fix(analytics): guard polling start so concurrent callers don't leak executors (SDK-81)

[tylerjroach]

Summary

startPollingForDefinitions had no lock around the create-and-assign of pollingExecutor. Two concurrent callers would each allocate a fresh ScheduledExecutorService, the field would point at the second, and the first executor's worker thread + scheduled task would silently leak (still alive, still polling, no longer reachable for shutdown).

Add a private pollingLock guarding the executor's lifecycle:

• startPollingForDefinitions: under the lock, bail early if pollingExecutor != null (idempotent), otherwise create + schedule.
• stopPollingForDefinitions: snapshot the executor under the lock and clear the field, then run shutdown / awaitTermination outside the lock so a long shutdown can't block a concurrent start for 5s.

Context

Linear: SDK-81. Same audit-driven cross-SDK push; Java is one of three affected (Java, Ruby, Node). Python uses a not self._polling_task check that's safe because both async and sync paths share it under the GIL; Go uses CompareAndSwap.

Test plan

• All 54 LocalFlagsProviderTest tests pass (mvn test)
• New testConcurrentStartPollingDoesNotLeakExecutors spins up 8 contender threads behind a CountDownLatch start gate. Counts JVM threads named mixpanel-flags-poller before and after; asserts exactly one new poller exists, then asserts stop cleans it up. Before this fix the count would be 8.

🤖 Generated with Claude Code

[linear-code]

SDK-85

SDK-81

[tylerjroach]

@greptileai

[greptile-apps]

Confidence Score: 5/5

Safe to merge — the lock/snapshot pattern is correct, shutdown happens outside the lock, and the fix is well-scoped to the executor lifecycle.

The production code change is straightforward and mechanically correct: idempotent create under lock, snapshot-clear-then-shutdown pattern for stop. No pre-existing contract is broken and no new race conditions are introduced.

No files require special attention.

Important Files Changed

Filename	Overview
src/main/java/com/mixpanel/mixpanelapi/featureflags/provider/LocalFlagsProvider.java	Adds pollingLock to guard executor lifecycle; start is now idempotent under the lock, stop snapshots-and-clears under the lock then shuts down outside it — correct pattern.
src/test/java/com/mixpanel/mixpanelapi/featureflags/provider/LocalFlagsProviderTest.java	Adds a concurrency regression test using a CountDownLatch start gate; thread-count assertion after stopPollingForDefinitions relies on a 100 ms sleep which is a fragile timing heuristic.

Sequence Diagram

%%{init: {'theme': 'neutral'}}%%
sequenceDiagram
    participant T1 as Thread 1
    participant T2 as Thread 2
    participant Lock as pollingLock
    participant Exec as ScheduledExecutorService

    T1->>T1: fetchDefinitions()
    T2->>T2: fetchDefinitions()
    T1->>Lock: synchronized(pollingLock)
    Note over Lock: T2 waits for lock
    T1->>T1: "pollingExecutor == null? true"
    T1->>Exec: create + scheduleAtFixedRate
    T1->>Lock: release lock
    T2->>Lock: synchronized(pollingLock)
    T2->>T2: "pollingExecutor != null, return early"
    T2->>Lock: release lock
    Note over T1,Exec: Only one executor created
    T1->>Lock: stopPollingForDefinitions: synchronized(pollingLock)
    T1->>T1: "snapshot toShutdown, pollingExecutor = null"
    T1->>Lock: release lock
    T1->>Exec: toShutdown.shutdown() + awaitTermination

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sequenceDiagram
    participant T1 as Thread 1
    participant T2 as Thread 2
    participant Lock as pollingLock
    participant Exec as ScheduledExecutorService

    T1->>T1: fetchDefinitions()
    T2->>T2: fetchDefinitions()
    T1->>Lock: synchronized(pollingLock)
    Note over Lock: T2 waits for lock
    T1->>T1: "pollingExecutor == null? true"
    T1->>Exec: create + scheduleAtFixedRate
    T1->>Lock: release lock
    T2->>Lock: synchronized(pollingLock)
    T2->>T2: "pollingExecutor != null, return early"
    T2->>Lock: release lock
    Note over T1,Exec: Only one executor created
    T1->>Lock: stopPollingForDefinitions: synchronized(pollingLock)
    T1->>T1: "snapshot toShutdown, pollingExecutor = null"
    T1->>Lock: release lock
    T1->>Exec: toShutdown.shutdown() + awaitTermination

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_{Reviews (2): Last reviewed commit: "test(flags): fix region comment issue ke..." | Re-trigger Greptile}

[tylerjroach]

Pushed 6116ece — region comment now reads SDK-81.