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Fedora 31 Self-Contained Change proposal: DNF Make Best Mode the Default

<a href="https://fedoraproject.org/wiki/Changes/DNF_Default_Best" title="https://fedoraproject.org/wiki/Changes/DNF_Default_Best">https://fedoraproject.org/wiki/Changes/DNF_Default_Best</a>

== Summary ==
Currently, DNF prefers clean dependency resolution over package updates;
a package (almost) silently won't get updated to a newer version if the new
version has dependency problems. DNF will be changed to prefer updates and fail
if they have dependency resolution issues, while the failure has a
temporal or permanent workaround
hint for users who want to use the older behavior.

== Owner ==
* Name: [[User:jmracek| Jaroslav Mracek]]
* Email: <a href="mailto: ... at redhat dot com"> ... at redhat dot com</a>

== Detailed Description ==
Change the built-in default value of the `best` configuration option
from `0` (false) to `1` (true).

As a result, unless `best` is overridden in the `/etc/dnf/dnf.conf`
file or using `--setopt`, it will default to `1`. As a convenience, we
will also put the explicit `best=1` assignment in the shipped
`/etc/dnf/dnf.conf` file for better transparency, and introduce the
new `--nobest` command-line switch.

The purpose of the `--nobest` switch (as a shorthand for
`--setopt=best=0`) is to make it easy for the user to override the
default setting when needed, and it will also be
[https://github.com/rpm-software-management/dnf/pull/1311/commits/9a3e8fd0da49291d30fd1fef527cffb0bf3f047d#diff-6c823931c6d150295e5011fac6529ab9R144
suggested] in the DNF output when a dependency error occurs.

Relevant excerpt from the updated `dnf.conf(5)`:
<pre>
best boolean
When upgrading a package, always try to install its highest version
available, even only to find out some of its deps are not satisfiable.
Enable this if you want to experience broken dependencies in the
repositories firsthand. The default is True.
</pre>

Relevant excerpt from the updated `dnf(8)`:
<pre>
--nobest
Set best option as false, therefore transactions are not limited to
only best candidates.
</pre>

'''Change in DNF output - missing vim-enhanced-2:8.1.1561-1.fc30'''

Original output. DNF succeed with return code 0:
<pre>
sudo dnf upgrade
Last metadata expiration check: 2:16:40 ago on Mon 24 Jun 2019 04:27:16 PM CEST.
Dependencies resolved.

Problem: package vim-enhanced-2:8.1.1471-1.fc30.x86_64 requires
vim-common = 2:8.1.1471-1.fc30, but none of the providers can be
installed
- cannot install both vim-common-2:8.1.1561-1.fc30.x86_64 and
vim-common-2:8.1.1471-1.fc30.x86_64
- problem with installed package vim-enhanced-2:8.1.1471-1.fc30.x86_64
- cannot install the best update candidate for package
vim-common-2:8.1.1471-1.fc30.x86_64
- package vim-enhanced-2:8.1.1561-1.fc30.x86_64 is excluded
===================================================================================================================================
Package Architecture Version
Repository Size
===================================================================================================================================
Skipping packages with conflicts:
(add '--best --allowerasing' to command line to force their upgrade):
vim-common x86_64
2:8.1.1561-1.fc30 updates 6.7
M

Transaction Summary
===================================================================================================================================
Skip 1 Package

Nothing to do.
Complete!
</pre>

Output after the change. DNF fails with return code 1, but proposing
`--nobest` option as an option to resolve the issue:
<pre>
sudo dnf upgrade
Last metadata expiration check: 2:16:36 ago on Mon 24 Jun 2019 04:27:16 PM CEST.
Error:
Problem: package vim-enhanced-2:8.1.1471-1.fc30.x86_64 requires
vim-common = 2:8.1.1471-1.fc30, but none of the providers can be
installed
- cannot install both vim-common-2:8.1.1561-1.fc30.x86_64 and
vim-common-2:8.1.1471-1.fc30.x86_64
- problem with installed package vim-enhanced-2:8.1.1471-1.fc30.x86_64
- cannot install the best update candidate for package
vim-common-2:8.1.1471-1.fc30.x86_64
- package vim-enhanced-2:8.1.1561-1.fc30.x86_64 is excluded
(try to add '--allowerasing' to command line to replace conflicting
packages or '--skip-broken' to skip uninstallable packages or
'--nobest' to use not only best candidate packages)
</pre>

