SOLR-18089: Configure zookeeper.maxCnxns for embedded ZkServer (#4173)

Thus avoiding the warning log from o.a.z.s.ServerCnxnFactory about this not being configured.

Co-authored-by: Kamlendra <kamlendrachauhan21@gmail.com>

Author: openworld-maker

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

@@ -41,6 +41,9 @@ public class SolrZkServer {
   private static final Logger log = LoggerFactory.getLogger(MethodHandles.lookup().lookupClass());
 
   public static final String ZK_WHITELIST_PROPERTY = "zookeeper.4lw.commands.whitelist";
+  public static final String ZK_MAX_CNXNS_PROPERTY = "zookeeper.maxCnxns";
+  // Per ZooKeeper, "0" means no limit for max client connections.
+  public static final String ZK_MAX_CNXNS_DEFAULT = "0";
 
   boolean zkRun = false;
   String zkHost;
@@ -138,6 +141,7 @@ public void start() {
       return;
     }
 
+    ensureZkMaxCnxnsConfigured();
     if (System.getProperty(ZK_WHITELIST_PROPERTY) == null) {
       System.setProperty(ZK_WHITELIST_PROPERTY, "ruok, mntr, conf");
     }
@@ -208,6 +212,10 @@ public void stop() {
     }
     zkThread.interrupt();
   }
+
+  static void ensureZkMaxCnxnsConfigured() {
+    System.getProperties().putIfAbsent(ZK_MAX_CNXNS_PROPERTY, ZK_MAX_CNXNS_DEFAULT);
+  }
 }
 
 // Allows us to set a default for the data dir before parsing

solr/test-framework/src/java/org/apache/solr/cloud/ZkTestServer.java

@@ -483,6 +483,7 @@ public void run() throws InterruptedException, IOException {
 
   public void run(boolean solrFormat) throws InterruptedException, IOException {
     log.info("STARTING ZK TEST SERVER");
+    SolrZkServer.ensureZkMaxCnxnsConfigured();
     ensureStatCommandWhitelisted();
 
     AtomicReference<Throwable> zooError = new AtomicReference<>();