ZOOKEEPER-4377: KeeperException.create has NullPointerException when low version client requests the high version server

- When low version client accessed the high version server which has some new added error code, the client will get a NPE:
```
 java.lang.NullPointerException
at org.apache.zookeeper.KeeperException.create(KeeperException.java:94)
at org.apache.zookeeper.KeeperException.create(KeeperException.java:54)
at org.apache.zookeeper.ZooKeeper.create(ZooKeeper.java:1538)
```
- How to reproduce this issue？For example:
```
the client version we using is 3.6.0, and server version we using is 3.7.0 which has a new added error code QUOTAEXCEEDED(-125),
we set quota at server side and use the client to create znodes which exceeds the quota,
the client will get a NPE
```
- Apply this patch, we will get the following:
```
 java.lang.IllegalArgumentException: The current client version cannot lookup this code:-125
 at org.apache.zookeeper.KeeperException$Code.get(KeeperException.java:449)
 at org.apache.zookeeper.ZooKeeper.create(ZooKeeper.java:1347)
```
- we should backport this PR to all branches, making the client has upward compatibility
- more details in the [ZOOKEEPER-4377](https://issues.apache.org/jira/browse/ZOOKEEPER-4377)

Author: maoling <maoling@apache.org>

Reviewers: Enrico Olivelli <eolivelli@apache.org>, ruanwenjun <wenjun@apache.org>

Closes #1764 from maoling/ZOOKEEPER-4377

zookeeper-server/src/main/java/org/apache/zookeeper/KeeperException.java

@@ -154,7 +154,7 @@ public abstract class KeeperException extends Exception {
             return new ThrottledOpException();
         case OK:
         default:
-            throw new IllegalArgumentException("Invalid exception code");
+            throw new IllegalArgumentException("Invalid exception code:" + code.code);
         }
     }
 
@@ -441,10 +441,14 @@ public abstract class KeeperException extends Exception {
         /**
          * Get the Code value for a particular integer error code
          * @param code int error code
-         * @return Code value corresponding to specified int code, or null
+         * @return Code value corresponding to specified int code, if null throws IllegalArgumentException
          */
         public static Code get(int code) {
-            return lookup.get(code);
+            Code codeVal = lookup.get(code);
+            if (codeVal == null) {
+                throw new IllegalArgumentException("The current client version cannot lookup this code:" + code);
+            }
+            return codeVal;
         }
     }
 

zookeeper-server/src/test/java/org/apache/zookeeper/ZooKeeperTest.java

@@ -18,9 +18,11 @@
 
 package org.apache.zookeeper;
 
+import static org.apache.zookeeper.KeeperException.Code.NOAUTH;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 import static org.junit.jupiter.api.Assertions.fail;
 import java.io.ByteArrayOutputStream;
@@ -31,6 +33,7 @@ import java.util.Arrays;
 import java.util.List;
 import java.util.concurrent.atomic.AtomicInteger;
 import org.apache.zookeeper.AsyncCallback.VoidCallback;
+import org.apache.zookeeper.KeeperException.Code;
 import org.apache.zookeeper.ZooDefs.Ids;
 import org.apache.zookeeper.cli.CliCommand;
 import org.apache.zookeeper.cli.CliException;
@@ -802,4 +805,14 @@ public class ZooKeeperTest extends ClientBase {
 
     }
 
+    @Test
+    public void testKeeperExceptionCreateNPE() {
+        // One existing code
+        KeeperException k1 = KeeperException.create(Code.get(NOAUTH.intValue()));
+        assertTrue(k1 instanceof KeeperException.NoAuthException);
+
+        // One impossible code
+        assertThrows(IllegalArgumentException.class, () -> KeeperException.create(Code.get(Integer.MAX_VALUE)));
+    }
+
 }