cjf2xn opened a new pull request, #19480:
URL: https://github.com/apache/kafka/pull/19480
## Problem
The `Position` class uses multi-threaded data structures and safe
publication which suggests to users that it's thread safe. The `Position#merge`
method is not thread-safe due to the check-then-act logic:
```java
if (!partitionMap.containsKey(partition) || partitionMap.get(partition) <
offset) {
partitionMap.put(partition, offset);
}
```
The `partitionMap`'s entry for `partition` can change between the
conditional check and the `put` call.
The multi-threaded unit test reproduces the problem and fails ~80% of the
time without the fix.
## Solution
Delegate mutation to `withComponent` which is thread-safe since it relies on
`ConcurrentHashMap#compute` which uses synchronization internally to atomically
execute the remapping function.
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