Return noop continuation when not async propagating

Author: mcculls

dd-trace-core/src/main/java/datadog/trace/core/scopemanager/ContinuableScope.java

@@ -5,6 +5,7 @@
 import datadog.trace.api.scopemanager.ScopeListener;
 import datadog.trace.bootstrap.instrumentation.api.AgentScope;
 import datadog.trace.bootstrap.instrumentation.api.AgentSpan;
+import datadog.trace.bootstrap.instrumentation.api.AgentTracer;
 import datadog.trace.bootstrap.instrumentation.api.AttachableWrapper;
 import datadog.trace.bootstrap.instrumentation.api.ScopeSource;
 import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;
@@ -132,13 +133,14 @@ public final void setAsyncPropagation(final boolean value) {
   /**
    * The continuation returned must be closed or activated or the trace will not finish.
    *
-   * @return The new continuation, or null if this scope is not async propagating.
+   * @return The new continuation, or {@link AgentTracer#noopContinuation()} if this scope is not
+   *     async propagating.
    */
   @Override
-  public final ScopeContinuation capture() {
+  public final Continuation capture() {
     return isAsyncPropagating
         ? new ScopeContinuation(scopeManager, span, source()).register()
-        : null;
+        : AgentTracer.noopContinuation();
   }
 
   @Override

internal-api/src/main/java/datadog/trace/bootstrap/instrumentation/api/AgentTracer.java

@@ -203,6 +203,18 @@ public static AgentScope noopScope() {
     return NoopScope.INSTANCE;
   }
 
+  /**
+   * Returns the noop continuation instance.
+   *
+   * <p>This instance will always be the same, and can be safely tested using object identity (ie
+   * {@code ==}).
+   *
+   * @return the noop continuation instance.
+   */
+  public static AgentScope.Continuation noopContinuation() {
+    return NoopContinuation.INSTANCE;
+  }
+
   public static final TracerAPI NOOP_TRACER = new NoopTracerAPI();
 
   private static volatile TracerAPI provider = NOOP_TRACER;
@@ -619,28 +631,6 @@ public <C> AgentSpanContext.Extracted extract(final C carrier, final ContextVisi
     }
   }
 
-  static class NoopContinuation implements AgentScope.Continuation {
-    static final NoopContinuation INSTANCE = new NoopContinuation();
-
-    @Override
-    public AgentScope.Continuation hold() {
-      return this;
-    }
-
-    @Override
-    public AgentScope activate() {
-      return NoopScope.INSTANCE;
-    }
-
-    @Override
-    public AgentSpan getSpan() {
-      return NoopSpan.INSTANCE;
-    }
-
-    @Override
-    public void cancel() {}
-  }
-
   public static class NoopAgentTraceCollector implements AgentTraceCollector {
     public static final NoopAgentTraceCollector INSTANCE = new NoopAgentTraceCollector();
 

internal-api/src/main/java/datadog/trace/bootstrap/instrumentation/api/NoopContinuation.java

@@ -0,0 +1,25 @@
+package datadog.trace.bootstrap.instrumentation.api;
+
+final class NoopContinuation implements AgentScope.Continuation {
+  static final NoopContinuation INSTANCE = new NoopContinuation();
+
+  private NoopContinuation() {}
+
+  @Override
+  public AgentScope.Continuation hold() {
+    return this;
+  }
+
+  @Override
+  public AgentScope activate() {
+    return NoopScope.INSTANCE;
+  }
+
+  @Override
+  public AgentSpan getSpan() {
+    return NoopSpan.INSTANCE;
+  }
+
+  @Override
+  public void cancel() {}
+}

internal-api/src/main/java/datadog/trace/bootstrap/instrumentation/api/NoopScope.java

@@ -17,7 +17,7 @@ public byte source() {
 
   @Override
   public Continuation capture() {
-    return AgentTracer.NoopContinuation.INSTANCE;
+    return NoopContinuation.INSTANCE;
   }
 
   @Override