Share parent's PropagationTags for local child spans

dd-trace-core/src/main/java/datadog/trace/core/CoreTracer.java

@@ -2017,7 +2017,9 @@ protected static final DDSpanContext buildSpanContext(
           requestContextDataIast = null;
           ciVisibilityContextData = null;
         }
-        propagationTags = tracer.propagationTagsFactory.empty();
+        // Local children share the parent's PropagationTags (trace-level state; reads route to the
+        // root) instead of allocating an unused empty() per span.
+        propagationTags = ddsc.getPropagationTags();
       } else {
         long endToEndStartTime;
 

dd-trace-core/src/test/java/datadog/trace/core/PropagationTagsChildSpanTest.java

@@ -0,0 +1,98 @@
+package datadog.trace.core;
+
+import static datadog.trace.api.TracePropagationStyle.DATADOG;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import datadog.trace.api.DDTraceId;
+import datadog.trace.api.sampling.PrioritySampling;
+import datadog.trace.bootstrap.instrumentation.api.AgentSpan;
+import datadog.trace.core.propagation.ExtractedContext;
+import datadog.trace.core.propagation.PropagationTags;
+import java.util.Collections;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Characterization test for the open question in {@code ptags-allocation-findings.md}: when a
+ * distributed trace arrives carrying {@code _dd.p.*} propagation tags, does a <b>non-root (local
+ * child)</b> span still carry those inbound tags when it injects?
+ *
+ * <p>Inbound {@code _dd.p.*} live on the root's {@link PropagationTags} (inherited from the {@link
+ * ExtractedContext}, {@code CoreTracer} ~line 2031), but a local child currently receives a fresh
+ * {@code propagationTagsFactory.empty()} instance (~line 2020). If {@link
+ * #localChildCarriesInboundDdpTags()} <b>fails</b>, non-root injection is dropping inbound {@code
+ * _dd.p.*} — a latent correctness bug, not merely the known per-span allocation waste. If it
+ * <b>passes</b>, there is reconciliation and the per-span empties are pure waste (sharing the
+ * parent's instance is then a safe allocation win).
+ *
+ * <p>Either way this test is the gate + safety net for the planned "share the parent's
+ * PropagationTags for local children" fix.
+ */
+class PropagationTagsChildSpanTest extends DDCoreJavaSpecification {
+
+  private static final String INBOUND_HEADER = "_dd.p.dm=934086a686-4,_dd.p.anytag=value";
+  private static final String INBOUND_TAG = "_dd.p.anytag=value";
+
+  private CoreTracer tracer;
+
+  @BeforeEach
+  void setup() {
+    tracer = tracerBuilder().build();
+  }
+
+  private static ExtractedContext extractedWithDdpTags() {
+    return new ExtractedContext(
+        DDTraceId.ONE,
+        2,
+        PrioritySampling.SAMPLER_KEEP,
+        null,
+        0,
+        Collections.<String, String>emptyMap(),
+        Collections.<String, Object>emptyMap(),
+        null,
+        PropagationTags.factory()
+            .fromHeaderValue(PropagationTags.HeaderType.DATADOG, INBOUND_HEADER),
+        null,
+        DATADOG);
+  }
+
+  /** What the Datadog codec would inject for {@code x-datadog-tags} from this span. */
+  private static String injectedDdpHeader(AgentSpan span) {
+    return ((DDSpanContext) span.context())
+        .getPropagationTags()
+        .headerValue(PropagationTags.HeaderType.DATADOG);
+  }
+
+  /** Baseline: the root span (built directly from the extracted context) carries inbound tags. */
+  @Test
+  void rootSpanCarriesInboundDdpTags() {
+    AgentSpan root = tracer.buildSpan("test", "root").asChildOf(extractedWithDdpTags()).start();
+    try {
+      String header = injectedDdpHeader(root);
+      assertTrue(
+          header != null && header.contains(INBOUND_TAG),
+          "root injected _dd.p.* header should contain inbound tag; was: " + header);
+    } finally {
+      root.finish();
+    }
+  }
+
+  /**
+   * THE open question: a local child of the root. Inbound {@code _dd.p.*} must survive injection
+   * from the child, or downstream services lose the distributed tags on every non-root hop.
+   */
+  @Test
+  void localChildCarriesInboundDdpTags() {
+    AgentSpan root = tracer.buildSpan("test", "root").asChildOf(extractedWithDdpTags()).start();
+    AgentSpan child = tracer.buildSpan("test", "child").asChildOf(root.context()).start();
+    try {
+      String header = injectedDdpHeader(child);
+      assertTrue(
+          header != null && header.contains(INBOUND_TAG),
+          "local child injected _dd.p.* header should contain inbound tag; was: " + header);
+    } finally {
+      child.finish();
+      root.finish();
+    }
+  }
+}