szetszwo commented on code in PR #958:
URL: https://github.com/apache/ratis/pull/958#discussion_r1378988763
##########
ratis-server/src/main/java/org/apache/ratis/server/impl/ReadRequests.java:
##########
@@ -95,6 +95,13 @@ CompletableFuture<Long> waitToAdvance(long readIndex) {
if (stateMachine.getLastAppliedTermIndex().getIndex() >= readIndex) {
return CompletableFuture.completedFuture(readIndex);
}
- return readIndexQueue.add(readIndex);
+ final CompletableFuture<Long> f = readIndexQueue.add(readIndex);
+
+ // double check here to avoid potential data race conditions on
LastAppliedTermIndex
+ if (stateMachine.getLastAppliedTermIndex().getIndex() >= readIndex) {
+ return CompletableFuture.completedFuture(readIndex);
+ } else {
+ return f;
+ }
Review Comment:
- Let's call `readIndexQueue.complete(..)`?
- BTW, I found that it is more correct to complete the future in line 96
with `lastAppliedIndex` instead of `readIndex`, although it is not used.
Then, the code becomes:
```java
CompletableFuture<Long> waitToAdvance(long readIndex) {
final long lastApplied =
stateMachine.getLastAppliedTermIndex().getIndex();
if (lastApplied >= readIndex) {
return CompletableFuture.completedFuture(lastApplied);
}
final CompletableFuture<Long> f = readIndexQueue.add(readIndex);
final long current = stateMachine.getLastAppliedTermIndex().getIndex();
if (current > lastApplied) {
readIndexQueue.complete(current);
}
return f;
}
```
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