F39 Change Proposal: Allow Removal of tzdata (System-Wide)

[Aoife Moloney]

https://fedoraproject.org/wiki/Changes/AllowRemovalOfTzdata

This document represents a proposed Change. As part of the Changes
process, proposals are publicly announced in order to receive
community feedback. This proposal will only be implemented if approved
by the Fedora Engineering Steering Committee.

== Summary ==
Allow the removal of tzdata especially on containers in order to minimize size.

== Owner ==
* Name: Patsy Griffin (Franklin)

* Email: [e-mail]

== Detailed Description ==
This change will allow the removal of tzdata.  When tzdata is removed,
the system will default to UTC. In order to reduce overhead, many
container installations now remove the data associated with tzdata but
cannot fully remove the package due to dependencies by other packages.
This results in confusion regarding the expected timezone info.

In order for this to work, we need packages that use tzdata at run
time to switch from Require'ing tzdata to Recommend'ing tzdata.  These
packages should also gracefully handle instances where tzdata has been
removed.  For example, this would require recognizing that tzdata is
not present and providing an appropriate error, such as recommending
that the user install tzdata.

== Feedback ==
In June of 2021, we proposed creating a new tzdata sub-package that
would only provide the UTC timezone.  As part of the discussion around
this proposal, it was recommended that we completely remove tzdata. We
appreciated this input and welcome additional feedback.

== Benefit to Fedora ==
This change will allow tzdata to be removed from containers without
leaving inconsistent package remnants.

== Scope ==
* Proposal owners: No changes are needed to tzdata.

* Other developers: Some packages need to change their spec files from
`Requires: tzdata` to `Recommends: tzdata`. It would be beneficial if
all packages switched in this way, but it is not required. Supporting
optional tzdata installation for as many workloads as possible allows
those workloads to minimize their container image size.
List of packages which need to be changed:
* glibc (glibc-common)
* gcc (libstdc++)
* python3.XX (3.9, 3.10, 3.11, 3.12)
List of packages which would be beneficial to be changed:
* python3-dateutil
* python3-pytz
* libical
Upon acceptance of the change request we will file bugs to fix each of
these packages for Fedora 39.

* Release engineering: No changes needed.

* Policies and guidelines: N/A (not needed for this Change)

* Trademark approval: N/A (not needed for this Change)

* Alignment with Community Initiatives:

== Upgrade/compatibility impact ==
If tzdata is already installed, then it continues to be installed on the system.

Following the Fedora Weak Dependencies Policy dnf will treat the
Recommends on tzdata as if it were a Requires and tzdata will always
be installed in a default system
(https://docs.fedoraproject.org/en-US/packaging-guidelines/WeakDependencies/)

However, the recommends will allow tzdata to be correctly uninstalled
in a container build file rather than having to use ‘rm -rf’ to delete
the zone files to recover space.

== How To Test ==
Language runtimes were installed and A/B tests carried out with tzdata
present and tzdata removed. The intent of these tests was to ensure
that the system can use the language frameworks without tzdata present
and that when the data was required that meaningful errors were
presented to the user.

Packages tested were C (glibc), C++ (libstdc++), Python (Python 3.11),
using their time and date APIs. This testing led to the correction of
the libstdc++ implementation as noted here:
https://gcc.gnu.org/cgit/gcc/commit/?id=4abd5bc600193e821fbc41995a0b8d9ea42b...

Developers can test this by installing Rawhide and uninstalling tzdata
and verifying their package operates as expected.

If tzdata cannot be uninstalled then we recommend filling a bug
against the package that Requires: tzdata and having a discussion with
the maintainer to make tzdata optional e.g. Recommends: tzdata.

== User Experience ==
The user can remove tzdata to minimize the container size.

With tzdata removed the system is UTC only.

== Dependencies ==
Fixing glibc, gcc, and python3.xx at a minimum to make tzdata
Recommends instead of Requires.

== Contingency Plan ==
* Contingency mechanism: Back out the change. Don’t do it.
* Contingency deadline:  Can be backed out at the last minute since we
are only dealing with conversions from Requires to Recommends.
* Blocks release? No

== Documentation ==
Document that the tzdata package may be removed if needed to reduce
space on containers.  With tzdata removed, the system will default to
UTC.

== Release Notes ==