install/54745: Sysinst generates strange i386 partition table

[gson%gson.org@localhost (Andreas Gustafsson)]

>Number:         54745
>Category:       install
>Synopsis:       Sysinst generates strange i386 partition table
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    install-manager
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat Dec 07 15:20:00 +0000 2019
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current, source date 2019.11.30.04.06.22
>Organization:
>Environment:
System: NetBSD
Architecture: i386
Machine: i386
>Description:

When I try to install a week-old NetBSD-current/i386 on an unused
MBR partition of a laptop with an existing Linux installation,
I end up with the following sysinst screen.  It's confusing me
because there is no swap partition, the NetBSD portion of the
disk is called "b" rather than "c", and the whole disk is called
"c" rather than "d":

   We now have your disklabel partitions for wd1 below.  This is your last 
   chance to change them. 

   Flags: (N)ewfs.  Total size: 75G, free: 128M

        Start (MB)     End (MB)    Size (MB)  FS type Flag Filesystem
      ------------ ------------ ------------ -------- ---- ----------------
   a:        76294       152459        76166    FFSv2 N    /
   b:        76294       152587        76294 NetBSD partition
   c:            0       238474       238475 Whole disk
      ------------ ------------ ------------ -------- ---- ----------------
   e: Add a partition
   f: Change input units (sectors/cylinders/MB/GB)
   g: Edit name of the disk
   h: Clone external partition(s)
   i: Cancel
  >x: Partition sizes ok

>How-To-Repeat:

Build a NetBSD-current/i386 install image using "build.sh
install-image" and uncompress it into NetBSD-9.99.18-i386-install.img.

Save the following uuencoded copy of the actual MBR from the laptop as
"mbr.uu":

begin 644 mbr
MZV.0$([0O`"PN```CMB.P/N^`'R_``:Y``+SI.HA!@``OKX'.`1U"X/&$('^
M_@=U\^L6M`*P`;L`?+*`BG0!BTP"S1/J`'P``.O^````````````````````
M`(`!`````````/_ZD)#VPH!T!?;"<'0"LH#J>7P``#'`CMB.T+P`(/N@9'P\
M_W0"B,)2OH!]Z!<!O@5\M$&[JE7-$UI2<CV!^U6J=3>#X0%T,C'`B40$0(A$
M_XE$`L<$$`!FBQY<?&:)7`AFBQY@?&:)7`S'1`8`<+1"S1-R!;L`<.MVM`C-
M$W,-]L*`#X38`+Z+?>F"`&8/ML:(9/]`9HE$!`^VT<'B`HCHB/1`B40(#[;"
MP.@"9HD$9J%@?&8)P'5.9J%<?&8QTF;W-(C1,=)F]W0$.T0(?3?^P8C%,,#!
MZ`((P8C06HC&NP!PCL,QV[@!`LT3<AZ,PV`>N0`!CMLQ]K\`@([&_/.E'V'_
M)EI\OH9]ZP.^E7WH-`"^FGWH+@#-&.O^1U)50B``1V5O;0!(87)D($1I<VL`
M4F5A9``@17)R;W(-"@"[`0"T#LT0K#P`=?3#``````````"N;@@```"`("$`
M@_[__P`(````*%`)`/[__X/^__\`,%`)`#!0"0#^__^#_O__`&"@$@`P4`D`
1_O__@O[__P"0\!L`R"L!5:H`
`
end

Create a hard disk image of the same size as the laptop disk, as a
sparse file so that it won't take up too much acutal space:

  uudecode mbr.uu
  cp mbr hd
  dd if=/dev/zero of=hd bs=512 seek=488397167 count=1

Boot the install image in qemu:

  qemu-system-i386 -nographic -snapshot -hda NetBSD-9.99.18-i386-install.img -hdb hd

Choose your terminal type

Choose your keyboard type

Answer the other sysinst prompts as follows:

a: Install NetBSD to hard disk  
Shall we continue? b: Yes
a: wd1 (232G)
a: This is the correct geometry 
a: Edit the MBR partition table
b:         76294         76294      Linux native
a:        type : Linux native
b: NetBSD
x: Partition OK
x: Partition table OK
Do you want to install the NetBSD bootcode? a: Yes
a: Set sizes of NetBSD partitions

You will now end up at the strange partition table shown at beginning
of the PR.

>Fix: