postgres_ffi: make WalGenerator generic over record generator

[erikgrinaker]

Problem

Benchmarks need more control over the WAL generated by WalGenerator. In particular, they need to vary the size of logical messages.

Summary of changes

• Make WalGenerator generic over RecordGenerator, which constructs WAL records.
• Add LogicalMessageGenerator which emits logical messages, with a configurable payload.
• Minor tweaks and code reorganization.

There are no changes to the core logic or emitted WAL.

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[github-actions]

5406 tests run: 5177 passed, 7 failed, 222 skipped (full report)

---

Failures on Postgres 17

• test_pg_regress[None]: release-x86-64, release-arm64, debug-x86-64
• test_pg_regress[4]: release-x86-64, release-arm64, debug-x86-64
• test_retried_detach_ancestor_after_failed_reparenting: debug-x86-64

# Run all failed tests locally:
scripts/pytest -vv -n $(nproc) -k "test_pg_regress[release-pg17-None] or test_pg_regress[release-pg17-None] or test_pg_regress[debug-pg17-None] or test_pg_regress[release-pg17-4] or test_pg_regress[release-pg17-4] or test_pg_regress[debug-pg17-4] or test_retried_detach_ancestor_after_failed_reparenting[debug-pg17]"

Flaky tests (2)

Postgres 17

• test_sharding_split_smoke: debug-x86-64
• test_retried_detach_ancestor_after_failed_reparenting: debug-x86-64

Test coverage report is not available

_{The comment gets automatically updated with the latest test results
270a9dd at 2024-11-03T17:04:33.162Z :recycle:}