epugh commented on code in PR #3904:
URL: https://github.com/apache/solr/pull/3904#discussion_r2659748313


##########
solr/core/src/test/org/apache/solr/cloud/DeleteCoreRemnantsOnCreateTest.java:
##########
@@ -0,0 +1,327 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.solr.cloud;
+
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.List;
+import java.util.Optional;
+import org.apache.solr.client.solrj.request.CollectionAdminRequest;
+import org.apache.solr.client.solrj.request.CoreAdminRequest;
+import org.apache.solr.client.solrj.response.json.JsonMapResponseParser;
+import org.apache.solr.common.cloud.DocCollection;
+import org.apache.solr.common.cloud.Replica;
+import org.apache.solr.common.cloud.Slice;
+import org.apache.solr.core.CoreDescriptor;
+import org.apache.solr.core.SolrCore;
+import org.apache.solr.embedded.JettySolrRunner;
+import org.junit.Before;
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+/**
+ * Solr occasionally gets into an inconsistent state with its cores lifecycle 
where remnant files
+ * are left on disk after various operations that delete a core. Examples 
include deleting a
+ * collection operation that doesn't properly finish, or maybe the Solr 
process unexpectedly gets
+ * killed. The system property "solr.cloud.delete.unknown.cores.enabled" is an 
expert setting that

Review Comment:
   I think part of why I wrote this is because we don't actually have anything 
in the ref guide beyond a short mention in `solr-properties.adoc`, so it ended 
up here.  I'll look for a place in Ref Guide to move this advice.



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