Cache GitHub API information between Actions runs

[holly-cummins]

We would ideally cache information between Actions runs, using the Gatsby cache facility and the GitHub actions cache facility

We would want a short expiry on the cache, so that issue counts are current; or we would re-query issue counts, even if the cache is populated. That's the technique we now use for repositories with labels. We would also want to reflect image changes reasonably quickly.

[holly-cummins]

We are currently seeing grey icons again, because of secondary rate limits on the GitHub API, so this is clearly needed. We will want a couple of coordinated cache mechanisms, one at the actions level (see commits on #300), and another using gatsby or a node library to cache results for individual repos, with eviction.

[holly-cummins]

Gatsby offers a built-in cache, which seems like a very tidy solution, but there are a few reasons not to use it:
There’s no time-to-live feature, so any TTL would need to be home-rolled. The GitHub data definitely needs to be refreshed occasionally.
Persisting the Gatsby cache between builds would also drag in caches for a number of other things that we maybe don’t want to cache in a production build.

On the other hand, the available persistent cache libraries are rather unsatisfactory:

They write to disk on every set, which will slow builds down and isn’t needed
They have low Snyk scores (partly because they’re so simple)

I’ve ended up going for a hybrid solution, where I use the GitHub caching mechanism to carry the Gatsby cache forward between builds, and write the API output to that cache; but I also include a layer of a normal in-memory cache to handle evicting stale results.

I realised that because we do the initial read in a big batch, if we have a fixed TTL we’ll end up with a single build needing to refresh the whole cache, which we don’t want. To avoid this I’ve put a jitter on the default time to live.

[holly-cummins]

Hurray! The images are all back after the overnight build (which had a warm cache). The overall build was 6m, with only 45s being for the actual gatsby build phase.