Issue 4800: Reuse metadata driver for over-replicated ledger GC

[gaozhangmin]

Descriptions of the changes in this PR:

Main Issue: #xyz

Motivation

Over-replicated ledger cleanup currently instantiates a metadata bookie driver each time the scan-and-compare garbage collector checks for over-replicated ledgers. This repeatedly creates metadata client resources during GC cycles, which is unnecessary and can add overhead to the cleanup path.

Changes

• Reuse a metadata bookie driver and ledger manager factory inside ScanAndCompareGarbageCollector for over-replicated ledger cleanup.
• Make ScanAndCompareGarbageCollector closeable and release the cached metadata driver when GarbageCollectorThread shuts down.
• Keep the existing GC metadata operation rate limiter behavior while reusing the metadata driver.
• Add a focused unit test to verify the metadata driver is reused across calls and closed correctly.

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[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

This PR adds lifecycle management for metadata resources used during over-replicated ledger GC by reusing a single MetadataBookieDriver/LedgerManagerFactory and explicitly closing them during shutdown.

Changes:

• Cache and reuse MetadataBookieDriver/LedgerManagerFactory in ScanAndCompareGarbageCollector instead of recreating them per GC run.
• Add close() to ScanAndCompareGarbageCollector and invoke it from GarbageCollectorThread.shutdown().
• Add a unit test validating driver reuse and cleanup behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
bookkeeper-server/src/test/java/org/apache/bookkeeper/bookie/GarbageCollectorThreadTest.java	Adds a test to ensure the metadata driver is reused and then closed/reset.
bookkeeper-server/src/main/java/org/apache/bookkeeper/bookie/ScanAndCompareGarbageCollector.java	Introduces cached metadata driver/factory, lazy initialization, and explicit close logic.
bookkeeper-server/src/main/java/org/apache/bookkeeper/bookie/GarbageCollectorThread.java	Ensures the garbage collector’s metadata resources are closed during thread shutdown.

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[gaozhangmin]

@hangc0276 PTAL

[void-ptr974]

Thanks for improving this path and avoiding repeated metadata driver initialization.

One thing that might be worth considering is whether we can wire in the bookie server’s existing MetadataBookieDriver or LedgerManagerFactory here. Since the server already creates and owns those resources through its lifecycle, GC could potentially borrow the factory and only create/close the per-run LedgerUnderreplicationManager. That would keep the ownership/close semantics centralized while still achieving the reuse goal.