[feat][broker] PIP-470: Close inactive topics without deleting their data

[codelipenghui]

PIP: #TBD (see pip/pip-470.md in this PR)

Motivation

Deployments with very large numbers of mostly-idle topics suffer broker memory pressure and per-topic metrics cardinality, even when they explicitly do not want the topic data deleted. Today the only mechanisms are:

1. brokerDeleteInactiveTopicsEnabled — detects inactivity the right way, but deletes the data. Not acceptable when data must be retained.
2. Manual pulsar-admin topics unload driven by an external cron — reimplements the broker's inactivity detection outside the broker.

This PR adds a first-class broker option that reuses the existing inactivity detection and closes (unloads) the inactive topic instead of deleting it. The topic's BookKeeper ledgers, subscriptions, cursors and policies are all preserved; only the in-memory PersistentTopic/NonPersistentTopic and its broker-cache entry are released. The next producer/consumer reconnect transparently reloads the topic.

Modifications

• ServiceConfiguration: new dynamic config brokerCloseInactiveTopicsEnabled (default false).
• PulsarService.start(): fail-fast validation — brokerDeleteInactiveTopicsEnabled and brokerCloseInactiveTopicsEnabled are mutually exclusive.
• BrokerService.startInactivityMonitor(): schedule checkGC when either flag is on.
• AbstractTopic.isCloseWhileInactive(): broker-level helper reading the new flag.
• PersistentTopic.checkGC() / NonPersistentTopic.checkGC(): when close mode is active (and delete is not), after the same inactivity and replication-producer checks as today, call close(true, false). That path already disconnects clients, closes the managed ledger without deleting ledgers, and removes the topic from the broker cache via disposeTopic → removeTopicFromCache. The retention-window guard is bypassed in the close branch because it exists only to prevent data loss, which is moot when nothing is deleted.
• Partitioned-topic metadata cleanup is not run in close mode (nothing is deleted).
• Added pip/pip-470.md describing motivation, design, and interactions.

Detection inputs are unchanged and reused: brokerDeleteInactiveTopicsMode, brokerDeleteInactiveTopicsFrequencySeconds, brokerDeleteInactiveTopicsMaxInactiveDurationSeconds.

Verifying this change

This change added tests and can be verified as follows:

• New InactiveTopicCloseTest:
  • testCloseInactiveTopicKeepsDataAndEvictsFromCache — produces a message, drops the subscription, waits for the inactivity window, asserts the topic is evicted from the broker cache (getTopicReference empty) but still listed in metadata, and a fresh consumer on a new subscription reads the original message from earliest — proving data is preserved.
  • testActiveTopicIsNotClosed — topic with an active subscription must not be closed.
  • testMutualExclusionWithDeleteInactive — broker startup fails when both flags are enabled.
• Existing InactiveTopicDeleteTest (20 tests) — all pass; delete-path behavior is unchanged.

Does this pull request potentially affect one of the following parts:

• The default values of configurations — a new configuration brokerCloseInactiveTopicsEnabled is introduced with default false; no existing configuration default is changed.

Documentation

• doc-required — when this feature merges, the inactive-topic-management docs should be updated to describe the close-without-delete mode alongside the existing delete mode.

Matching PR in forked repository

PR in forked repository: https://github.com/codelipenghui/incubator-pulsar/tree/penghui/jolly-nightingale-e2e980