stevenschlansker opened a new issue, #11913:
URL: https://github.com/apache/lucene/issues/11913
### Description
In the lucene-replicator module, the PrimaryNode does some initialization
work in the constructor. It starts with an IndexWriter provided by the
application author. At line 92:
```
writer.getConfig().setMergedSegmentWarmer(new
PreCopyMergedSegmentWarmer(this));
```
In this line, the IndexWriter is mutated to indirectly reference
`PrimaryNode.this` before the constructor initialization has completed. At this
point, important fields are not set yet (`mgr` for example is not set), and
further initialization is not done carefully with regards to thread safety
(`mgr` is not volatile, for example).
Meanwhile, a background thread calls `IndexWriter.commit()`, not realizing
the `PrimaryNode` is not yet initialized. Doing so causes a merge, which causes
the `PreCopyMergedSegmentWarmer` to then invoke
`PrimaryNode.preCopyMergedSegmentFiles`. Now, another thread is calling an
instance method on an incompletely initialized PrimaryNode.
In our case, this leads to a `NullPointerException` reading a field of our
PrimaryNode subclass, despite it being `final` and initialized in the
constructor. Essentially, this code can fail, but only in rare circumstances:
```
public class MyNode extends PrimaryNode {
private final ImportantField importantField;
public MyNode(IndexWriter writer) {
super(writer, ...); // leaks `this` into `writer`
importantField = new ImportantField();
}
protected void preCopyMergedSegmentFiles(...) {
// this can be called before constructor finishes! danger!
importantField.getInformation(); // very surprising
NullPointerException!
}
}
```
I am not entirely sure what the fix here is. Initialization is clearly a
tricky problem.
Generally, it might be nice to move any code beyond setting fields out of
the PrimaryNode constructor into a start() method, but this is likely not a
compatible change.
Leaking a `this` reference from an object constructor into shared state is
an inherently risky thing to do.
### Version and environment details
Lucene 9.4.1, Java 17+19, platform-independent
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