This looks suspicious (in mock-core-configs):

* Fri Feb 07 2020 Pavel Raiskup <praiskup> 32.0-2
- solve yum.conf vs. dnf.conf inconsistency in config and code



IMHO you do NOT change these kinds of things in a released version, you change them in rawhide.
This breaks setups.

Can you please upload the config file which doesn't work for you now?

Ok, I see:
dnf install -y /etc/mock/fedora-31-x86_64-rpmfusion_free.cfg

https://github.com/rpm-software-management/mock/pull/486

With F30 fully updated and new mock-rpmfusion-free-32.1-1.fc30.noarch.rpm from koji.rpmfusion  I am still getting

{{{
INFO: enabled HW Info plugin
No matches found for the following disable plugin patterns: local, spacewalk
file:///tmp/tmp.Cc8w9LMQ9U/results/fedora-30-x86_64-rpmfusion_free                                                  0.0  B/s |   0  B     00:00    
Errors during downloading metadata for repository 'local_build_repo':
  - Curl error (37): Couldn't read a file:// file for file:///tmp/tmp.Cc8w9LMQ9U/results/fedora-30-x86_64-rpmfusion_free/repodata/repomd.xml [Couldn't open file /tmp/tmp.Cc8w9LMQ9U/results/fedora-30-x86_64-rpmfusion_free/repodata/repomd.xml]
Error: Failed to download metadata for repo 'local_build_repo': Cannot download repomd.xml: Cannot download repodata/repomd.xml: All mirrors were tried
ERROR: Exception(/tmp/tmp.jMGtAYkkPu/mythtv-32.Pre.102.g90a86400ae-100.src.rpm) Config(fedora-30-x86_64) 0 minutes 30 seconds
INFO: Results and/or logs in: /tmp/tmp.Cc8w9LMQ9U/results/fedora-30-x86_64-rpmfusion_free/mythtv-32.Pre.102.g90a86400ae-100
INFO: End chain build: /tmp/tmp.jMGtAYkkPu/mythtv-32.Pre.102.g90a86400ae-100.src.rpm

$ rpm -qa | grep mock
mock-rpmfusion-nonfree-31.2-1.fc30.noarch
mock-rpmfusion-free-32.1-1.fc30.noarch
mock-core-configs-32.1-1.fc30.noarch
mock-2.0-2.fc30.noarch
python2-requests-mock-1.5.2-1.fc29.noarch

}}}

konqueror has no problem in displaying the 'unreadable' 3.0 KiB .xml file.

google suggests that this is a gpg signing error.  Maybe the rpmfusion file from koji, or...?

mock-rpmfusion-free-32.1-1.fc30.noarch  is now in the rpmfusion testing repo.  I reinstalled it from there but still get the

Curl error (37): Couldn't read a file:// file...  Couldn't open file...  when that file appears to be viewable by all.

> I reinstalled it from there but still get the Curl error (37): Couldn't
> read a file:// file...  Couldn't open file...

You seem to be mounting stuff from /tmp directory into bootstrap,
but that doesn't work because systemd-nspawn over-mounts /tmp by
empty directory.  See the docs:

https://github.com/rpm-software-management/mock/wiki/Feature-bootstrap#using-bootstrap-with-local-repositories

I'd suggest to use /var/tmp, or /var/cache, or something which is
not handled by systemd-nspawn.  (also --isolation=simple should help)

I filled this
https://github.com/rpm-software-management/mock/issues/502

Anyways this is different bug, it should have separate bug ID.

Copied from my post today in the initial thread on the Fedora users list: 

This thread was about the failure of a script that uses mock --chain to 
build rpm packages from more than one src.rpm.

There had been a recent update of mock itself, and some of its config 
packages too.  More updates have followed, and my system appears to be 
working again, with a small work-around in the script.

https://github.com/rpm-software-management/mock/issues/501#issuecomment-5...

FEDORA-2020-e331425193 has been submitted as an update to Fedora 30. https://bodhi.fedoraproject.org/updates/FEDORA-2020-e331425193

FEDORA-EPEL-2020-5a84e15907 has been submitted as an update to Fedora EPEL 8. https://bodhi.fedoraproject.org/updates/FEDORA-EPEL-2020-5a84e15907

FEDORA-EPEL-2020-88ef4b4d66 has been submitted as an update to Fedora EPEL 7. https://bodhi.fedoraproject.org/updates/FEDORA-EPEL-2020-88ef4b4d66

mock-2.1-1.fc32, mock-core-configs-32.4-1.fc32 has been pushed to the Fedora 32 testing repository. If problems still persist, please make note of it in this bug report.
See https://fedoraproject.org/wiki/QA:Updates_Testing for
instructions on how to install test updates.
You can provide feedback for this update here: https://bodhi.fedoraproject.org/updates/FEDORA-2020-d32d395e18

mock-2.1-1.el8, mock-core-configs-32.4-1.el8 has been pushed to the Fedora EPEL 8 testing repository. If problems still persist, please make note of it in this bug report.
See https://fedoraproject.org/wiki/QA:Updates_Testing for
instructions on how to install test updates.
You can provide feedback for this update here: https://bodhi.fedoraproject.org/updates/FEDORA-EPEL-2020-5a84e15907

mock-2.1-1.fc30, mock-core-configs-32.4-1.fc30 has been pushed to the Fedora 30 testing repository. If problems still persist, please make note of it in this bug report.
See https://fedoraproject.org/wiki/QA:Updates_Testing for
instructions on how to install test updates.
You can provide feedback for this update here: https://bodhi.fedoraproject.org/updates/FEDORA-2020-e331425193

mock-2.1-1.fc31, mock-core-configs-32.4-1.fc31 has been pushed to the Fedora 31 testing repository. If problems still persist, please make note of it in this bug report.
See https://fedoraproject.org/wiki/QA:Updates_Testing for
instructions on how to install test updates.
You can provide feedback for this update here: https://bodhi.fedoraproject.org/updates/FEDORA-2020-d952a70712

mock-2.1-1.el7, mock-core-configs-32.4-1.el7 has been pushed to the Fedora EPEL 7 testing repository. If problems still persist, please make note of it in this bug report.
See https://fedoraproject.org/wiki/QA:Updates_Testing for
instructions on how to install test updates.
You can provide feedback for this update here: https://bodhi.fedoraproject.org/updates/FEDORA-EPEL-2020-88ef4b4d66

