[ldelvoye]

part of INFRENG-846

Adds a pytest plugin that counts, for each transaction the code under test opens, the savepoints that hold a Postgres subtransaction id. A test fails once one transaction goes past 64 of them, the point where Postgres overflows its subtransaction cache and every query on the database slows down while that transaction stays open. One function opening 6 or more savepoints in a single transaction is listed in a savepoint loops section at the end of the run, without failing anything.

It lives in sentry.testutils.pytest, so getsentry's tests run it too. Like stale_database_reads, a test that needs to can opt out with self.savepoint_usage.clear().

Code that overflows today goes in a savepoint_baseline list in each repo's pyproject.toml: its overflows are listed in a savepoint baseline section instead of failing, and an entry nothing hits is flagged for removal. Sentry's one entry is the self-hosted relocation import, which runs the whole import in one transaction under monolith databases. getsentry's entry comes from getsentry/getsentry#23235.

It only sees code the tests run, at the sizes the tests use, so a loop over 3 test rows will not reach 64 even if production feeds it thousands. How long a transaction stays open is left to production monitoring.

The current list of baselined and reported code: https://www.notion.so/4178b10e4b5d81fc8baac1545b732fe6

Merge order:

1. getsentry/getsentry#23235: getsentry baseline entry, plus a filter for the option sentry does not register yet
2. feat(testutils): fail tests that overflow Postgres's subtransaction cache #131123: the savepoint check; then wait for the bot to bump sentry-version in getsentry
3. getsentry/getsentry#23236: drop the filter

[linear-code]

INFRENG-846

[cursor]

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