Regresssion: .ics Calendar subscription does not contain any past data anymore

[MacLemon]

Summary:

Regresssion: .ics Calendar subscriotion does not contain any past data anymore

Steps to Reproduce:

1. Subscribe to .ics calendar in your favorite calendaring application
2. https://metalab.at/calendar/export/ical/
3. Try to check what event happened yesterday.
4. Find out, that according to the calendar, nothing ever happened at Metalab at all.

Expected Results:

Past Events should be visible for being able to lookup what happened in the past. That includes recent event as well as historical data.

Actual Results:

No past data is in that calendar anymore at all, making planning, lookups and historical checks impossible.
It's a common use-case to lookup data around that time last year, or before Corona, or when was the first time this event took place. It's even impossible to lookup yesterdays event, for example if one wants to contact an organiser.

Regression:

This info was fully available up until a recent change in MOS.

Notes:

The calendar export does't need to be current up-to-the-minute. MOS knows when a change has been made to the calendar and can create a new export on-change. There's no need to freshly export upon each request which wouldn't make sense from a performance point of view.
The prerendered .ics file can be cached efficiently by the reverse proxy in front of MOS.

Version Information:

MOS version that was live at 2024-04-19 has the problem.

[luto]

easiest fix: expand the .ics file to a few months before current date, instead of just after current date.

more involved fix: don't generate the .ics on every request but cache/store it somewhere (includes free cache invalidation challenge, difficulty level: medium-easy).