feat(profiling): emit SpanExecutionThread JFR event on span finish

Author: kaahos

dd-java-agent/agent-profiling/profiling-ddprof/src/main/java/com/datadog/profiling/ddprof/DatadogProfilingIntegration.java

@@ -117,6 +117,18 @@ public void onSpanFinished(AgentSpan span) {
         span.getDurationNano(),
         ctx.getEncodedOperationName(),
         ctx.getEncodedResourceName());
+    // Emit the actual execution thread captured in finishAndAddToTrace() so the backend can
+    // correctly attribute each span to the thread that ran it, rather than the event loop thread
+    // that calls CoreTracer.write() and commits the SpanNode event above.
+    long executionThreadId = ctx.getExecutionThreadId();
+    String executionThreadName = ctx.getExecutionThreadName();
+    if (executionThreadId > 0 && executionThreadName != null && !executionThreadName.isEmpty()) {
+      SpanExecutionThreadEvent event = new SpanExecutionThreadEvent();
+      event.spanId = ctx.getSpanId();
+      event.executionThreadId = executionThreadId;
+      event.executionThreadName = executionThreadName;
+      event.commit();
+    }
   }
 
   @Override

dd-java-agent/agent-profiling/profiling-ddprof/src/main/java/com/datadog/profiling/ddprof/SpanExecutionThreadEvent.java

@@ -0,0 +1,33 @@
+package com.datadog.profiling.ddprof;
+
+import jdk.jfr.Category;
+import jdk.jfr.Event;
+import jdk.jfr.Label;
+import jdk.jfr.Name;
+import jdk.jfr.StackTrace;
+
+/**
+ * Pure-Java JFR event that records the actual execution thread for each span. Emitted from {@code
+ * DatadogProfilingIntegration.onSpanFinished()} using the thread information captured in {@code
+ * DDSpan.finishAndAddToTrace()} — on the span's own finishing thread, not from the event loop that
+ * calls {@code CoreTracer.write()}.
+ *
+ * <p>The profiling backend ({@code CausalDagExtractor}) reads this event to override the incorrect
+ * {@code EVENT_THREAD} on {@code datadog.SpanNode} events (which are emitted at trace completion
+ * from the event loop thread, causing all DAG nodes to appear as event-loop threads).
+ */
+@Name("datadog.SpanExecutionThread")
+@Label("Span Execution Thread")
+@Category("Datadog")
+@StackTrace(false)
+class SpanExecutionThreadEvent extends Event {
+
+  @Label("Span ID")
+  long spanId;
+
+  @Label("Execution Thread ID")
+  long executionThreadId;
+
+  @Label("Execution Thread Name")
+  String executionThreadName;
+}