DataDog · dougqh · Jun 22, 2026
@@ -79,8 +79,6 @@ public interface Factory {
 
   public abstract CharSequence getLastParentId();
 
-  public abstract void updateLastParentId(CharSequence lastParentId);
-
   /**
    * Gets the original <a href="https://www.w3.org/TR/trace-context/#tracestate-header">W3C
    * tracestate header</a> value.
@@ -104,6 +102,15 @@ public interface Factory {
    */
   public abstract String headerValue(HeaderType headerType);
 
+  /**
+   * Like {@link #headerValue(HeaderType)} but uses {@code lastParentIdOverride} for the W3C {@code
+   * p:} (last-parent-id) instead of the stored {@link #getLastParentId() last-parent-id}. Used at
+   * inject so the injecting span's id is supplied as a parameter rather than mutated into these
+   * (possibly trace-level, shared) tags — keeping transient per-injection identity out of shared
+   * state. A {@code null} override falls back to {@link #headerValue(HeaderType)}.
+   */
+  public abstract String headerValue(HeaderType headerType, CharSequence lastParentIdOverride);
+
   /**
    * Fills a provided tagMap with valid propagated _dd.p.* tags and possibly a new sampling decision
    * tags _dd.p.dm (root span only) based on the current state, or sets only an error tag if the

@@ -80,8 +80,11 @@ private <C> void injectTraceParent(DDSpanContext context, C carrier, CarrierSett
 
     private <C> void injectTraceState(DDSpanContext context, C carrier, CarrierSetter<C> setter) {
       PropagationTags propagationTags = context.getPropagationTags();
-      propagationTags.updateLastParentId(DDSpanId.toHexStringPadded(context.getSpanId()));
-      String tracestate = propagationTags.headerValue(W3C);
+      // Supply the injecting span's id for the W3C `p:` as a parameter rather than mutating it into
+      // the (possibly trace-level, shared) tags — keeps transient per-injection identity out of
+      // shared state, so concurrent sibling injects can't race on it.
+      String tracestate =
+          propagationTags.headerValue(W3C, DDSpanId.toHexStringPadded(context.getSpanId()));
       if (tracestate != null && !tracestate.isEmpty()) {
         setter.set(carrier, TRACE_STATE_KEY, tracestate);
       }

@@ -25,14 +25,18 @@ abstract class PTagsCodec {
   protected static final TagKey UPSTREAM_SERVICES_DEPRECATED_TAG = TagKey.from("upstream_services");
 
   static String headerValue(PTagsCodec codec, PTags ptags) {
+    return headerValue(codec, ptags, null);
+  }
+
+  static String headerValue(PTagsCodec codec, PTags ptags, CharSequence lastParentIdOverride) {
     int estimate = codec.estimateHeaderSize(ptags);
     if (estimate == 0) {
       return "";
     }
 
     // No encoding validation here because we don't allow arbitrary tag change
     StringBuilder sb = new StringBuilder(estimate);
-    int size = codec.appendPrefix(sb, ptags);
+    int size = codec.appendPrefix(sb, ptags, lastParentIdOverride);
     if (!ptags.isPropagationTagsDisabled()) {
       if (ptags.getDecisionMakerTagValue() != null) {
         size = codec.appendTag(sb, DECISION_MAKER_TAG, ptags.getDecisionMakerTagValue(), size);
@@ -174,6 +178,14 @@ static int calcXDatadogTagsSize(int size, TagKey tagKey, TagValue tagValue) {
 
   protected abstract int appendPrefix(StringBuilder sb, PTags ptags);
 
+  /**
+   * Encode the prefix, using {@code lastParentIdOverride} for the W3C {@code p:} when non-null
+   * (inject-time). Codecs without a last-parent-id (e.g. Datadog) ignore the override.
+   */
+  protected int appendPrefix(StringBuilder sb, PTags ptags, CharSequence lastParentIdOverride) {
+    return appendPrefix(sb, ptags);
+  }
+
   protected abstract int appendTag(StringBuilder sb, TagElement key, TagElement value, int size);
 
   protected abstract int appendSuffix(StringBuilder sb, PTags ptags, int size);

@@ -420,14 +420,6 @@ public CharSequence getLastParentId() {
       return lastParentId;
     }
 
-    @Override
-    public void updateLastParentId(CharSequence lastParentId) {
-      if (!Objects.equals(this.lastParentId, lastParentId)) {
-        clearCachedHeader(W3C);
-        this.lastParentId = TagValue.from(lastParentId);
-      }
-    }
-
     @Override
     @SuppressWarnings("StringEquality")
     @SuppressFBWarnings("ES_COMPARING_STRINGS_WITH_EQ")
@@ -448,6 +440,18 @@ public String headerValue(HeaderType headerType) {
       return header;
     }
 
