[GitHub] [pulsar] lhotari opened a new issue #8293: Race condition in updating ManagedCursorImpl.readPosition

[GitBox <gi...@apache.org>]

lhotari opened a new issue #8293:
URL: https://github.com/apache/pulsar/issues/8293


   **Describe the bug**
   #8229 seems to have been caused by a race condition in updating `ManagedCursorImpl.readPosition`
   
   **To Reproduce**
   Since this is a concurrency issue, it's hard to reproduce and there isn't yet a publicly shared way to reproduce.
   
   **Expected behavior**
   Updates to `ManagedCursorImpl.readPosition` field should not lead to inconsistent state. It's not clear without understanding the code how concurrent updates should be handled.
   
   **Additional context**
   Please refer to #8229 for additional context. There's a link to a Slack thread for more discussions.
   
   There's a fix for #8229 which prevents the infinite loop: #8284 . This fix doesn't specifically address the race condition that happens in updating the ManagedCursorImpl.readPosition field. 
   
   There seems to be quite a few past issues where a race condition in updating readPosition has been an issue. For example https://github.com/apache/pulsar/pull/1478 , https://github.com/apache/pulsar/pull/3015 & https://github.com/apache/pulsar/pull/287 .
   
   There is also a change https://github.com/apache/pulsar/pull/6606 which adds `READ_POSITION_UPDATER` for `ManagedCursorImpl.readPosition`.
   
   Regarding the race condition in #8229, it seems that `ManagedCursorImpl.readPosition` could get out of sync from `OpReadEntry.readPosition` if `ManagedCursorImpl.readPosition` gets updated after the `OpReadEntry` has been created since `OpReadEntry`'s `readPosition` gets initialized from `ManagedCursorImpl.readPosition`.
   
   The race condition seems to happen in this code in the `setAcknowledgePosition` method:
   
   https://github.com/apache/pulsar/blob/825fdd4222dd65ef3099f1a975a1555226297379/managed-ledger/src/main/java/org/apache/bookkeeper/mledger/impl/ManagedCursorImpl.java#L1512-L1523
   
   In other locations, whenever `readPosition` field is modified, it is locked. In this particular location, there is no lock.
   
   However https://github.com/apache/pulsar/pull/6606 introduced another method for handling race condition. So there are 2 ways to handle race conditions for `ManagedCursorImpl.readPosition` field: `ManagedCursorImpl.lock.writeLock()` and there's also `ManagedCursorImpl.READ_POSITION_UPDATER` which is used in one location to update  `ManagedCursorImpl.readPosition`.


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[GitHub] [pulsar] merlimat closed issue #8293: Race condition in updating ManagedCursorImpl.readPosition

[GitBox <gi...@apache.org>]

merlimat closed issue #8293:
URL: https://github.com/apache/pulsar/issues/8293


   


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