pageserver: exclude archived timelines from freeze+flush on shutdown

[jcsp]

Problem

If offloading races with normal shutdown, we get a "failed to freeze and flush: cannot flush frozen layers when flush_loop is not running, state is Exited". This is harmless but points to it being quite strange to try and freeze and flush such a timeline. flushing on shutdown for an archived timeline isn't useful.

Related: #10389

Summary of changes

• During Timeline::shutdown, ignore ShutdownMode::FreezeAndFlush if the timeline is archived

[github-actions]

7440 tests run: 7088 passed, 0 failed, 352 skipped (full report)

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Flaky tests (8)

Postgres 17

• test_pgdata_import_smoke[None-1024-RelBlockSize.MULTIPLE_RELATION_SEGMENTS]: release-arm64-without-lfc, release-arm64-with-lfc, debug-x86-64-without-lfc
• test_pgdata_import_smoke[8-1024-RelBlockSize.MULTIPLE_RELATION_SEGMENTS]: release-arm64-with-lfc, release-arm64-without-lfc, debug-x86-64-without-lfc
• test_scrubber_tenant_snapshot[4]: debug-x86-64-without-lfc
• test_timeline_archive[4]: debug-x86-64-without-lfc

Code coverage* (full report)

• functions: 33.4% (8515 of 25532 functions)
• lines: 49.1% (71509 of 145554 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
7543de9 at 2025-01-31T11:04:23.704Z :recycle:}

[jcsp]

need to check out test_timeline_archive[debug-pg17-4] flake, since it's related code to this change

[jcsp]

Confirmed the test_timeline_archive flake matches pre-existing ones