+    @Override
+    public String headerValue(HeaderType headerType, CharSequence lastParentIdOverride) {
+      if (lastParentIdOverride == null) {
+        return headerValue(headerType);
+      }
+      // Inject-time path: encode fresh with the override; do NOT cache — the W3C `p:` is
+      // per-injecting-span and these tags may be shared across sibling spans.
+      String header =
+          PTagsCodec.headerValue(factory.getDecoderEncoder(headerType), this, lastParentIdOverride);
+      return (header == null || header.isEmpty()) ? null : header;
+    }
+
     @Override
     public void fillTagMap(Map<String, String> tagMap) {
       PTagsCodec.fillTagMap(this, tagMap);

@@ -226,6 +226,11 @@ protected int estimateHeaderSize(PTags pTags) {
 
   @Override
   protected int appendPrefix(StringBuilder sb, PTags ptags) {
+    return appendPrefix(sb, ptags, null);
+  }
+
+  @Override
+  protected int appendPrefix(StringBuilder sb, PTags ptags, CharSequence lastParentIdOverride) {
     sb.append(DATADOG_MEMBER_KEY);
     // Append sampling priority (s)
     if (ptags.getSamplingPriority() != PrioritySampling.UNSET) {
@@ -246,7 +251,8 @@ protected int appendPrefix(StringBuilder sb, PTags ptags) {
       }
     }
     // append last ParentId (p)
-    CharSequence lastParent = ptags.getLastParentId();
+    CharSequence lastParent =
+        lastParentIdOverride != null ? lastParentIdOverride : ptags.getLastParentId();
     if (lastParent != null) {
       if (sb.length() > EMPTY_SIZE) {
         sb.append(';');

@@ -0,0 +1,61 @@
+package datadog.trace.core.propagation;
+
+import static datadog.trace.core.propagation.PropagationTags.HeaderType.W3C;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import org.junit.jupiter.api.Test;
+
+/**
+ * The inject-time W3C last-parent-id ({@code p:}) is supplied as a parameter to {@link
+ * PropagationTags#headerValue(PropagationTags.HeaderType, CharSequence)} rather than mutated into
+ * the (possibly trace-level, shared) tags. This keeps transient per-injection identity out of
+ * shared state: a sibling span's inject can't pollute another span's header, and the stored inbound
+ * last-parent-id is never overwritten.
+ */
+class PropagationTagsLastParentIdTest {
+
+  private static final String SPAN_A = "00000000000000aa";
+  private static final String SPAN_B = "00000000000000bb";
+
+  private static PropagationTags w3c(String header) {
+    return PropagationTags.factory().fromHeaderValue(W3C, header);
+  }
+
+  @Test
+  void overrideSuppliesW3cLastParentId() {
+    PropagationTags tags = w3c("dd=s:1;o:rum");
+    assertTrue(tags.headerValue(W3C, SPAN_A).contains("p:" + SPAN_A));
+  }
+
+  @Test
+  void overrideDoesNotMutateSharedTags_noCrossTalk() {
+    // One tags instance, two sibling spans injecting through it (the shared-root scenario).
+    PropagationTags shared = w3c("dd=s:1;o:rum"); // no inbound p:
+
+    String headerA = shared.headerValue(W3C, SPAN_A);
+    String headerB = shared.headerValue(W3C, SPAN_B);
+    String headerAagain = shared.headerValue(W3C, SPAN_A);
+
+    assertTrue(headerA.contains("p:" + SPAN_A));
+    assertTrue(headerB.contains("p:" + SPAN_B));
+    // Injecting B did not change what A injects — no shared mutation.
+    assertEquals(headerA, headerAagain, "a sibling inject must not change another span's header");
+    // The override is never written into the shared tags (no-override header has no p:).
+    assertFalse(shared.headerValue(W3C).contains("p:"), "override must not mutate the stored tags");
+  }
+
+  @Test
+  void inboundLastParentIdPreservedAndUnmutatedByOverride() {
+    PropagationTags tags = w3c("dd=s:1;p:" + SPAN_A); // arrived carrying a last-parent-id
+
+    // No-override path (e.g. span-link traceState) keeps the inbound p:.
+    assertTrue(tags.headerValue(W3C).contains("p:" + SPAN_A));
+    // An inject override replaces it for that produced header...
+    assertTrue(tags.headerValue(W3C, SPAN_B).contains("p:" + SPAN_B));
+    // ...without mutating the stored inbound value.
+    assertTrue(
+        tags.headerValue(W3C).contains("p:" + SPAN_A), "inbound p: must survive override use");
+  }
+}