Add optional in-memory cache of HelmRepository index files

[souleb]

If implemented, will provide users with a way to cache index files.

This addresses issues where the index file is loaded and unmarshalled in
concurrent reconciliation resulting in a heavy memory footprint.

The caching strategy used is cache aside, and the cache is a k/v store
with expiration.

The cache number of entries and ttl for entries are configurable.

The cache is optional and is disabled by default

Signed-off-by: Soule BA soule@weave.works

Fix: fluxcd/flux2#2072

[tehlers320]

.DS_Store should be checked in here.

[stefanprodan]

Do we remove the cache entry on HelmRepository finalization?

[souleb]

Do we remove the cache entry on HelmRepository finalization?

My assumption was that if a source is unavailable, the chart reconciliation fails early, before attempting to load the index. hence we just wait for the corresponding cache entry expiration and deletion.

[hiddeco]

Nit:

[..]
The caching strategy used is cache aside, and the cache is a k/v store
with expiration.
[..]
The cache is optional and is disabled by default

General comment, but not blocking for introduction:

I feel that the introduction of this cache could partly eliminate the need of some of the simpler and naive caching approaches introduced by me, thereby simplifying the overall code. This might be worth looking into as a follow up.

[souleb]

This PR is to introduce the cache. There will be follow up work, but for now I want to merge it so users can start testing.

[stefanprodan]

LGTM

Thanks @souleb 🥇

[hiddeco]

Happy with this for experimental introduction, thanks a lot @souleb, and looking forward to the further improvements 🥇 🙇

[stefanprodan]

After we release flux with this change, we need to update the bootstrap cheatsheet to show users how to enable the cache.

[souleb]

After we release flux with this change, we need to update the bootstrap cheatsheet to show users how to enable the cache.

I will create an issue for that.

[darkowlzz]

LGTM!