Optimise Python unit test runtime (~7x speedup)

[alexander-beedie]

Update

• Each of the 88 tests was recreating a fresh Database from scratch, reloading/parsing all data, etc.
• Optimised this by creating as a read-only DB fixture once, and caching it:
  • conn_db_readonly.
• Only the 3 tests that require a writable DB get a different database; there are two writable fixtures, one of which is also cached (and this is the one that gets used, as the current set of tests requiring write-access don't step on each other):
  • conn_db_writable_cached
  • conn_db_writable_new.

Speedup

(These timings were generated locally on Apple Silicon M2)

Before: ~37 secs
 After: ~5½ secs 🚀 

Note: not worth adding xdist parallelism, as the initial setup cost leads to a higher total runtime (8-9 secs).

[mewim]

@alexander-beedie Thanks for your contribution. I think it makes a lot of sense to cache database for testing instead of create a new one per test. For write tests though, I think there might be some risks in caching the database. It is not easy to tell whether a write test would conflict with other ones or not and may cause confusing errors when modifications on a test causes other tests to fail. So I changed the code a bit to only keep conn_db_readonly and conn_db_readwrite (which always create a new database for writing). I also added a minor workaround and notes as read only mode is not currently supported properly on Windows. I think this is now ready to be merged.

[alexander-beedie]

@alexander-beedie I changed the code a bit to only keep conn_db_readonly and conn_db_readwrite (which always create a new database for writing).

Good call; as there are only three tests that need it the performance difference is marginal, and it's better to always do the right thing rather than chase a few % more speed in the tests ;)