caching: pool trimmed append-only mem leases

[snissn]

Summary

• when trimAppendOnlyMemLeases drops strong lease references, return the reset memtables to appendOnlyMemPool instead of discarding them
• release any stale direct-arena lease association before pooling
• add TestTrimAppendOnlyMemLeases_DroppedLeasesReturnToPool

Why

Previously, trim dropped append-only memtables on the floor after hard reset. That loses reusable entry-slice backing arrays and increases reallocation pressure on subsequent rotate cycles.

Validation

• go test ./TreeDB/caching -run TestTrimAppendOnlyMemLeases_DroppedLeasesReturnToPool -count=50
• go test ./TreeDB/caching ./TreeDB/internal/memtable -count=1
• go vet ./TreeDB/caching ./TreeDB/internal/memtable
• unified bench:
  • /home/mikers/tmp/perf-cap-trim-1773579157
  • /home/mikers/tmp/perf-lease-trim-pool-1773579424

[Copilot]

Pull request overview

Improves TreeDB caching’s post-flush/checkpoint trimming so that append-only memtables dropped from the strong “lease” set are reset and returned to the appendOnlyMemPool (and any stale direct-arena lease association is released) to reduce reallocation pressure on subsequent rotations.

Changes:

• Pool dropped append-only memtables during trimAppendOnlyMemLeases instead of discarding them.
• Release any associated append-only direct arena lease before pooling the memtable.
• Add a unit test asserting trimmed leases are returned to the memtable pool.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
TreeDB/caching/db.go	Returns dropped append-only memtables to appendOnlyMemPool after hard reset and direct-arena lease release.
TreeDB/caching/retained_arena_trim_test.go	Adds a test intended to validate that trimmed append-only leases are pooled for reuse.

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[Copilot]

Pull request overview

This PR improves memory reuse in the caching layer by ensuring append-only memtables dropped during lease trimming are returned to the per-DB appendOnlyMemPool, and by releasing any associated direct-arena leases before pooling.

Changes:

• Return dropped append-only memtable leases to db.appendOnlyMemPool instead of discarding them.
• Release any append-only direct-arena lease association before pooling dropped memtables.
• Add a unit test intended to validate that trimmed leases are returned to the pool.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
TreeDB/caching/db.go	Pools trimmed append-only memtables and releases direct-arena lease associations before pooling.
TreeDB/caching/retained_arena_trim_test.go	Adds a test for verifying trimmed append-only leases are returned to the pool.

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[Copilot]

Pull request overview

Improve append-only memtable lease trimming by pooling reset memtables (instead of discarding) and adding a regression test to ensure dropped leases are reusable, reducing allocation pressure during rotate cycles.

Changes:

• Return dropped append-only memtables to appendOnlyMemPool after a hard reset.
• Release stale direct-arena lease association before pooling.
• Add a unit test covering the “dropped leases returned to pool” behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
TreeDB/caching/db.go	Pools dropped append-only memtables during trimming and releases direct-arena lease associations.
TreeDB/caching/retained_arena_trim_test.go	Adds a regression test asserting trimmed leases become available for reuse via the mem pool.

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