DataDog · amarziali · Jun 5, 2026
@@ -13,7 +13,6 @@
 import datadog.context.ContextScope;
 import datadog.trace.agent.tooling.Instrumenter;
 import datadog.trace.bootstrap.InstrumentationContext;
-import datadog.trace.bootstrap.instrumentation.api.Java8BytecodeBridge;
 import net.bytebuddy.asm.Advice;
 import net.bytebuddy.description.type.TypeDescription;
 import net.bytebuddy.matcher.ElementMatcher;
@@ -53,21 +52,11 @@ public static class PublisherSubscribeAdvice {
     @Advice.OnMethodEnter(suppress = Throwable.class)
     public static ContextScope onSubscribe(
         @Advice.This final Publisher self, @Advice.Argument(value = 0) final Subscriber s) {
-
-      final Context context =
-          InstrumentationContext.get(Publisher.class, Context.class).remove(self);
-      final Context activeContext = Java8BytecodeBridge.getCurrentContext();
-      if (s == null || (context == null && activeContext == null)) {
-        return null;
-      }
-      final Context current =
-          InstrumentationContext.get(Subscriber.class, Context.class)
-              .putIfAbsent(s, context != null ? context : activeContext);
-      if (current != null && activeContext != current) {
-        return current.attach();
-      }
-
-      return null;
+      return ReactiveStreamsContextPropagation.captureOnSubscribe(
+          self,
+          s,
+          InstrumentationContext.get(Publisher.class, Context.class),
+          InstrumentationContext.get(Subscriber.class, Context.class));
     }
 
     @Advice.OnMethodExit(suppress = Throwable.class, onThrowable = Throwable.class)

@@ -0,0 +1,66 @@
+package datadog.trace.instrumentation.reactivestreams;
+
+import datadog.context.Context;
+import datadog.context.ContextScope;
+import datadog.trace.bootstrap.ContextStore;
+import datadog.trace.bootstrap.instrumentation.api.Java8BytecodeBridge;
+import org.reactivestreams.Publisher;
+import org.reactivestreams.Subscriber;
+
+public final class ReactiveStreamsContextPropagation {
+
+  private ReactiveStreamsContextPropagation() {}
+
+  public static ContextScope captureOnSubscribe(
+      final Publisher<?> publisher,
+      final Subscriber<?> subscriber,
+      final ContextStore<Publisher, Context> publisherContexts,
+      final ContextStore<Subscriber, Context> subscriberContexts) {
+    if (subscriber == null) {
+      return null;
+    }
+
+    final Context contextFromPublisher = publisherContexts.remove(publisher);
+    final Context activeContext = Java8BytecodeBridge.getCurrentContext();
+    final Context context =
+        contextFromPublisher != null ? contextFromPublisher : nonRootContext(activeContext);
+    if (context == null) {
+      return null;
+    }
+
+    final Context subscriberContext = subscriberContexts.putIfAbsent(subscriber, context);
+    // A context captured on the publisher (cross-thread propagation) must win even when the
+    // current thread already carries a non-root active context
+    return attachIfRequired(subscriberContext, activeContext, true);
+  }
+
+  public static ContextScope activateOnSignal(
+      final Subscriber<?> subscriber, final ContextStore<Subscriber, Context> subscriberContexts) {
+    final Context activeContext = Java8BytecodeBridge.getCurrentContext();
+    if (nonRootContext(activeContext) != null) {
+      return null;
+    }
+    return attachIfRequired(subscriberContexts.get(subscriber), activeContext, false);
+  }
+
+  public static ContextScope activateOnComplete(
+      final Subscriber<?> subscriber, final ContextStore<Subscriber, Context> subscriberContexts) {
+    return attachIfRequired(
+        subscriberContexts.get(subscriber), Java8BytecodeBridge.getCurrentContext(), true);
+  }
+
+  private static ContextScope attachIfRequired(
+      final Context context, final Context activeContext, final boolean allowReplacingActive) {
+    if (nonRootContext(context) == null || context == activeContext) {
+      return null;
+    }
+    if (!allowReplacingActive && nonRootContext(activeContext) != null) {
+      return null;
+    }
+    return context.attach();
+  }
+
+  private static Context nonRootContext(final Context context) {
+    return context == null || context == Java8BytecodeBridge.getRootContext() ? null : context;
+  }
+}
@@ -19,6 +19,7 @@ public ReactiveStreamsModule() {
   @Override
   public String[] helperClassNames() {
     return new String[] {
+      packageName + ".ReactiveStreamsContextPropagation",
       packageName + ".ReactiveStreamsAsyncResultExtension",
       packageName + ".ReactiveStreamsAsyncResultExtension$WrappedPublisher",
       packageName + ".ReactiveStreamsAsyncResultExtension$WrappedSubscriber",

