Keep User's Change when Update solr.xml

Problem
In our product, user is able to create additional core: Solr will update solr.xml to add extra core information. But at some time, we need update solr.xml to add our extra cores. 
This is a common requirement: we need upgrade solr config files in future release, but at same time keep customer's change.

For solrconfig.xml or solrschema.xml, we can use xi:include to put our change in one xml(for example solrconfig-upgrade.xml), user should only change solrconfig.xml either manually or via GUI.
Please refer to http://wiki.apache.org/solr/SolrConfigXml#XInclude

But this doesn't work for solr.xml, as if we define xi:include in solr.xml:

<solr persistent="true" sharedLib="lib" shareSchema="true">
 <cores adminPath="/admin/cores" defaultCoreName="collection1" 

host="${host:}" hostPort="${jetty.port:}" hostContext="${hostContext:}" zkClientTimeout="${zkClientTimeout:15000}">

<core name="collection1" instanceDir="collection1"/>
  <!-- this doesn't work -->
   <xi:include href="solr_extracores.xml"/>
 </cores>
</solr>

later if user creates another solr core, solr will update solr.xml: it will delete <xi:include> and overwrite cores section to include all cores.
Solution
We can define extra cores in a property file: cvanalytics_solr_extra.propertie like below:

# support corename_name, corename_instanceDir, corename_schemaName, corename_configName
extra_cores=extracore1, extracore2
extracore1_name=extracore1
extracore1_instanceDir=extracore1
extracore1_schemaName=extracore1_schema.xml
extracore1_configName=extracore1_config.xml
....

We will iterate all core names in extra_cores, if it's already loaded: CoreContainer.getCoreFromAnyList(core_name) returns not null, we ignore, Otherwise, we create a SolrCore and register it.

Implementation Code

org.apache.solr.core.CoreContainer.Initializer
private void initExtraCores(CoreContainer cores) throws RuntimeException {
  try {
 Properties cvProps = cores.getCVProperties();
 String solrxml_extra = cvProps.getProperty("solrxml_extra");
 if (!StringUtils.isBlank(solrxml_extra)) {
   File solrxml_extraFile = new File(cores.getSolrHome(), solrxml_extra);
   Properties solrxml_extraProps = readProperties(solrxml_extraFile);
   
   String extra_cores = solrxml_extraProps.getProperty("extra_cores");
   if (!StringUtils.isBlank(extra_cores)) {
  String[] extra_coresArr = extra_cores.split(",");
  for (String extra_core : extra_coresArr) {
    extra_core = extra_core.trim();
    if (cores.getCoreFromAnyList(extra_core) == null) {
   String coreName = solrxml_extraProps.getProperty(extra_core
    + "_name");
   if (StringUtils.isBlank(coreName)) {
     coreName = extra_core;
   }
   String instanceDir = solrxml_extraProps.getProperty(extra_core
    + "_instanceDir");
   if (StringUtils.isBlank(instanceDir)) {
     throw new RuntimeException("No instanceDir defined for "
      + coreName + "in " + solrxml_extraFile);
   }
   CoreDescriptor cd = new CoreDescriptor(cores, coreName,
    instanceDir);
   String schemaName = solrxml_extraProps.getProperty(extra_core
    + "_schemaName");
   if (!StringUtils.isBlank(schemaName)) {
     cd.setSchemaName(schemaName);
   }
   String configName = solrxml_extraProps.getProperty(extra_core
    + "_configName");
   if (!StringUtils.isBlank(configName)) {
     cd.setConfigName(configName);
   }
   SolrCore core = cores.create(cd);
   cores.register(core, false);
    }
  }
   }
 }
  } catch (Exception e) {
 throw new RuntimeException(e);
  }
}
public CoreContainer initialize() {
  CoreContainer cores = null;
  String solrHome = SolrResourceLoader.locateSolrHome();
  cores = new CoreContainer(solrHome);
  
  if (fconf.exists()) {
 cores.load(solrHome, fconf);
  } else {
 log.info("no solr.xml file found - using default");
 try {
   cores.load(solrHome, new InputSource(new ByteArrayInputStream(DEF_SOLR_XML.getBytes("UTF-8"))));
 } catch (Exception e) {
   throw new SolrException(ErrorCode.SERVER_ERROR,
    "CoreContainer.Initialize failed when trying to load default solr.xml file", e);
 }
 cores.configFile = fconf;
  }
  // call initExtraCores
  initExtraCores(cores);
  return cores;
}