometimes configures _both_ interfaces with _both_ addresses:

# ifconfig -a
...
ure0: flags=0x8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
...
        inet6 2001:db8::123/128 flags 0
        inet6 2001:db8::456/128 flags 0
...
urtwn0: flags=0x8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
...
        inet6 2001:db8::456/128 flags 0
        inet6 2001:db8::123/128 flags 0
...
>How-To-Repeat:
not sure
>Fix:
Yes, please!