My understanding is that there is explicitly no supported upgrade path for dynamically switching a host between slow stream and fast stream virt modules, or vica-verca. IIUC, the expectation is that the admin is responsible for fully uninstalling any packages related to the virt module being disabled, *before* trying to enable & use a different virt module stream.

This doesn't mean we shouldn't still fix the upgrade issue wrt to the nbdkit package, especially if this might still affect slow stream on its own if we later rebase nbdkit in the slow stream.

I asked Ming to file this bug because I think we should try to make the upgrade path work here.

What happened is that we renamed nbdkit -> nbdkit-server.  The nbdkit package in 1.12 is now
a virtual package which depends on nbdkit-server and a few others.

The server binary (/usr/sbin/nbdkit) and manual page (/usr/share/man/man1/nbdkit.1.gz) moved from
nbdkit to nbdkit-server.

Looking at the dnf output it seems like before the transaction, (old) nbdkit is
installed, containing the server and the man page.  Then libguestfs is upgraded
which only depends on the minimal nbdkit-server package.  But dnf doesn't
either remove or upgrade nbdkit.  In other words dnf is trying to upgrade to:

  nbdkit-server 1.12
  nbdkit 1.4

which of course causes a conflict.

I'm not sure how to solve this without going back in time and fixing the 1.4 package.

If the 'nbdkit-1.12' package actually exists in the repos, that should be considered an upgrade path from 'nbdkit-1.4', which would mean everything should "just work".

From this conflict, it looks like 'nbdkit-1.12' doesn't exist in the repos, meaning that when yum tries to pull in nbd-server for libguestfs, things become unresolvable.

I wonder if this has something to do with CRB not being enabled.  mxie can you check if
it's possible to install just the nbdkit 1.12 package on its own?  Also which repository
that package comes from?

Verify the bug with builds
nbdkit-1.4.2-5.module+el8.1.0+3531+2918145b.x86_64
nbdkit-1.12.1-3.module+el8.1.0+3795+00d3a468.x86_64

Steps:
1.Prepare a rhel8 host which has installed virt-v2v and slow stream virt module
# yum module info virt
Name             : virt
Stream           : rhel [d][e][a]
Version          : 8010020190723195906
Context          : cdc1202b
Architecture     : x86_64
Profiles         : common [d]
Default profiles : common
Repo             : slow-module
Summary          : Virtualization module
Description      : A virtualization module

# rpm -q virt-v2v nbdkit
virt-v2v-1.38.4-13.module+el8.1.0+3531+2918145b.x86_64
nbdkit-1.4.2-5.module+el8.1.0+3531+2918145b.x86_64


2.Update the virt module from slow train to fast train on rhel8 host

2.1 Enable the repo of fast stream and disable the repo of slow stream

2.2 # yum module reset virt -y
=================================================
 Package                Architecture          Version                   Repository              Size
==================================================
Disabling module profiles:
 virt/common                                                                                        
Resetting modules:
 virt                                                                                               

Transaction Summary
==================================================

Complete!

2.3 Enable fast virt module
# yum module enable virt -y

=======================================================
 Package                Architecture          Version                   Repository              Size
========================================================
Enabling module streams:
 virt                                         8.1                                                   

Transaction Summary
==============================================

Complete!


3.Install fast stream virt module 
# yum module install virt -y
.....
.....
Installed:
  libvirt-daemon-driver-storage-iscsi-direct-5.5.0-2.module+el8.1.0+3773+7dd501bf.x86_64
  nbdkit-basic-filters-1.12.1-3.module+el8.1.0+3795+00d3a468.x86_64             
  nbdkit-basic-plugins-1.12.1-3.module+el8.1.0+3795+00d3a468.x86_64             
  nbdkit-python-plugin-1.12.1-3.module+el8.1.0+3795+00d3a468.x86_64             
  nbdkit-server-1.12.1-3.module+el8.1.0+3795+00d3a468.x86_64                    
  nbdkit-vddk-plugin-1.12.1-3.module+el8.1.0+3795+00d3a468.x86_64    

Result:
  Upgrade smoothly between the two nbdkit versions (slow and fast) during virt module installation,so move the bug from ON_QA to 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, 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/RHBA-2019:3723