## Problem
The cache was introduced as a hackathon project and the only supported
limit was the number of entries.
The basebackup entry size may vary. We need to have more control over
disk space usage to ship it to production.
- Part of https://github.com/neondatabase/cloud/issues/29353
## Summary of changes
- Store the size of entries in the cache and use it to limit
`max_total_size_bytes`
- Add the size of the cache in bytes to metrics.
## Problem
- `test_basebackup_cache` fails in
https://github.com/neondatabase/neon/pull/11712 because after the
timelines on safekeepers are managed by storage controller, they do
contain proper start_lsn and the compute_ctl tool sends the first
basebackup request with this LSN.
- `Failed to prepare basebackup` log messages during timeline
initialization, because the timeline is not yet in the global timeline
map.
- Relates to https://github.com/neondatabase/cloud/issues/29353
## Summary of changes
- Account for `timeline_onto_safekeepers` storcon's option in the test.
- Do not trigger basebackup prepare during the timeline initialization.
## Problem
Basebackup cache is on the hot path of compute startup and is generated
on every request (may be slow).
- Issue: https://github.com/neondatabase/cloud/issues/29353
## Summary of changes
- Add `BasebackupCache` which stores basebackups on local disk.
- Basebackup prepare requests are triggered by
`XLOG_CHECKPOINT_SHUTDOWN` records in the log.
- Limit the size of the cache by number of entries.
- Add `basebackup_cache_enabled` feature flag to TenantConfig.
- Write tests for the cache
## Not implemented yet
- Limit the size of the cache by total size in bytes
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Co-authored-by: Aleksandr Sarantsev <aleksandr@neon.tech>
