Commits on Oct 29, 2024

1. Move QPY tests to GitHub Actions and increase inter-symengine tests …

   This commit has two major goals:
   
   - fix the caching of the QPY files for both the `main` and `stable/*`
     branches
   
   - increase the number of compatibility tests between the different
     symengine versions that might be involved in the generation and
     loading of the QPY files.
   
   Achieving both of these goals also means that it is sensible to move the
   job to GitHub Actions at the same time, since it will put more pressure
   on the Azure machine concurrency we use.
   
   Caching
   -------
   
   The previous QPY tests attempted to cache the generated files for each
   historical version of Qiskit, but this was unreliable.  The cache never
   seemed to hit on backport branches, which was a huge slowdown in the
   critical path to getting releases out.  The cache restore keys were also
   a bit lax, meaning that we might accidentally have invalidated files in
   the cache by changing what we wanted to test, but the restore keys
   wouldn't have changed.
   
   The cache files would fail to restore as a side-effect of ed79d42
   (Qiskitgh-11526); QPY was moved to be on the tail end of the lint run, rather
   than in a test run.  This meant that it was no longer run as part of the
   push event when updating `main` or one of the `stable/*` branches.  In
   Azure (and GitHub Actions), the "cache" action accesses a _scoped_
   cache, not a universal one for the repository [^1][^2].  Approximately,
   base branches each have their own scope, and PR events open a new scope
   that is a child of the target branch, the default branch, and the source
   branch, if appropriate.  A cache task can read from any of its parent
   scopes, but write events go to the most local scope.  This means that we
   haven't been writing to long-standing caches for some time now.  PRs
   would typically miss the cache on the first attempt, hit their
   cache for updates, then miss again once entering the merge queue.
   
   The fix for this is to run the QPY job on branch-update events as well.
   The post-job cache action will then write out to a reachable cache for
   all following events.
   
   Cross-symengine tests
   ---------------------
   
   We previously were just running a single test with differing versions of
   symengine between the loading and generation of the QPY files.  This
   refactors the QPY `run_tests.sh` script to run a full pairwise matrix of
   compatibility tests, to increase the coverage.
   
   [^1]: https://docs.github.com/en/actions/writing-workflows/choosing-what-your-workflow-does/caching-dependencies-to-speed-up-workflows#restrictions-for-accessing-a-cache
   [^2]: https://learn.microsoft.com/en-us/azure/devops/pipelines/release/caching?view=azure-devops#cache-isolation-and-security

   

   jakelishman committed Oct 29, 2024
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