Fix missing alternate full snapshots

[shreyas-s-rao]

Signed-off-by: Shreyas Rao shreyas.sriganesh.rao@sap.com

What this PR does / why we need it:
This PR fixes missing full snapshots on shoot unhibernation. Setting the previous full snapshot existence check to 23.5 hours provides a buffer of 30 minutes for the pod to become ready to take the first full snapshot everyday in the case where full snapshot schedule lies within the shoot's hibernation period.

Which issue(s) this PR fixes:
Fixes #258

Special notes for your reviewer:

Release note:

Fix missing alternate full snapshots for some unhibernating shoots.

[amshuman-kr]

Thanks for the PR, @shreyas-s-rao! The change itself looks good. But as discussed elsewhere, how about addressing this by tweaking the gardener to consider hibernation schedule while computing the backup schedule string? It is already considering maintenance schedule.

[shreyas-s-rao]

@amshuman-kr as discussed, we can go ahead with this PR as it's a quick win, while we do acknowledge that it's only a temporary solution, and we need a more generic approach to determine whether a "recent enough" previous full snapshot was taken, according to the given cron schedule.

[amshuman-kr]

LGTM. Have you tested the case? I haven't :-)

[shreyas-s-rao]

@amshuman-kr I have tested this locally with a helm chart, where I simulated a delayed startup (by 10 minutes) of etcd on day 1. Without this PR, the day 2 etcdbr startup missed the full snapshot. With the PR, it was able to account for that 10-minute delay the previous day, and took the full snapshot on etcdbr startup. Not sure if we can have an automated test case for this, since the config values are more Gardener-specific (like 24 hours). Until we automate that, I don't see a way to add a test case for this scenario. I'll open an issue to move away from the hard-coded 24-hour value here towards a more cron-schedule-aware method to calculate the period between two consecutive full snapshots. For the time-being, I think we can go ahead and merge this PR. WDYT?