Transport: read/write support for list of strings

Add support for reading and writng string lists to existing StreamInput and StreamOutput class.

src/main/java/org/elasticsearch/common/io/stream/StreamInput.java

@@ -335,6 +335,21 @@ public abstract class StreamInput extends InputStream {
         return ret;
     }
 
+    /**
+     * Read in a list of strings. List can be empty but not {@code null}.
+     */
+    public List<String> readStringList() throws IOException {
+        int size = readVInt();
+        if (size == 0) {
+            return Collections.emptyList();
+        }
+        List<String> ret = new ArrayList<>(size);
+        for (int i = 0; i < size; i++) {
+            ret.add(readString());
+        }
+        return ret;
+    }
+
     @Nullable
     public Map<String, Object> readMap() throws IOException {
         return (Map<String, Object>) readGenericValue();

src/main/java/org/elasticsearch/common/io/stream/StreamOutput.java

@@ -283,6 +283,16 @@ public abstract class StreamOutput extends OutputStream {
         }
     }
 
+    /**
+     * Write a list of strings. List can be empty but not {@code null}.
+     */
+    public void writeStringList(List<String> stringList) throws IOException {
+        writeVInt(stringList.size());
+        for (String s : stringList) {
+            writeString(s);
+        }
+    }
+
     /**
      * Writes a string array, for nullable string, writes it as 0 (empty string).
      */

src/test/java/org/elasticsearch/common/io/streams/BytesStreamsTests.java

@@ -28,6 +28,8 @@ import org.elasticsearch.test.ElasticsearchTestCase;
 import org.junit.Ignore;
 import org.junit.Test;
 
+import java.util.Arrays;
+
 import static org.hamcrest.Matchers.closeTo;
 import static org.hamcrest.Matchers.equalTo;
 
@@ -282,6 +284,7 @@ public class BytesStreamsTests extends ElasticsearchTestCase {
         out.writeGenericValue(doubleArray);
         out.writeString("hello");
         out.writeString("goodbye");
+        out.writeStringList(Arrays.asList(new String[]{"Hello", "Again"}));
         out.writeGenericValue(BytesRefs.toBytesRef("bytesref"));
         BytesStreamInput in = new BytesStreamInput(out.bytes().toBytes());
         assertThat(in.readBoolean(), equalTo(false));
@@ -299,6 +302,7 @@ public class BytesStreamsTests extends ElasticsearchTestCase {
         assertThat(in.readGenericValue(), equalTo((Object)doubleArray));
         assertThat(in.readString(), equalTo("hello"));
         assertThat(in.readString(), equalTo("goodbye"));
+        assertThat(in.readStringList(), equalTo(Arrays.asList(new String[]{"Hello", "Again"})));
         assertThat(in.readGenericValue(), equalTo((Object)BytesRefs.toBytesRef("bytesref")));
         in.close();
         out.close();