Greatly reduce allocations by treating existing context containers as no-op scopes

Author: mcculls

components/context/src/main/java/datadog/context/NoopContextContinuation.java

@@ -1,8 +1,9 @@
 package datadog.context;
 
 /** {@link ContextContinuation} that has no effect on execution units. */
-final class NoopContextContinuation implements ContextContinuation {
-  static final ContextContinuation ROOT_CONTINUATION = new NoopContextContinuation(Context.root());
+final class NoopContextContinuation implements ContextContinuation, ContextScope {
+  static final NoopContextContinuation ROOT_CONTINUATION =
+      new NoopContextContinuation(Context.root());
 
   private final Context context;
 
@@ -22,9 +23,12 @@ public Context context() {
 
   @Override
   public ContextScope resume() {
-    return NoopContextScope.create(context);
+    return this; // acts as no-op scope, avoiding allocation
   }
 
   @Override
   public void release() {}
+
+  @Override
+  public void close() {}
 }

components/context/src/main/java/datadog/context/NoopContextScope.java

@@ -32,8 +32,9 @@ ContextScope doAttach(Context context, @Nullable ContextContinuationImpl continu
       if (continuation != null) {
         // already attached, safe to release early to avoid resource leak
         continuation.releaseOnScopeClose();
+        return continuation; // acts as no-op scope, avoiding allocation
       }
-      return NoopContextScope.create(previous);
+      return holder; // acts as no-op scope, avoiding allocation
     }
 
     ContextListener[] ls = listeners;
@@ -196,7 +197,7 @@ public void close() {
     }
   }
 
-  private static final class ContextContinuationImpl implements ContextContinuation {
+  private static final class ContextContinuationImpl implements ContextContinuation, ContextScope {
 
     private static final AtomicIntegerFieldUpdater<ContextContinuationImpl> COUNT =
         AtomicIntegerFieldUpdater.newUpdater(ContextContinuationImpl.class, "count");
@@ -253,7 +254,7 @@ public ContextScope resume() {
       } else {
         // continuation released or too many resumes; rollback count
         COUNT.decrementAndGet(this);
-        return NoopContextScope.create(context);
+        return this; // acts as no-op scope, avoiding allocation
       }
     }
 
@@ -284,9 +285,20 @@ void releaseOnScopeClose() {
         release();
       } /* else there are outstanding resumes or hold is in place */
     }
+
+    @Override
+    public void close() {}
   }
 
-  private static final class ContextHolder {
+  private static final class ContextHolder implements ContextScope {
     Context current = Context.root();
+
+    @Override
+    public Context context() {
+      return current;
+    }
+
+    @Override
+    public void close() {}
   }
 }

components/context/src/main/java/datadog/context/ThreadLocalContextManager.java

@@ -73,7 +73,7 @@ public Context current() {
 
           @Override
           public ContextScope attach(Context context) {
-            return NoopContextScope.ROOT_SCOPE;
+            return NoopContextContinuation.ROOT_CONTINUATION;
           }
 
           @Override