port-amd64/54430: installboot(8) fails with GPT PMBR

[martin%NetBSD.org@localhost]

>Number:         54430
>Category:       port-amd64
>Synopsis:       installboot(8) fails with GPT PMBR
>Confidential:   no
>Severity:       critical
>Priority:       high
>Responsible:    port-amd64-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri Aug 02 11:00:00 +0000 2019
>Originator:     Martin Husemann
>Release:        NetBSD 9.99.2
>Organization:
The NetBSD Foundation, Inc.
>Environment:
System: NetBSD seven-days-to-the-wolves.aprisoft.de 9.99.2 NetBSD 9.99.2 (GENERIC) #293: Thu Aug 1 08:25:48 CEST 2019 martin%seven-days-to-the-wolves.aprisoft.de@localhost:/work/src/sys/arch/amd64/compile/GENERIC amd64
Architecture: x86_64
Machine: amd64
>Description:

I have a setup where installing bootblocks fails. This is a BIOS and GPT
install:

# /usr/sbin/installboot -o console=pc,speed=38400 /dev/rdk0 /usr/mdec/bootxx_ffsv2
installboot: Insufficient reserved space before FAT (512 bytes available), use -f (may invalidate filesystem)
installboot: Set bootstrap operation failed

and gpt show says:

      start       size  index  contents
          0          1         PMBR
          1          1         Pri GPT header
          2         32         Pri GPT table
         34         30         Unused
         64    8388608      1  GPT part - NetBSD FFSv1/FFSv2
    8388672         64         Unused
    8388736   33552384      2  GPT part - NetBSD swap
   41941120         64         Unused
   41941184   82831293      3  GPT part - NetBSD FFSv1/FFSv2
  124772477          3         Unused
  124772480   31457119      4  GPT part - NetBSD FFSv1/FFSv2
  156229599         32         Sec GPT table
  156229631          1         Sec GPT header


Of course there is no FAT involved anywhere.

>How-To-Repeat:

Not sure, ran into this accidently and am not sure how to replicate it yet.

>Fix:
n/a (will look, but feel free to beat me to it)