BewareMyPower commented on code in PR #25110:
URL: https://github.com/apache/pulsar/pull/25110#discussion_r2672329694
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managed-ledger/src/main/java/org/apache/bookkeeper/mledger/impl/ManagedCursorImpl.java:
##########
@@ -1243,14 +1243,17 @@ public boolean hasMoreEntries() {
@Override
public long getNumberOfEntries() {
- if (readPosition.compareTo(ledger.getLastPosition().getNext()) > 0) {
+ Position readPos = readPosition;
+ Position lastPosition = ledger.getLastPosition();
+ Position nextPosition = lastPosition.getNext();
Review Comment:
Copying the positions into local variables is not enough if these variables
can be modified in a different thread. We should add a lock to guarantee thread
safety.
Here is an example of a very simple case:
```java
class Range {
private int start = 0;
private int end = 0;
public int getNumberOfEntries() {
return end - start;
}
public void produce(int n) {
CompletableFuture.runAsync(() -> {
end += n;
});
}
public void consume(int n) {
CompletableFuture.runAsync(() -> {
if (start + n > end) {
throw new IllegalStateException("Cannot consume more than
produced");
}
start += n;
});
}
}
```
Even if you changed `getNumberOfEntries()` to:
```java
private volatile int start = 0;
private volatile int end = 0;
public int getNumberOfEntries() {
int end = this.end;
int start = this.start;
return end - start;
}
```
`getNumberOfEntries` is still not safe
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