Senrian opened a new pull request, #10235:
URL: https://github.com/apache/rocketmq/pull/10235
## Bug Description
Issue: #10214
In `MQClientInstance.java`, `brokerVersionTable` (a `ConcurrentHashMap`) is
accessed using a non-atomic check-then-act pattern that can cause
NullPointerException under concurrent access.
### Locations Fixed
1. **sendHeartbeatToBroker()** - Race between containsKey() and get()
2. **sendHeartbeatToBrokerV2()** - Same issue
3. **findBrokerVersion()** - Triple get() calls with potential NPE
### Fix Pattern
Use `computeIfAbsent` for atomic get-or-create:
```java
// Before (race condition):
if (!this.brokerVersionTable.containsKey(brokerName)) {
this.brokerVersionTable.put(brokerName, new ConcurrentHashMap<>(4));
}
this.brokerVersionTable.get(brokerName).put(addr, version);
// After (thread-safe):
this.brokerVersionTable.computeIfAbsent(brokerName, k -> new
ConcurrentHashMap<>(4))
.put(addr, version);
```
For findBrokerVersion(), use a local variable to avoid NPE if entry is
removed between calls.
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