Cache loaded repos in ProvisioningContext

[laeubi]

Currently if fetching updates the provision context is queried multiple times leading to the repositories loaded multiple times, even though this is usually faster as repo are cached this still produces unnecessary workload, especially if there is one failed repository it will be tried to be loaded over and over again.

This now caches the repositories loaded on first access to speed that up.

[github-actions]

Test Results

  375 files  ±0    375 suites  ±0   47m 54s ⏱️ +59s
1 895 tests ±0  1 891 ✅  - 1  3 💤 ±0  1 ❌ +1 
6 685 runs  ±0  6 673 ✅  - 3  9 💤 ±0  3 ❌ +3 

For more details on these failures, see this check.

Results for commit 5bc1d2d. ± Comparison against base commit 385d266.

♻️ This comment has been updated with latest results.

[merks]

I guess this same problem doesn't arise for ProvisioningContext.getLoadedArtifactRepositories(IProgressMonitor)?

One concern I have here is that one can call org.eclipse.equinox.p2.engine.ProvisioningContext.setMetadataRepositories(URI...) in the API and result of getLoadedMetadataRepositories depends on that value so probably it would be most proper to clear the cache when that setter is called.

[laeubi]

The ProvisionContext is kind of messy... e.g. it says

The provisioning context has a distinct lifecycle, whereby the metadata and artifact repositories to be used are determined when the client retrieves retrieves the metadata queryable. Clients should not reset the list of metadata repository cations or artifact repository locations once the metadata queryable has been retrieved.

in fact the artifact repositories are only loaded fully when the metadata is queried and so on.

Of course this "distinct lifecycle" is no where described :-\

[merks]

Looks fine.

repositorie is spelled wrong, but it was that way before. 😱