@@ -10,7 +10,6 @@
 import datadog.trace.agent.tooling.bytebuddy.matcher.HierarchyMatchers;
 import datadog.trace.agent.tooling.bytebuddy.matcher.NameMatchers;
 import datadog.trace.bootstrap.InstrumentationContext;
-import datadog.trace.bootstrap.instrumentation.api.Java8BytecodeBridge;
 import net.bytebuddy.asm.Advice;
 import net.bytebuddy.description.type.TypeDescription;
 import net.bytebuddy.matcher.ElementMatcher;
@@ -51,12 +50,8 @@ public ElementMatcher<TypeDescription> hierarchyMatcher() {
   public static class SubscriberDownStreamAdvice {
     @Advice.OnMethodEnter(suppress = Throwable.class)
     public static ContextScope before(@Advice.This final Subscriber self) {
-      final Context currentContext = Java8BytecodeBridge.getCurrentContext();
-      if (currentContext != null && currentContext != Java8BytecodeBridge.getRootContext()) {
-        return null;
-      }
-      final Context context = InstrumentationContext.get(Subscriber.class, Context.class).get(self);
-      return context == null || context == currentContext ? null : context.attach();
+      return ReactiveStreamsContextPropagation.activateOnSignal(
+          self, InstrumentationContext.get(Subscriber.class, Context.class));
     }
 
     @Advice.OnMethodExit(onThrowable = Throwable.class, suppress = Throwable.class)
@@ -75,8 +70,8 @@ public static void closeScope(@Advice.Enter final ContextScope scope) {
   public static class SubscriberCompleteAdvice {
     @Advice.OnMethodEnter(suppress = Throwable.class)
     public static ContextScope before(@Advice.This final Subscriber self) {
-      final Context context = InstrumentationContext.get(Subscriber.class, Context.class).get(self);
-      return context == null ? null : context.attach();
+      return ReactiveStreamsContextPropagation.activateOnComplete(
+          self, InstrumentationContext.get(Subscriber.class, Context.class));
     }
 
     @Advice.OnMethodExit(onThrowable = Throwable.class, suppress = Throwable.class)

