[CaffeinatedCoder]

Closes #148

Problem

QueryTranslationVariantTests caches one open in-memory SqliteConnection per model, which is seeded once and then only read. Since #142, SqlServerQueryVariantTests reads that same cache through QueryTranslationVariantTests.Run from its own collection (SQL Server), in parallel with the SQLite class. A SqliteConnection isn't thread-safe, so two commands at once on one connection fail with SQLite Error 5: 'database is locked'.

This only fails on the Linux leg, the only one that runs the Needs=Docker classes. It failed twice on #141's run 40889709782, with a different case each time. Windows passed both times.

Change

• QueryTranslationVariantTests.Run holds the cached connection's lock while it builds and runs a case's query. Within the SQLite class the cases already ran one after another, so the lock only orders this class's reads against the SQL Server class's.
• Databases' summary says why.

Tests

• New A_seeded_database_read_by_many_cases_at_once_answers_each_alike: 400 reads of one seeded database in parallel, each holding the same rows.
  • Seeded with the lock made a no-op (lock (new object())): fails with database is locked. Restored: passes.
• QueryTranslationVariantTests locally: 9,610 passed, 462 skipped, in 52 s.
• The SQL Server leg runs in CI's Linux job.

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