Fix DefaultChannelId#asLongText NPE

Motivation:
The DefaultChannelId class is serializable, but it does not account for variations in the length of machineId.

Modifications:
Adjusted code to accommodate varying lengths of MachineId.

Result:
Fixed NPE.
Respects varying length of machineId.

transport/src/main/java/io/netty/channel/DefaultChannelId.java

@@ -21,6 +21,7 @@
 import io.netty.util.internal.MacAddressUtil;
 import io.netty.util.internal.PlatformDependent;
 import io.netty.util.internal.SystemPropertyUtil;
+import io.netty.util.internal.ThreadLocalRandom;
 import io.netty.util.internal.logging.InternalLogger;
 import io.netty.util.internal.logging.InternalLoggerFactory;
 
@@ -53,7 +54,11 @@ public final class DefaultChannelId implements ChannelId {
      * Returns a new {@link DefaultChannelId} instance.
      */
     public static DefaultChannelId newInstance() {
-        return new DefaultChannelId();
+        return new DefaultChannelId(MACHINE_ID,
+                                    PROCESS_ID,
+                                    nextSequence.getAndIncrement(),
+                                    Long.reverse(System.nanoTime()) ^ System.currentTimeMillis(),
+                                    ThreadLocalRandom.current().nextInt());
     }
 
     static {
@@ -189,28 +194,31 @@ static int defaultProcessId() {
     private transient String shortValue;
     private transient String longValue;
 
-    private DefaultChannelId() {
-        final byte[] data = new byte[MACHINE_ID.length + PROCESS_ID_LEN + SEQUENCE_LEN + TIMESTAMP_LEN + RANDOM_LEN];
+    /**
+     * Visible for testing
+     */
+    DefaultChannelId(final byte[] machineId, final int processId, final int sequence,
+                             final long timestamp, final int random) {
+        final byte[] data = new byte[machineId.length + PROCESS_ID_LEN + SEQUENCE_LEN + TIMESTAMP_LEN + RANDOM_LEN];
         int i = 0;
 
         // machineId
-        System.arraycopy(MACHINE_ID, 0, data, i, MACHINE_ID.length);
-        i += MACHINE_ID.length;
+        System.arraycopy(machineId, 0, data, i, machineId.length);
+        i += machineId.length;
 
         // processId
-        writeInt(data, i, PROCESS_ID);
+        writeInt(data, i, processId);
         i += Integer.BYTES;
 
         // sequence
-        writeInt(data, i, nextSequence.getAndIncrement());
+        writeInt(data, i, sequence);
         i += Integer.BYTES;
 
         // timestamp (kind of)
-        writeLong(data, i, Long.reverse(System.nanoTime()) ^ System.currentTimeMillis());
+        writeLong(data, i, timestamp);
         i += Long.BYTES;
 
         // random
-        int random = PlatformDependent.threadLocalRandom().nextInt();
         writeInt(data, i, random);
         i += Integer.BYTES;
         assert i == data.length;
@@ -264,9 +272,10 @@ public String asLongText() {
     }
 
     private String newLongValue() {
-        StringBuilder buf = new StringBuilder(2 * data.length + 5);
+        final StringBuilder buf = new StringBuilder(2 * data.length + 5);
+        final int machineIdLen = data.length - PROCESS_ID_LEN - SEQUENCE_LEN - TIMESTAMP_LEN - RANDOM_LEN;
         int i = 0;
-        i = appendHexDumpField(buf, i, MACHINE_ID.length);
+        i = appendHexDumpField(buf, i, machineIdLen);
         i = appendHexDumpField(buf, i, PROCESS_ID_LEN);
         i = appendHexDumpField(buf, i, SEQUENCE_LEN);
         i = appendHexDumpField(buf, i, TIMESTAMP_LEN);

transport/src/test/java/io/netty/channel/DefaultChannelIdTest.java

@@ -20,10 +20,13 @@
 import io.netty.buffer.ByteBufInputStream;
 import io.netty.buffer.ByteBufOutputStream;
 import io.netty.buffer.Unpooled;
+import org.assertj.core.api.Assertions;
 import org.junit.jupiter.api.Test;
 
 import java.io.ObjectInputStream;
 import java.io.ObjectOutputStream;
+import java.util.Arrays;
+import java.util.Comparator;
 
 import static org.hamcrest.CoreMatchers.is;
 import static org.hamcrest.CoreMatchers.not;
@@ -33,6 +36,7 @@
 
 @SuppressWarnings("DynamicRegexReplaceableByCompiledPattern")
 public class DefaultChannelIdTest {
+
     @Test
     public void testShortText() {
         String text = DefaultChannelId.newInstance().asShortText();
@@ -84,4 +88,33 @@ public void testSerialization() throws Exception {
         assertThat(a, is(not(sameInstance(b))));
         assertThat(a.asLongText(), is(b.asLongText()));
     }
+
+    @Test
+    public void testDeserialization() throws Exception {
+        // DefaultChannelId with 8 byte machineId
+        final DefaultChannelId c8 = new DefaultChannelId(
+                new byte[] {
+                        (byte) 0x01, (byte) 0x23, (byte) 0x45, (byte) 0x67,
+                        (byte) 0x89, (byte) 0xab, (byte) 0xcd, (byte) 0xef
+                },
+                0x000052af,
+                0x00000000,
+                0x06504f638eb4c386L,
+                0xd964df5e);
+
+        // DefaultChannelId with 6 byte machineId
+        final DefaultChannelId c6 =
+                new DefaultChannelId(
+                        new byte[] {
+                                (byte) 0x01, (byte) 0x23, (byte) 0x45, (byte) 0x67,
+                                (byte) 0x89, (byte) 0xab,
+                        },
+                        0xce005283,
+                        0x00000001,
+                        0x069e6dce9eb4516fL,
+                        0x721757b7);
+
+        assertThat(c8.asLongText(), is("0123456789abcdef-000052af-00000000-06504f638eb4c386-d964df5e"));
+        assertThat(c6.asLongText(), is("0123456789ab-ce005283-00000001-069e6dce9eb4516f-721757b7"));
+    }
 }