This package has changed maintainer in the Fedora.
Reassigning to the new maintainer of this component.

mock-2.1-1.fc32, mock-core-configs-32.4-1.fc32 has been pushed to the Fedora 32 stable repository. If problems still persist, please make note of it in this bug report.

mock-2.1-1.fc31, mock-core-configs-32.4-1.fc31 has been pushed to the Fedora 31 stable repository. If problems still persist, please make note of it in this bug report.

FEDORA-EPEL-2020-88ef4b4d66 has been submitted as an update to Fedora EPEL 7. https://bodhi.fedoraproject.org/updates/FEDORA-EPEL-2020-88ef4b4d66

FEDORA-EPEL-2020-5a84e15907 has been submitted as an update to Fedora EPEL 8. https://bodhi.fedoraproject.org/updates/FEDORA-EPEL-2020-5a84e15907

FEDORA-2020-e331425193 has been pushed to the Fedora 30 stable repository.
If problem still persists, please make note of it in this bug report.

mock --chain is now working for me in fc30, as it was before this BZ was opened.  But it now needs a workaround as noted in my Comment 9 above.  The command line I am using now includes '--no-bootstrap-chroot --isolation simple'

> The command line I am using now includes '--no-bootstrap-chroot --isolation simple'

Why?  It shouldn't be needed.  Please provide `mock -r <...> --debug-config` output.

Because it works.  My test case just failed on starting the second build in the chain, with the original curl error.

The main build works with the --no-bootstrap-chroot but leaves a bootstrap folder in the builddir.  With the --isolation simple option that doesn't happen. 

I suppose it may be because /tmp is still being used.  --localrepo=/tmp/x

FEDORA-EPEL-2020-5a84e15907 has been pushed to the Fedora EPEL 8 testing repository.

You can provide feedback for this update here: https://bodhi.fedoraproject.org/updates/FEDORA-EPEL-2020-5a84e15907

See also https://fedoraproject.org/wiki/QA:Updates_Testing for more information on how to test updates.

> I suppose it may be because /tmp is still being used.  --localrepo=/tmp/x

Yes.  Makes sense.  My recommendation is to move to different directory
(instead of disabling nspawn/bootstrap).  At least until we realize what
to do about this problem (perhaps we could fix mock, so /tmp would be 
allowed - or at least fail as soon as possible with clear message).
Thanks for all the feedback!

FEDORA-EPEL-2020-88ef4b4d66 has been pushed to the Fedora EPEL 7 testing repository.

You can provide feedback for this update here: https://bodhi.fedoraproject.org/updates/FEDORA-EPEL-2020-88ef4b4d66

See also https://fedoraproject.org/wiki/QA:Updates_Testing for more information on how to test updates.

In fact the build script that I am using has moved the localrepo to /var/tmp but also applies the no-bootstrap and isolation options.  I'll try without.  Thanks.

After moving the localrepo from /tmp to /var/tmp my test build completed *without* the no-bootstrap-chroot and isolation options.  But perhaps I should look for simpler srpms...

$ mkdir /var/tmp/x ; mock --root=fedora-30-x86_64-rpmfusion_free --chain --localrepo=/var/tmp/x ../m2vrequantiser-20030929-12.fc30.src.rpm ../dvbcut-0.6.1-29.svn179.fc30.src.rpm

I confirm that with the localrepo in /var/tmp my builds of rpms for MythTV succeeded when all calls to mock were made *without* the no-bootstrap-chroot and isolation workarounds.  rpm sets built under fc30 are now installed and running under both fc30 and el7.   Thanks for the help.

FEDORA-EPEL-2020-5a84e15907 has been submitted as an update to Fedora EPEL 8. https://bodhi.fedoraproject.org/updates/FEDORA-EPEL-2020-5a84e15907

FEDORA-EPEL-2020-88ef4b4d66 has been submitted as an update to Fedora EPEL 7. https://bodhi.fedoraproject.org/updates/FEDORA-EPEL-2020-88ef4b4d66

FEDORA-EPEL-2020-5a84e15907 has been pushed to the Fedora EPEL 8 testing repository.

You can provide feedback for this update here: https://bodhi.fedoraproject.org/updates/FEDORA-EPEL-2020-5a84e15907

See also https://fedoraproject.org/wiki/QA:Updates_Testing for more information on how to test updates.

FEDORA-EPEL-2020-88ef4b4d66 has been pushed to the Fedora EPEL 7 testing repository.

You can provide feedback for this update here: https://bodhi.fedoraproject.org/updates/FEDORA-EPEL-2020-88ef4b4d66

See also https://fedoraproject.org/wiki/QA:Updates_Testing for more information on how to test updates.

FEDORA-EPEL-2020-5a84e15907 has been pushed to the Fedora EPEL 8 stable repository.
If problem still persists, please make note of it in this bug report.

FEDORA-EPEL-2020-88ef4b4d66 has been pushed to the Fedora EPEL 7 stable repository.
If problem still persists, please make note of it in this bug report.