SOLR-15960: Use ConcurrentHashMap for EnvUtils.camelCaseToDotsMap

[pjmcarthur]

https://issues.apache.org/jira/browse/SOLR-15960

Description

The test run in #2126 (and #2202, I think) has failed for some Collection creation tests. The stack trace is
Caused by: java.lang.NullPointerException: key can't be null at java.base/java.lang.System.checkKey(System.java:956) at java.base/java.lang.System.getProperty(System.java:826) at org.apache.solr.common.util.EnvUtils.getPropWithCamelCaseFallback(EnvUtils.java:169) at org.apache.solr.common.util.EnvUtils.getProp(EnvUtils.java:153) at org.apache.solr.common.util.PropertiesUtil.substitute(PropertiesUtil.java:68)

It seems that getPropWithCamelCaseFallback has returned a null value for a key name. I suspect this may be caused by a concurrency issue causing an inconsistent view of the map state between threads, as the tests that failed use multiple nodes.

Solution

Changed camelCaseToDotsMap to be a ConcurrentHashMap to eliminate possible race conditions.

Tests

I have not been able to reproduce this reliably in a test.

Checklist

Please review the following and check all that apply:

• I have reviewed the guidelines for How to Contribute and my code conforms to the standards described there to the best of my ability.
• I have created a Jira issue and added the issue ID to my pull request title. (I attached this to the existing Jira)
• I have given Solr maintainers access to contribute to my PR branch. (optional but recommended)
• I have developed this patch against the main branch.
• I have run ./gradlew check.
• I have added tests for my changes.
• I have added documentation for the Reference Guide

[dsmiley]

I think camelCaseToDotSeparated should be implemented using computeIfAbsent