install/60265: sysinst gets confused by "broken" GPTs

["martin%NetBSD.org@localhost via gnats" <gnats-admin%NetBSD.org@localhost>]

>Number:         60265
>Category:       install
>Synopsis:       sysinst gets confused by "broken" GPTs
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    install-manager
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri May 15 12:20:00 +0000 2026
>Originator:     Martin Husemann
>Release:        NetBSD 11.99.6
>Organization:
The NetBSD Foundation, Inc.
>Environment:
System: NetBSD big-apple.aprisoft.de 11.99.6 NetBSD 11.99.6 (POWERMAC_G5.MP) #160: Tue May 12 10:25:34 CEST 2026 martin%seven-days-to-the-wolves.aprisoft.de@localhost:/work/src/sys/arch/macppc/compile/POWERMAC_G5.MP macppc
Architecture: powerpc
Machine: macppc
>Description:

When trying to install to a disk (or USB stick) with a broken GPT,
sysinst will get confused and unable to recover. It will try to create
a new GPT which fails:

	gpt: /dev/rsd0: Device already contains a GPT, destroy it first

>How-To-Repeat:

Unclear, would always happen when "gpt show" does not list a secondary
header at the end of the disk (internally: "has_gpt" is then not set
in the disk's partition information within sysinst).

>Fix:
Recognize broken GPTs and offer to repair them?
Recognize failure to create a GPT and offer wiping before retrying?