feat(pageserver): add aux file v2 write path

[skyzh]

Problem

#7462

Summary of changes

Add the write path of aux file v2. We first retrieve the value of the corresponding hash key. If the key exists (which indicates a hash collision), modify the value according to the type of file operation (i.e., update or insert). One value can store multiple files, though the chance of storing multiple files is low.

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]

2766 tests run: 2646 passed, 0 failed, 120 skipped (full report)

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Flaky tests (2)

Postgres 16

• test_compute_pageserver_connection_stress: release

Postgres 14

• test_delete_timeline_client_hangup: debug

Code coverage* (full report)

• functions: 28.1% (6484 of 23051 functions)
• lines: 47.0% (46107 of 98080 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
82327a8 at 2024-04-24T20:26:31.239Z :recycle:}

[VladLazar]

Hm. I don't feel like we should merge dead code. IMO there should be at least a unit test exercising it.

[skyzh]

I'll need to finish the read path #7468 before I could merge this pull request. That one doesn't seem simple because there will need to be some significant changes in image layer creation.

[skyzh]

added an encode/decode test, need to wait #7505 to put the logic behind a proper feature flag.

The aux file write path can only be tested when the basebackup read path is implemented, so I'd rather leave it untested in this pull request.

[VladLazar]

The aux file write path can only be tested when the basebackup read path is implemented, so I'd rather leave it untested in this pull request.

Can you not test with just Timeline::get calls or am I missing something?

[skyzh]

I assume we will need a compute node to write AUX files into the page server, and in that case, if we want to test read a single file without read path functionalities implemented (i.e., get single file, or generate basebackup), the test (in Python regression test) will need to compute the hash of the file and encode the value in order to do the comparison, or hardcode the key/value, which duplicates the implementation in the page server side.

Maybe I will have a single pull request with both read+write path for AUX file later so that we can merge some code with good test coverage.

For the generic case, I have #7468 that tests writing arbitrary keys and read them.