Fix: render execution_time through a per-test generator instead of patching the shared dialect

[niklasdohmen]

Problem

ModelTest implements execution_time by patching two process-global dictionaries on the dialect's generator class — TRANSFORMS and SQLGlot's cached dispatch table — inside _concurrent_render_context. The lock there covers only the model and CTE renders; setUp (fixture CREATE VIEW) and tearDown render SQL outside it.

With concurrent_tasks > 1 and at least one test that sets execution_time, another thread can hit the window in which patch.dict restores the dispatch table (clear, then refill) and fail with

ValueError: Unsupported expression type Create

while creating its fixture views. Less visibly, any other thread that renders CURRENT_DATE/CURRENT_TIMESTAMP during the patched window gets the first test's frozen time. We saw the crash on a 106-test Databricks project running 16-wide and had to serialise the whole suite (15 minutes instead of about 1).

Fix

• Build the frozen-time transforms into a per-test generator subclass (_frozen_time_generator_class, cached per generator/time/dialect) instead of patching the shared class. SQLGlot caches one dispatch table per generator class, so the subclass gets its own and the shared one is never touched.
• SQL model tests render the model query and the CTE queries through that subclass (_generate_sql). _concurrent_render_context keeps the lock only for what is genuinely process-global: time_machine.travel.
• Python model tests can run arbitrary SQL through the engine adapter, whose generator cannot be swapped per test, so they keep the previous shared-dialect patch under the lock (patch_shared_dialect=True). That path is unchanged in behaviour.

Test

test_freeze_time_does_not_patch_shared_generator: inside a frozen test's render context, another thread renders CREATE VIEW and CURRENT_DATE through the shared duckdb dialect 200 times; asserts no error, that the shared dialect still renders CURRENT_DATE, that the test's own generator renders the frozen cast, and that the shared TRANSFORMS and dispatch table are byte-identical afterwards. On main the shared dialect renders the frozen cast inside the context, which this test rejects; passes with this change. tests/core/test_test.py otherwise unchanged in outcome.