'''Q&A'''

Can be a default of the best configuration option overwritten easily
and permanently by user?
Yes, just add `best=false` to `/etc/dnf/dnf.conf`
<pre>
[main]
best=False
</pre>

Can be a default of the best configuration option overwritten easily
from commandline?
Yes, just add `--nobest` to command
<pre>
dnf upgrade --nobest
</pre>

What about PackageKit and Gnome Software?
<pre>
PackageKit and Gnome Software will be not affected by the change. In
case that the same behavior will be desired for PackageKit, It will
require changes in PackageKit code.
</pre>

What about Microdnf?
<pre>
Microdnf will be not affected by the change. There is a plan to unify
functional parity and behavior DNF with Microdnf but not before Fedora
33.
</pre>

== Benefit to Fedora ==

This change allows the users to be properly notified when a package
cannot be upgraded to the latest version, instead of silently ignoring
it as an upgrade candidate.

Right now, when DNF runs in `best=0` mode, if a package cannot be
upgraded due to dependency problems, it is skipped and a warning is
printed in the transaction summary table. However, this poses a risk
of important security fixes being overlooked by the user in case they
are broken for some reason, such as due to a repository
misconfiguration or inconsistency within the metadata itself.

Moreover, since DNF always exits with the return code `0` (success)
when in `best=0` mode, this mode is especially risky in automated
scripts invoking DNF in `assumeyes` mode in which case such
unsuccessful package upgrades could easily go unnoticed unless the
logs are manually examined after the fact.

The new behavior is also more in line with the generally accepted
software development practice of failing early and failing fast.

As a secondary benefit, broken upgrade paths in the Fedora
repositories will hopefully be noticed, reported and therefore fixed
sooner. Although, we would prefer if such problems would be detected
before we ship them to users.

'''Summary of benefits:'''

# No silently passed problems with updates
# Broken dependencies faster disappear from Fedora distribution
# Problems will be reported more often - opportunity to fix issues
# Increase in stability of Fedora distribution
# Less issues after branching
# Identical behavior of DNF in all distributions - Fedora/RHEL/Mageia/OpenSuse

== Scope ==
* Proposal owners:
The change is already part of the upstream (dnf-4.1.0) and reverted in
Fedora downstream. The change was composed by following pull requests:

<a href="https://github.com/rpm-software-management/libdnf/pull/678" title="https://github.com/rpm-software-management/libdnf/pull/678">https://github.com/rpm-software-management/libdnf/pull/678</a><br>
<a href="https://github.com/rpm-software-management/dnf/pull/1311" title="https://github.com/rpm-software-management/dnf/pull/1311">https://github.com/rpm-software-management/dnf/pull/1311</a><br>
<a href="https://github.com/rpm-software-management/dnf/pull/1316" title="https://github.com/rpm-software-management/dnf/pull/1316">https://github.com/rpm-software-management/dnf/pull/1316</a><br>
<a href="https://github.com/rpm-software-management/dnf/pull/1319" title="https://github.com/rpm-software-management/dnf/pull/1319">https://github.com/rpm-software-management/dnf/pull/1319</a>

We would like to stop the reverting the changes.

* Other developers: N/A (not a System Wide Change)
* Policies and guidelines: N/A (not a System Wide Change)
* Trademark approval: N/A (not needed for this Change)

== Upgrade/compatibility impact ==
N/A (not a System Wide Change)

== How To Test ==
N/A (not a System Wide Change)

== User Experience ==
Broken upgrades are recognized early, enabling the users to act upon
them by double-checking their repository configuration or filing bugs,
instead of assuming no upgrades are available.

== Dependencies ==
N/A (not a System Wide Change)

== Contingency Plan ==
If there is a massive negative feedback by the rawhide and pre-beta
users, we can revert the
change at the beta freeze. If there is a massive negative feedback by
the beta users, we can
revert the change at final freeze.

* Contingency mechanism: (What to do? Who will do it?) N/A (not a
System Wide Change)
* Contingency deadline: N/A (not a System Wide Change)
* Blocks release? N/A (not a System Wide Change), Yes/No

== Documentation ==
N/A (not a System Wide Change)

== Release Notes ==