Avoid NPE if snapshotMgr failed during init

[AndreyBozhko]

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

Description

Avoid NPE when SolrCore is closing.

If the snapshotMgr failed to initialize (e.g., because it didn't have write permission to create the snapshot metadata folder), SolrCore begins its close routine and may encounter a NPE because snapshotMgr is null.

11:07:29.001 [coreLoadExecutor-12-thread-1] ERROR org.apache.solr.core.SolrCore - Error while closing
java.lang.NullPointerException: null
	at org.apache.solr.core.SolrCore.doClose(SolrCore.java:1879) ~[solr-core-9.4.0.jar:9.4.0 71e101bb37497f730078d9afe1991b60d10bfe96 - stillalex - 2023-10-10 19:10:39]
	at org.apache.solr.core.SolrCore.close(SolrCore.java:1755) ~[solr-core-9.4.0.jar:9.4.0 71e101bb37497f730078d9afe1991b60d10bfe96 - stillalex - 2023-10-10 19:10:39]
	at org.apache.solr.core.SolrCore.<init>(SolrCore.java:1223) ~[solr-core-9.4.0.jar:9.4.0 71e101bb37497f730078d9afe1991b60d10bfe96 - stillalex - 2023-10-10 19:10:39]
	at org.apache.solr.core.SolrCore.<init>(SolrCore.java:1056) ~[solr-core-9.4.0.jar:9.4.0 71e101bb37497f730078d9afe1991b60d10bfe96 - stillalex - 2023-10-10 19:10:39]
	at org.apache.solr.core.CoreContainer.createFromDescriptor(CoreContainer.java:1705) ~[solr-core-9.4.0.jar:9.4.0 71e101bb37497f730078d9afe1991b60d10bfe96 - stillalex - 2023-10-10 19:10:39]
	at org.apache.solr.core.CoreContainer.lambda$loadInternal$12(CoreContainer.java:1043) ~[solr-core-9.4.0.jar:9.4.0 71e101bb37497f730078d9afe1991b60d10bfe96 - stillalex - 2023-10-10 19:10:39]
	at com.codahale.metrics.InstrumentedExecutorService$InstrumentedCallable.call(InstrumentedExecutorService.java:234) ~[metrics-core-4.2.20.jar:4.2.20]
	at java.util.concurrent.FutureTask.run(FutureTask.java:264) ~[?:?]
	at org.apache.solr.common.util.ExecutorUtil$MDCAwareThreadPoolExecutor.lambda$execute$0(ExecutorUtil.java:294) ~[solr-solrj-9.4.0.jar:9.4.0 71e101bb37497f730078d9afe1991b60d10bfe96 - stillalex - 2023-10-10 19:10:39]
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128) [?:?]
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628) [?:?]
	at java.lang.Thread.run(Thread.java:829) [?:?]

Solution

Please provide a short description of the approach taken to implement your solution.

Tests

Please describe the tests you've developed or run to confirm this patch implements the feature or solves the problem.

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 have given Solr maintainers access to contribute to my PR branch. (optional but recommended, not available for branches on forks living under an organisation)
• 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]

Thanks. Ideally add a JIRA & CHANGES.txt entry but I'm personally fine with simply merging this trivial change as-is. So if nobody says anything or you don't bother, that's what I'll do by end of week.

[AndreyBozhko]

Thanks - it also seemed to me that the change is too minor to warrant a JIRA.

[dsmiley]

I pushed to 9x too: 119b986
Thanks.