[PR] [fix][broker] Avoid blocking the metrics thread on the pending-ack managed ledger [pulsar]

["merlimat (via GitHub)" <gi...@apache.org>]

merlimat opened a new pull request, #26062:
URL: https://github.com/apache/pulsar/pull/26062

   ### Motivation
   
   `TransactionAggregator.generate` (per-subscription transaction metrics) read the pending-ack store's managed ledger with `((PersistentSubscription) subscription).getPendingAckManageLedger().get()` — an **un-timed** `CompletableFuture.get()` running on the prometheus-stats executor (a bounded, 4-thread pool).
   
   The guard `checkIfPendingAckStoreInit()` only verifies `pendingAckStoreFuture.isDone()`; `getStoreManageLedger()` then composes `pendingAckStoreFuture.thenCompose(store -> store.getManagedLedger())`, so the *managed-ledger* future can still be resolving. A stuck future blocks that metrics thread, and enough of them exhaust the pool — hanging all metric scraping.
   
   ### Modifications
   
   Use `getNow(null)` instead of `get()`: if the managed ledger is already resolved it is used exactly as before; if not, this subscription's pending-ack-store stats are skipped for the current scrape and picked up on the next one. A future that completed exceptionally still throws into the existing `catch` (unchanged).
   
   ### Verifying this change
   
   Covered by existing tests, which pass: `TransactionMetricsTest.testManagedLedgerMetrics` (ledger ready → stats present) and `testManagedLedgerMetricsWhenPendingAckNotInit` (store not initialized → skipped).
   
   ### Does this pull request potentially affect one of the following parts:
   
   *If the box was checked, please highlight the changes*
   
   - [ ] Dependencies (add or upgrade a dependency)
   - [ ] The public API
   - [ ] The schema
   - [ ] The default values of configurations
   - [ ] The threading model
   - [ ] The binary protocol
   - [ ] The REST endpoints
   - [ ] The admin CLI options
   - [ ] The metrics
   - [ ] Anything that affects deployment
   


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Re: [PR] [fix][broker] Avoid blocking the metrics thread on the pending-ack managed ledger [pulsar]

["lhotari (via GitHub)" <gi...@apache.org>]

lhotari merged PR #26062:
URL: https://github.com/apache/pulsar/pull/26062


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