fix(llmobs): Avoid sending empty payloads to the LLMObs endpoint

communication/src/main/java/datadog/communication/serialization/FlushingBuffer.java

@@ -27,8 +27,12 @@ public boolean isDirty() {
 
   @Override
   public void mark() {
-    mark = buffer.position();
-    ++messageCount;
+    int current = buffer.position();
+    if (current != mark) {
+      // count only non-empty messages
+      ++messageCount;
+      mark = current;
+    }
   }
 
   @Override
@@ -101,4 +105,9 @@ public void reset() {
     buffer.limit(buffer.capacity());
     mark = 0;
   }
+
+  // for tests only
+  int getMessageCount() {
+    return messageCount;
+  }
 }

communication/src/test/groovy/datadog/communication/serialization/FlushingBufferTest.java

@@ -10,4 +10,45 @@ public class FlushingBufferTest {
   public void testBufferCapacity() {
     assertEquals(5, new FlushingBuffer(5, (messageCount, buffer) -> {}).capacity());
   }
+
+  @Test
+  public void testMessageCount() {
+    FlushingBuffer fb = new FlushingBuffer(10, (messageCount, buffer) -> {});
+
+    // initial counter
+    assertEquals(0, fb.getMessageCount());
+
+    fb.mark();
+    fb.mark();
+
+    // counter doesn't change if no data pushed into the buffer
+    assertEquals(0, fb.getMessageCount());
+
+    fb.put((byte) 1);
+    // still zero because the message counter increases on mark
+    assertEquals(0, fb.getMessageCount());
+
+    fb.mark();
+    // expect increased message counter
+    assertEquals(1, fb.getMessageCount());
+
+    fb.mark();
+    fb.mark();
+    // no change to the counter expected for consecutive mark calls
+
+    fb.putChar('a');
+    fb.putChar('b');
+    fb.putChar('c');
+    // no change to the message counter expected before mark call
+    assertEquals(1, fb.getMessageCount());
+
+    fb.mark();
+    // expect increased message counter
+    assertEquals(2, fb.getMessageCount());
+
+    fb.mark();
+    fb.mark();
+    // no change to the counter expected for consecutive mark calls
+    assertEquals(2, fb.getMessageCount());
+  }
 }

dd-trace-core/src/main/java/datadog/trace/common/writer/PayloadDispatcherImpl.java

@@ -111,13 +111,20 @@ public void accept(int messageCount, ByteBuffer buffer) {
       mapper.reset();
       if (response.success()) {
         if (log.isDebugEnabled()) {
-          log.debug("Successfully sent {} traces to the API", messageCount);
+          log.debug(
+              "Successfully sent {} traces {} bytes to the API {}",
+              messageCount,
+              buffer.position(),
+              mapper.endpoint());
         }
         healthMetrics.onSend(messageCount, sizeInBytes, response);
       } else {
         if (log.isDebugEnabled()) {
           log.debug(
-              "Failed to send {} traces of size {} bytes to the API", messageCount, sizeInBytes);
+              "Failed to send {} traces of size {} bytes to the API {}",
+              messageCount,
+              sizeInBytes,
+              mapper.endpoint());
         }
         healthMetrics.onFailedSend(messageCount, sizeInBytes, response);
       }

dd-trace-core/src/main/java/datadog/trace/llmobs/writer/ddintake/LLMObsSpanMapper.java

@@ -93,6 +93,11 @@ public void map(List<? extends CoreSpan<?>> trace, Writable writable) {
     List<? extends CoreSpan<?>> llmobsSpans =
         trace.stream().filter(LLMObsSpanMapper::isLLMObsSpan).collect(Collectors.toList());
 
+    if (llmobsSpans.isEmpty()) {
+      // do nothing if no llmobs spans in the trace
+      return;
+    }
+
     writable.startMap(3);
 
     writable.writeUTF8(EVENT_TYPE);

dd-trace-core/src/test/groovy/datadog/trace/llmobs/writer/ddintake/LLMObsSpanMapperTest.groovy

@@ -110,6 +110,36 @@ class LLMObsSpanMapperTest extends DDCoreSpecification {
     spanData["tags"].contains("language:jvm")
   }
 
+  def "test LLMObsSpanMapper writes no spans when none are LLMObs spans"() {
+    setup:
+    def mapper = new LLMObsSpanMapper()
+    def tracer = tracerBuilder().writer(new ListWriter()).build()
+
+    def regularSpan1 = tracer.buildSpan("http.request")
+      .withResourceName("GET /api/users")
+      .withTag("http.method", "GET")
+      .withTag("http.url", "https://example.com/api/users")
+      .start()
+    regularSpan1.finish()
+
+    def regularSpan2 = tracer.buildSpan("database.query")
+      .withResourceName("SELECT * FROM users")
+      .withTag("db.type", "postgresql")
+      .start()
+    regularSpan2.finish()
+
+    def trace = [regularSpan1, regularSpan2]
+    CapturingByteBufferConsumer sink = new CapturingByteBufferConsumer()
+    MsgPackWriter packer = new MsgPackWriter(new FlushingBuffer(1024, sink))
+
+    when:
+    packer.format(trace, mapper)
+    packer.flush()
+
+    then:
+    sink.captured == null
+  }
+
   static class CapturingByteBufferConsumer implements ByteBufferConsumer {
 
     ByteBuffer captured