HBASE-25323 Fix potential NPE when the zookeeper path of RegionServerTracker does not exist when start (#2702)

Signed-off-by: Duo Zhang <zhangduo@apache.org>
Signed-off-by: Guanghao Zhang <zghao@apache.org>

hbase-server/src/main/java/org/apache/hadoop/hbase/master/RegionServerTracker.java

@@ -19,6 +19,7 @@ package org.apache.hadoop.hbase.master;
 
 import java.io.IOException;
 import java.io.InterruptedIOException;
+import java.util.Collections;
 import java.util.HashSet;
 import java.util.Iterator;
 import java.util.List;
@@ -35,6 +36,7 @@ import org.apache.hadoop.hbase.zookeeper.ZKListener;
 import org.apache.hadoop.hbase.zookeeper.ZKUtil;
 import org.apache.hadoop.hbase.zookeeper.ZKWatcher;
 import org.apache.hadoop.hbase.zookeeper.ZNodePaths;
+import org.apache.hbase.thirdparty.org.apache.commons.collections4.CollectionUtils;
 import org.apache.yetus.audience.InterfaceAudience;
 import org.apache.zookeeper.KeeperException;
 import org.slf4j.Logger;
@@ -129,22 +131,25 @@ public class RegionServerTracker extends ZKListener {
     splittingServersFromWALDir.stream().filter(s -> !deadServersFromPE.contains(s)).
       forEach(s -> LOG.error("{} has no matching ServerCrashProcedure", s));
     //create ServerNode for all possible live servers from wal directory
-    liveServersFromWALDir.stream()
+    liveServersFromWALDir
         .forEach(sn -> server.getAssignmentManager().getRegionStates().getOrCreateServer(sn));
     watcher.registerListener(this);
     synchronized (this) {
       List<String> servers =
         ZKUtil.listChildrenAndWatchForNewChildren(watcher, watcher.getZNodePaths().rsZNode);
+      if (null != servers) {
         for (String n : servers) {
           Pair<ServerName, RegionServerInfo> pair = getServerInfo(n);
           ServerName serverName = pair.getFirst();
           RegionServerInfo info = pair.getSecond();
           regionServers.add(serverName);
-        ServerMetrics serverMetrics = info != null ? ServerMetricsBuilder.of(serverName,
+          ServerMetrics serverMetrics = info != null ?
-          VersionInfoUtil.getVersionNumber(info.getVersionInfo()),
+            ServerMetricsBuilder.of(serverName, VersionInfoUtil.getVersionNumber(info.getVersionInfo()),
-          info.getVersionInfo().getVersion()) : ServerMetricsBuilder.of(serverName);
+              info.getVersionInfo().getVersion()) :
+            ServerMetricsBuilder.of(serverName);
           serverManager.checkAndRecordNewServer(serverName, serverMetrics);
         }
+      }
       serverManager.findDeadServersAndProcess(deadServersFromPE, liveServersFromWALDir);
     }
   }
@@ -163,8 +168,9 @@ public class RegionServerTracker extends ZKListener {
       server.abort("Unexpected zk exception getting RS nodes", e);
       return;
     }
-    Set<ServerName> servers =
+    Set<ServerName> servers = CollectionUtils.isEmpty(names) ? Collections.emptySet() :
       names.stream().map(ServerName::parseServerName).collect(Collectors.toSet());
+
     for (Iterator<ServerName> iter = regionServers.iterator(); iter.hasNext();) {
       ServerName sn = iter.next();
       if (!servers.contains(sn)) {

hbase-zookeeper/src/main/java/org/apache/hadoop/hbase/zookeeper/MasterAddressTracker.java

@@ -297,7 +297,7 @@ public class MasterAddressTracker extends ZKNodeTracker {
   public static List<ServerName> getBackupMastersAndRenewWatch(
       ZKWatcher zkw) throws InterruptedIOException {
     // Build Set of backup masters from ZK nodes
-    List<String> backupMasterStrings = Collections.emptyList();
+    List<String> backupMasterStrings = null;
     try {
       backupMasterStrings = ZKUtil.listChildrenAndWatchForNewChildren(zkw,
           zkw.getZNodePaths().backupMasterAddressesZNode);

hbase-zookeeper/src/main/java/org/apache/hadoop/hbase/zookeeper/ZKUtil.java

@@ -445,16 +445,15 @@ public final class ZKUtil {
     } catch(KeeperException.NoNodeException ke) {
       LOG.debug(zkw.prefix("Unable to list children of znode " + znode + " " +
           "because node does not exist (not an error)"));
-      return null;
     } catch (KeeperException e) {
       LOG.warn(zkw.prefix("Unable to list children of znode " + znode + " "), e);
       zkw.keeperException(e);
-      return null;
     } catch (InterruptedException e) {
       LOG.warn(zkw.prefix("Unable to list children of znode " + znode + " "), e);
       zkw.interruptedException(e);
-      return null;
     }
+
+    return null;
   }
 
   /**