Only write raw compression array if compressable input

If compression type is INCOMPRESSIBLE, would previously end up writing
the entire raw working buffer, which is at least 4096 bytes long.

3 files changed, 22 insertions, 2 deletions

diff --git a/document/src/main/java/com/yahoo/document/serialization/VespaDocumentSerializer42.java b/document/src/main/java/com/yahoo/document/serialization/VespaDocumentSerializer42.java
index 211412be684..6095d4a1545 100644
--- a/document/src/main/java/com/yahoo/document/serialization/VespaDocumentSerializer42.java
+++ b/document/src/main/java/com/yahoo/document/serialization/VespaDocumentSerializer42.java
@@ -344,7 +344,7 @@ public class VespaDocumentSerializer42 extends BufferSerializer implements Docum
         }
 
         int pos = buf.position();
-        if (compression.data() != null) {
+        if (compression.data() != null && compression.type().isCompressed()) {
             put(null, compression.data());
         } else {
             put(null, buffer.getByteBuffer());
diff --git a/document/src/test/java/com/yahoo/document/serialization/VespaDocumentSerializerTestCase.java b/document/src/test/java/com/yahoo/document/serialization/VespaDocumentSerializerTestCase.java
index 329d1956e64..8399c0b280e 100644
--- a/document/src/test/java/com/yahoo/document/serialization/VespaDocumentSerializerTestCase.java
+++ b/document/src/test/java/com/yahoo/document/serialization/VespaDocumentSerializerTestCase.java
@@ -107,6 +107,26 @@ public class VespaDocumentSerializerTestCase {
 
         // Should _not_ throw any deserialization exceptions
         fixture.roundtripSerialize(doc);
+        // TODO check target values..!
+    }
+
+    @Test
+    public void incompressable_structs_are_serialized_without_buffer_size_overhead_bug() {
+        CompressionFixture fixture = new CompressionFixture();
+
+        Document doc = new Document(fixture.docType, "id:foo:map_of_structs::flarn");
+        Struct nested = new Struct(fixture.nestedType);
+        nested.setFieldValue("str", new StringFieldValue(fixture.compressableString()));
+
+        MapFieldValue<StringFieldValue, Struct> map = new MapFieldValue<StringFieldValue, Struct>(fixture.mapType);
+        // Only 1 struct added. Not enough redundant information that header struct containing map itself
+        // can be compressed.
+        map.put(new StringFieldValue("foo"), nested);
+        doc.setFieldValue("map", map);
+
+        GrowableByteBuffer buf = CompressionFixture.asSerialized(doc);
+        // Explanation of arbitrary value: buffer copy bug meant that incompressable structs were all serialized
+        // rounded up to 4096 bytes.
+        assertTrue(buf.remaining() < 4096);
     }
-    
 }
diff --git a/document/src/tests/data/serializejava-compressed.dat b/document/src/tests/data/serializejava-compressed.dat
index 86875d5f121..453abef81f1 100644
--- a/document/src/tests/data/serializejava-compressed.dat
+++ b/document/src/tests/data/serializejava-compressed.dat