SAI-4837 : In-memory synthetic SolrCore for QA node

solr/core/src/java/org/apache/solr/cloud/ZkConfigSetService.java

@@ -66,6 +66,20 @@ public ZkConfigSetService(SolrZkClient zkClient) {
     this.zkClient = zkClient;
   }
 
+  /**
+   * Do not perform Zk operations if configSetName is provided.
+   *
+   * <p>This is only used by {@link org.apache.solr.core.SyntheticSolrCore}, which is not registered
+   * with Zookeeper
+   */
+  @Override
+  protected SolrResourceLoader createCoreResourceLoader(CoreDescriptor cd, String configSetName) {
+    return configSetName != null
+        ? new ZkSolrResourceLoader(
+            cd.getInstanceDir(), configSetName, parentLoader.getClassLoader(), zkController)
+        : createCoreResourceLoader(cd);
+  }
+
   @Override
   public SolrResourceLoader createCoreResourceLoader(CoreDescriptor cd) {
     final String colName = cd.getCollectionName();

solr/core/src/java/org/apache/solr/core/ConfigSetService.java

@@ -251,8 +251,19 @@ public boolean isConfigSetTrusted(SolrResourceLoader coreLoader) throws IOExcept
    * @return a ConfigSet
    */
   public final ConfigSet loadConfigSet(CoreDescriptor dcore) {
+    return loadConfigSet(dcore, null);
+  }
+
+  /**
+   * Load the ConfigSet for a core with an explicit config set name
+   *
+   * @param dcore the core's CoreDescriptor
+   * @param configSetName an optional and explicit config set name
+   * @return a ConfigSet
+   */
+  final ConfigSet loadConfigSet(CoreDescriptor dcore, String configSetName) {
 
-    SolrResourceLoader coreLoader = createCoreResourceLoader(dcore);
+    SolrResourceLoader coreLoader = createCoreResourceLoader(dcore, configSetName);
 
     try {
       // ConfigSet properties are loaded from ConfigSetProperties.DEFAULT_FILENAME file.
@@ -391,6 +402,20 @@ protected NamedList<Object> loadConfigSetFlags(SolrResourceLoader loader) throws
    */
   protected abstract SolrResourceLoader createCoreResourceLoader(CoreDescriptor cd);
 
+  /**
+   * Create a SolrResourceLoader for a core with the provided configSetName.
+   *
+   * <p>By default, this will just call {@link
+   * ConfigSetService#createConfigSetService(CoreContainer)}. Child implementation might override
+   * this to make use of the configSetName directly
+   *
+   * @param cd the core's CoreDescriptor
+   * @param configSetName an optional config set name
+   * @return a SolrResourceLoader
+   */
+  protected SolrResourceLoader createCoreResourceLoader(CoreDescriptor cd, String configSetName) {
+    return createCoreResourceLoader(cd);
+  }
   /**
    * Return a name for the ConfigSet for a core to be used for printing/diagnostic purposes.
    *

solr/core/src/java/org/apache/solr/core/SolrCore.java

@@ -1255,7 +1255,7 @@ public void seedVersionBuckets() {
   }
 
   /** Set UpdateLog to buffer updates if the slice is in construction. */
-  private void bufferUpdatesIfConstructing(CoreDescriptor coreDescriptor) {
+  protected void bufferUpdatesIfConstructing(CoreDescriptor coreDescriptor) {
 
     if (coreContainer != null && coreContainer.isZooKeeperAware()) {
       if (reqHandlers.get("/get") == null) {

solr/core/src/java/org/apache/solr/core/SyntheticSolrCore.java

@@ -0,0 +1,66 @@
+package org.apache.solr.core;
+
+import java.nio.file.Paths;
+import java.util.HashMap;
+import java.util.Map;
+import org.apache.solr.common.SolrException;
+import org.apache.solr.common.params.CoreAdminParams;
+import org.apache.solr.rest.RestManager;
+
+/**
+ * A synthetic core that is created only in memory and not registered against Zookeeper.
+ *
+ * <p>This is only used in Coordinator node to support a subset of SolrCore functionalities required
+ * by Coordinator operations such as aggregating and writing out response and providing configset
+ * info.
+ *
+ * <p>There should only be one instance of SyntheticSolrCore per configset
+ */
+public class SyntheticSolrCore extends SolrCore {
+  public SyntheticSolrCore(CoreContainer coreContainer, CoreDescriptor cd, ConfigSet configSet) {
+    super(coreContainer, cd, configSet);
+  }
+
+  public static SyntheticSolrCore createAndRegisterCore(
+      CoreContainer coreContainer, String syntheticCoreName, String configSetName) {
+    Map<String, String> coreProps = new HashMap<>();
+    coreProps.put(CoreAdminParams.CORE_NODE_NAME, coreContainer.getHostName());
+    coreProps.put(CoreAdminParams.COLLECTION, syntheticCoreName);
+
+    CoreDescriptor syntheticCoreDescriptor =
+        new CoreDescriptor(
+            syntheticCoreName,
+            Paths.get(coreContainer.getSolrHome() + "/" + syntheticCoreName),
+            coreProps,
+            coreContainer.getContainerProperties(),
+            coreContainer.getZkController());
+
+    ConfigSet coreConfig =
+        coreContainer.getConfigSetService().loadConfigSet(syntheticCoreDescriptor, configSetName);
+    syntheticCoreDescriptor.setConfigSetTrusted(coreConfig.isTrusted());
+    SyntheticSolrCore syntheticCore =
+        new SyntheticSolrCore(coreContainer, syntheticCoreDescriptor, coreConfig);
+    coreContainer.registerCore(syntheticCoreDescriptor, syntheticCore, false, false);
+
+    return syntheticCore;
+  }
+
+  @Override
+  protected void bufferUpdatesIfConstructing(CoreDescriptor coreDescriptor) {
+    // no updates to SyntheticSolrCore
+  }
+
+  @Override
+  protected RestManager initRestManager() throws SolrException {
+    // returns an initialized RestManager. As init routines requires reading configname of the
+    // core's collection from ZK
+    // which synthetic core is not registered in ZK.
+    // We do not expect RestManager ops on Coordinator Nodes
+    return new RestManager();
+  }
+
+  @Override
+  public void close() {
+    super.close();
+  }
+}