[7.x] Add read/writeOptionalVLong to StreamInput/Output (#5314… (#53491)

The spirit of StreamInput/StreamOutput is that common I/O patterns should
be handled by these classes so that the persistence methods in
application classes can be kept short, which facilitates easy visual
comparison between read and write methods, and reduces risks of having
serialization issues due to mismatched implementations.

To this end, this change adds readOptionalVLong and writeOptionalVLong
methods to these classes as we have started to build up cases where
that conditional/null logic has been implemented directly in the read &
write methods.

Co-authored-by: Tim Vernum <tim.vernum@elastic.co>

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

@@ -315,6 +315,14 @@ public abstract class StreamInput extends InputStream {
         return i;
     }
 
+    @Nullable
+    public Long readOptionalVLong() throws IOException {
+        if (readBoolean()) {
+            return readVLong();
+        }
+        return null;
+    }
+
     public long readZLong() throws IOException {
         long accumulator = 0L;
         int i = 0;

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

@@ -301,6 +301,15 @@ public abstract class StreamOutput extends OutputStream {
         writeVLongNoCheck(i);
     }
 
+    public void writeOptionalVLong(@Nullable  Long l) throws IOException {
+        if (l == null) {
+            writeBoolean(false);
+        } else {
+            writeBoolean(true);
+            writeVLong(l);
+        }
+    }
+
     /**
      * Writes a long in a variable-length format without first checking if it is negative. Package private for testing. Use
      * {@link #writeVLong(long)} instead.

server/src/test/java/org/elasticsearch/common/io/stream/BytesStreamsTests.java

@@ -50,6 +50,7 @@ import static org.hamcrest.Matchers.endsWith;
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.hasSize;
 import static org.hamcrest.Matchers.is;
+import static org.hamcrest.Matchers.nullValue;
 
 /**
  * Tests for {@link BytesStreamOutput} paging behaviour.
@@ -275,6 +276,8 @@ public class BytesStreamsTests extends ESTestCase {
         out.writeLong(-3);
         out.writeVLong(4);
         out.writeOptionalLong(11234234L);
+        out.writeOptionalVLong(5L);
+        out.writeOptionalVLong(null);
         out.writeFloat(1.1f);
         out.writeDouble(2.2);
         int[] intArray = {1, 2, 3};
@@ -311,6 +314,8 @@ public class BytesStreamsTests extends ESTestCase {
         assertThat(in.readLong(), equalTo(-3L));
         assertThat(in.readVLong(), equalTo(4L));
         assertThat(in.readOptionalLong(), equalTo(11234234L));
+        assertThat(in.readOptionalVLong(), equalTo(5L));
+        assertThat(in.readOptionalVLong(), nullValue());
         assertThat((double)in.readFloat(), closeTo(1.1, 0.0001));
         assertThat(in.readDouble(), closeTo(2.2, 0.0001));
         assertThat(in.readGenericValue(), equalTo((Object) intArray));