fix(docs): Change year in copyright footer automatically to current year

[comawill]

Sphinx operates on environment variable SOURCE_DATE_EPOCH to provide reproducible builds.
This PR intends to change the Makefile to set SOURCE_DATE_EPOCH to the latest git revision

Additionally sphinx will only do this correctly, when there are no spaces between the numbers and dash. [src]

Checklist

• Add a new entry in an appropriate subdirectory of changelog.d
• Read and follow the PR guidelines

[comawill]

Do not merge: worked perfectly locally, but has no git in github action

[smatting]

@comawill Try adding pkgs.git to the list of available packages here https://github.com/wireapp/wire-server/blob/develop/nix/default.nix?plain=1#L28
I think @flokli mentioned before that adding git to nix envs could causes problems. But in this case maybe its fine?

[flokli]

This would mean the docs and all derivations that depend on them would not become binary reproducible anymore. Can we just drop the year numbers from the footer entirely?

[smatting]

@comawill I guess "date" inside the build returns unix epoch 0. I think the problem is that within a nix build you don't have access to the outside world's time. nix tries hard to make all builds deterministic on the input, so it forbids external inputs such as the time.
@flokli do you have an idea how to do this? pass the date from outside to script somehow?

[flokli]

As i wrote above, ideally we don't add a year number there at all. If we really want to, not make it automatic based on the current time, but manually, once per year.

[comawill]

@smatting using date +%s actually seems to work, but I agree with @flokli that this compromises reproducible builds.

My attempt was to depend on the git history here, but within nix-build we don't have the history available anymore.

Using the current year or a manual set one does not make a difference as the problems are caused by sphinx here when SOURCE_DATE_EPOCH is set. My proposed workaround is now to ignore SOURCE_DATE_EPOCH within sphinx.

[flokli]

We probably want to squash this before merge.

[flokli]

@comawill this broke builds outside the nix sandbox (make dev-run for example), where SOURCE_DATE_EPOCH env var is not set:

~/src/wire-server/docs $ make dev-run
rm -rf "/home/mf/src/wire-server/docs/build"
sphinx-autobuild \
        --port 3000 \
        --host 127.0.0.1 \
        -b html \
        -q \
        "/home/mf/src/wire-server/docs/src" "/home/mf/src/wire-server/docs/build"
[sphinx-autobuild] > sphinx-build -b html -q /home/mf/src/wire-server/docs/src /home/mf/src/wire-server/docs/build

Configuration error:
There is a programmable error in your configuration file:

Traceback (most recent call last):
  File "/nix/store/6gcnx7220g1aaz7d9y32yv8y0f1jg7q7-python3-3.10.9-env/lib/python3.10/site-packages/sphinx/config.py", line 350, in eval_config_file
    exec(code, namespace)
  File "/home/mf/src/wire-server/docs/src/conf.py", line 17, in <module>
    del os.environ['SOURCE_DATE_EPOCH']
  File "/nix/store/sp5x6s8n36gjlwck74xhj1i61p66vcpa-python3-3.10.9/lib/python3.10/os.py", line 696, in __delitem__
    raise KeyError(key) from None
KeyError: 'SOURCE_DATE_EPOCH'

Would you mind sending a followup PR to fix this?

[comawill]

@flokli wow, nice finding, sure, will make a PR

[flokli]

Already happened in #3110.