The check against null and usage of the field updateThread is typically protected by synchronization, but not in 1 location. [LUCENE-9306]

# Github Pull Request

I created a pull request on github for this issue at:

# Description

Checks against `null` and usages of the field `updateThread` are typically made atomic by synchronization on `this` (synchronized methods), e.g., at lines: [339-341](https://github.com/apache/lucene-solr/blob/531015245042507d71845b6d584e7e7389303093/lucene/replicator/src/java/org/apache/lucene/replicator/ReplicationClient.java#L339-L341), [357-358](https://github.com/apache/lucene-solr/blob/531015245042507d71845b6d584e7e7389303093/lucene/replicator/src/java/org/apache/lucene/replicator/ReplicationClient.java#L357-L358), and [380-381](https://github.com/apache/lucene-solr/blob/531015245042507d71845b6d584e7e7389303093/lucene/replicator/src/java/org/apache/lucene/replicator/ReplicationClient.java#L380-L381).

However, the `null` check at [line 387](https://github.com/apache/lucene-solr/blob/531015245042507d71845b6d584e7e7389303093/lucene/replicator/src/java/org/apache/lucene/replicator/ReplicationClient.java#L387) and the usage at [line 388](https://github.com/apache/lucene-solr/blob/531015245042507d71845b6d584e7e7389303093/lucene/replicator/src/java/org/apache/lucene/replicator/ReplicationClient.java#L388) are not protected by `synchronized`.

```java
public String toString() {
String res = "ReplicationClient";
if (updateThread != null) { <<<<<<<<<<<<<<<<<<<<<
res += " (" + updateThread.getName() + ")"; <<<<<<<<<<<<<<<<<
```

This check against `null` and usage of `updateThread` are in `toString()`.

However, the problem is not that the `toString()` will give a garbled string (i.e., a relatively minor issues).

The problem is that, in between the `null` check and the usage, `updateThread` can be set to `null` (e.g., by [line 369](https://github.com/apache/lucene-solr/blob/531015245042507d71845b6d584e7e7389303093/lucene/replicator/src/java/org/apache/lucene/replicator/ReplicationClient.java#L369)) and therefore the code can crash.

I.e., without `synchronized`, the `null` check does not protect the `updateThread.getName()` usage.

I don't know how `toString()` is called concurrently. However, it seems like a dangerous assumption to make that the callers of `toString()` know it should not be called concurrently with [line 369](https://github.com/apache/lucene-solr/blob/531015245042507d71845b6d584e7e7389303093/lucene/replicator/src/java/org/apache/lucene/replicator/ReplicationClient.java#L369), especially as the other methods do protect with synchronization the `null` check and usage.
# This Patch's Code

The fix is very simple: just make the method containing [lines 387-388](https://github.com/apache/lucene-solr/blob/531015245042507d71845b6d584e7e7389303093/lucene/replicator/src/java/org/apache/lucene/replicator/ReplicationClient.java#L387-L388) `synchronized`, just like the other methods containing `null` checks and usages of `updateThread`.

```java
public synchronized String toString() { <<<<<<<<<<<<<< added "synchronized" here
String res = "ReplicationClient";
if (updateThread != null) {
res += " (" + updateThread.getName() + ")";
```

---
Migrated from [LUCENE-9306](https://issues.apache.org/jira/browse/LUCENE-9306) by Adrian Nistor
Attachments: [LUCENE-9306.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-9306/LUCENE-9306.patch)

Read lucene/replicator/src/java/org/apache/lucene/replicator/ReplicationClient.java, focusing on toString() around lines 387-388 and the synchronized methods that access updateThread. Confirm that the change matches the synchronization used elsewhere and that the concurrent null-check concern is addressed; the issue already provides the proposed patch and links to an existing pull request.

Written by the indexing model from the issue text.