Note that the described behaviour is deliberate and (somewhat vaguely) documented in 'man virsh'

"When run without any arguments, this command prints all information types that are supported by the guest agent. You can limit the types of information that are  returned  by specifying  one or more flags.  If a requested information type is not supported, the processes will provide an exit code of 1.  Available information types flags are --user, --os, --timezone, --hostname, --filesystem, --disk and --interface."

The idea is that if you don't select any specific group of information, the command prints everything that it could get, which in this case is nothing.

If you think the documentation is unclear we can clarify that, but the code is deliberately and expectedly returning success.

Hi, Peter

(In reply to Peter Krempa from comment #1)
> Note that the described behaviour is deliberate and (somewhat vaguely)
> documented in 'man virsh'
> 
> "When run without any arguments, this command prints all information types
> that are supported by the guest agent. You can limit the types of
> information that are  returned  by specifying  one or more flags.  If a
> requested information type is not supported, the processes will provide an
> exit code of 1.  Available information types flags are --user, --os,
> --timezone, --hostname, --filesystem, --disk and --interface."
> 
> The idea is that if you don't select any specific group of information, the
> command prints everything that it could get, which in this case is nothing.
> 
I think the documentation is vague. I thought the documentation above talks about the
The information types guestinfo supports. If you do not point out, I would not think about
the circumstances when guestinfo can not get info(Here, the info is not some kind of unsupported info).

> If you think the documentation is unclear we can clarify that, but the code
> is deliberately and expectedly returning success.
Yes, please. Thanks.

Fixed upstream:

commit 755b16d10a90617b57dbaf900abe3ccadbe45324
Author: Peter Krempa <pkrempa>
Date:   Wed Jan 19 09:49:31 2022 +0100

    docs: man: virsh: Document more carefully that 'guestinfo' can return nothing
    
    When invoking 'virsh guestinfo $VM' without explicitly specifying a
    group of information to return, virsh always reports success even when
    the guest agent doesn't report any information in the current state.
    This is desired in situations when you are okay with stats being missing
    and avoids spurious errors being reported.
    
    Clarify that this is really desired in the man page.
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=2041665
    Signed-off-by: Peter Krempa <pkrempa>
    Reviewed-by: Andrea Bolognani <abologna>
    Signed-off-by: Peter Krempa <pkrempa>

v8.0.0-118-g755b16d10a

Tested with:
libvirt-8.2.0-1.fc35.x86_64

Check the doc

# man virsh
...
 When run without any arguments, this command prints all information types that are supported by the guest agent at that point, omitting unavailable ones.  Success  is always reported in this case.

Verified this bug with:
libvirt-8.2.0-1.el9.x86_64

The testing steps are the same with Comment #4.

The testing result matches with the expected result, mark the bug as verified.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory (Low: libvirt security, bug fix, and enhancement update), and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHSA-2022:8003