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Posted to issues@artemis.apache.org by "k-krawczyk (via GitHub)" <gi...@apache.org> on 2026/05/30 07:16:15 UTC
[PR] ARTEMIS-6085 Fix NPE from race in MQTTStateManager.getSessionState [artemis]
k-krawczyk opened a new pull request, #6480:
URL: https://github.com/apache/artemis/pull/6480
## What
Fixes a `NullPointerException` thrown from `MQTTProtocolHandler#handleLinkStealing()` (`getSessionState(clientID).getSession()`).
## Root cause
`MQTTStateManager.getSessionState(String)` used a non-atomic `containsKey()` + `get()` on a `ConcurrentHashMap`. During link stealing (two connections sharing a client ID), `removeSessionState()` can run on another thread between the two calls, so `get()` returns `null` and the caller dereferences it with `.getSession()`.
The race is still present on `main`.
## Fix
Replace the `containsKey()/get()/put()` block with an atomic `computeIfAbsent()` — a get-or-create that can never return `null`. Behaviour is otherwise unchanged.
## Test
Adds `MQTTStateManagerTest`, a concurrency regression test that runs `getSessionState()` against concurrent `removeSessionState()` on the same client ID and asserts a non-null result. It fails (NPE/null) on the pre-fix code in ~1s and passes after the fix. The full `artemis-mqtt-protocol` module suite is green (12/12).
JIRA: https://issues.apache.org/jira/browse/ARTEMIS-6085
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Re: [PR] ARTEMIS-6085 Fix NPE from race in MQTTStateManager.getSessionState [artemis]
Posted by "jbertram (via GitHub)" <gi...@apache.org>.
jbertram merged PR #6480:
URL: https://github.com/apache/artemis/pull/6480
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Re: [PR] ARTEMIS-6085 Fix NPE from race in MQTTStateManager.getSessionState [artemis]
Posted by "jbertram (via GitHub)" <gi...@apache.org>.
jbertram commented on PR #6480:
URL: https://github.com/apache/artemis/pull/6480#issuecomment-4594525760
@k-krawczyk many thanks for the great contribution!
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