@@ -0,0 +1,158 @@
+package datadog.trace.instrumentation.reactivestreams;
+
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertSame;
+
+import datadog.context.Context;
+import datadog.context.ContextKey;
+import datadog.context.ContextScope;
+import datadog.trace.bootstrap.ContextStore;
+import java.util.IdentityHashMap;
+import java.util.Map;
+import org.junit.jupiter.api.Test;
+import org.reactivestreams.Publisher;
+import org.reactivestreams.Subscriber;
+import org.reactivestreams.Subscription;
+
+class ReactiveStreamsContextPropagationTest {
+
+  private static final ContextKey<String> KEY = ContextKey.named("reactive-streams-test");
+
+  @Test
+  void publisherCapturedContextOverridesActiveContext() {
+    final Publisher<Object> publisher = subscriber -> {};
+    final Subscriber<Object> subscriber = new NoopSubscriber();
+    final ContextStore<Publisher, Context> publisherContexts = new MapContextStore<>();
+    final ContextStore<Subscriber, Context> subscriberContexts = new MapContextStore<>();
+
+    // A context was captured on the publisher (e.g. at assembly / cross-thread subscribe).
+    final Context captured = Context.root().with(KEY, "captured");
+    publisherContexts.put(publisher, captured);
+
+    // The current thread already carries a different, non-root active context.
+    final Context active = Context.root().with(KEY, "active");
+    try (ContextScope activeScope = active.attach()) {
+      assertSame(active, Context.current());
+
+      final ContextScope scope =
+          ReactiveStreamsContextPropagation.captureOnSubscribe(
+              publisher, subscriber, publisherContexts, subscriberContexts);
+      try {
+        // The captured context must win over the ambient active one
+        assertNotNull(scope, "captured context should be attached over the active context");
+        assertSame(captured, Context.current());
+      } finally {
+        if (scope != null) {
+          scope.close();
+        }
+      }
+
+      // Closing the scope restores the previously active context.
+      assertSame(active, Context.current());
+    }
+
+    // The captured context is remembered for the subscriber, and removed from the publisher store.
+    assertSame(captured, subscriberContexts.get(subscriber));
+    assertNull(publisherContexts.get(publisher));
+  }
+
+  @Test
+  void signalActivationIsSkippedWhenAnotherContextIsActive() {
+    final Subscriber<Object> subscriber = new NoopSubscriber();
+    final ContextStore<Subscriber, Context> subscriberContexts = new MapContextStore<>();
+    subscriberContexts.put(subscriber, Context.root().with(KEY, "stored"));
+
+    final Context active = Context.root().with(KEY, "active");
+    try (ContextScope activeScope = active.attach()) {
+      final ContextScope scope =
+          ReactiveStreamsContextPropagation.activateOnSignal(subscriber, subscriberContexts);
+      assertNull(scope, "must not override an already-active non-root context on a signal");
+      assertSame(active, Context.current());
+    }
+  }
+
+  @Test
+  void signalActivationAttachesStoredContextWhenIdle() {
+    final Subscriber<Object> subscriber = new NoopSubscriber();
+    final ContextStore<Subscriber, Context> subscriberContexts = new MapContextStore<>();
+    final Context stored = Context.root().with(KEY, "stored");
+    subscriberContexts.put(subscriber, stored);
+
+    final ContextScope scope =
+        ReactiveStreamsContextPropagation.activateOnSignal(subscriber, subscriberContexts);
+    try {
+      assertNotNull(scope);
+      assertSame(stored, Context.current());
+    } finally {
+      if (scope != null) {
+        scope.close();
+      }
+    }
+  }
+
+  private static final class NoopSubscriber implements Subscriber<Object> {
+    @Override
+    public void onSubscribe(final Subscription subscription) {}
+
+    @Override
+    public void onNext(final Object value) {}
+
+    @Override
+    public void onError(final Throwable throwable) {}
+
+    @Override
+    public void onComplete() {}
+  }
+
+  private static final class MapContextStore<K, C> implements ContextStore<K, C> {
+    private final Map<K, C> map = new IdentityHashMap<>();
+
+    @Override
+    public C get(final K key) {
+      return map.get(key);
+    }
+
+    @Override
+    public void put(final K key, final C context) {
+      map.put(key, context);
+    }
+
+    @Override
+    public C putIfAbsent(final K key, final C context) {
+      final C existing = map.get(key);
+      if (existing != null) {
+        return existing;
+      }
+      map.put(key, context);
+      return context;
+    }
+
+    @Override
+    public C putIfAbsent(final K key, final Factory<C> contextFactory) {
+      final C existing = map.get(key);
+      if (existing != null) {
+        return existing;
+      }
+      final C created = contextFactory.create();
+      map.put(key, created);
+      return created;
+    }
+
+    @Override
+    public C computeIfAbsent(final K key, final KeyAwareFactory<? super K, C> contextFactory) {
+      final C existing = map.get(key);
+      if (existing != null) {
+        return existing;
+      }
+      final C created = contextFactory.create(key);
+      map.put(key, created);
+      return created;
+    }
+
+    @Override
+    public C remove(final K key) {
+      return map.remove(key);
+    }
+  }
+}
@@ -41,11 +41,8 @@ public void methodAdvice(MethodTransformer transformer) {
   public static class BlockingAdvice {
     @Advice.OnMethodEnter(suppress = Throwable.class)
     public static ContextScope before(@Advice.This final Publisher self) {
-      final Context context = InstrumentationContext.get(Publisher.class, Context.class).get(self);
-      if (context == null) {
-        return null;
-      }
-      return context.attach();
+      return ReactorContextBridge.activateForBlocking(
+          self, InstrumentationContext.get(Publisher.class, Context.class));
     }
 
     @Advice.OnMethodExit(suppress = Throwable.class, onThrowable = Throwable.class)