bug#34276: ‘guix system disk-im age’ successfully builds a bad image

[Brice Waegeneire <brice@waegenei.re>]

Hello Ludovic,

> I investigated a bit.  I managed to get our code to cause a kernel 
> panic
> upon failure (patch below).  However I fail to turn that guest kernel
> panic into a different QEMU exit code.
> 
> I tried to use the “pvpanic” paravirtualized device (the ‘pvpanic.ko’
> module in the guest, and “-device pvpanic” on the QEMU command line),
> but unfortunately that thing is almost undocumented and I can’t get it
> to turn the panic into a non-zero exit code, nor do I know if it’s
> possible.
> 
> Thoughts anyone?

I looked a little into it and I have found how to use pvpanic.
Unfortunately it's not as straight forward as getting a non-zero exit
code form qemu. When pvpanic is loaded in a VṂ, as you did with 
“-device
pvpanic”, generate events[0] on the QMP interface when a crash happen
and qemu either shutdown or pause when using --no-shutdown[1].

(gnu build marionette) which use the “-monitor” interface could be
recycled to use “-qmp” a machine interface using JSON.

Following is log of a QMP session where the guest panicked[2]:
--8<---------------cut here---------------start------------->8---
{
     "QMP": {
         "version": {
             "qemu": {
                 "micro": 0,
                 "minor": 2,
                 "major": 4
             },
             "package": ""
         },
         "capabilities": [
             "oob"
         ]
     }
}
{ "execute": "qmp_capabilities" }
{
     "return": {
     }
}
{
     "timestamp": {
         "seconds": 1584645026,
         "microseconds": 936550
     },
     "event": "GUEST_PANICKED",
     "data": {
         "action": "pause"
     }
}
{
     "timestamp": {
         "seconds": 1584645026,
         "microseconds": 936675
     },
     "event": "GUEST_PANICKED",
     "data": {
         "action": "poweroff"
     }
}
{
     "timestamp": {
         "seconds": 1584645026,
         "microseconds": 936776
     },
     "event": "SHUTDOWN",
     "data": {
         "guest": true,
         "reason": "guest-panic"
     }
}
--8<---------------cut here---------------end--------------->8---


[0]: 
https://github.com/qemu/qemu/blob/9ced5c7c20cb16dff0c2fa3242c3ee96b68cec2a/qapi/run-state.json#L339-L355
[1]: 
https://github.com/qemu/qemu/blob/4dd6517e369828171290b65e11f6a45aeeed15af/softmmu/vl.c#L1423-L1427
[2]: 
https://git.qemu.org/?p=qemu.git;a=blob_plain;f=docs/interop/qmp-intro.txt;hb=HEAD

Brice.