Allow contexts to wrap themselves as scopes without attaching.

This helps reduce allocations when attaching the same context multiple times.

Author: mcculls

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

@@ -168,4 +168,13 @@ default Context with(@Nullable ImplicitContextKeyed value) {
     }
     return value.storeInto(this);
   }
+
+  /**
+   * Wraps context as a scope without attaching it to the current execution unit.
+   *
+   * @return a scope that has no effect on execution units.
+   */
+  default ContextScope asScope() {
+    return new NoopContextScope(this);
+  }
 }

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

@@ -5,7 +5,7 @@
 import javax.annotation.Nullable;
 
 /** {@link Context} containing no values. */
-final class EmptyContext implements Context {
+final class EmptyContext implements SelfScopedContext {
   static final Context INSTANCE = new EmptyContext();
 
   private EmptyContext() {}

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

@@ -8,7 +8,7 @@
 import javax.annotation.Nullable;
 
 /** {@link Context} containing many values. */
-final class IndexedContext implements Context {
+final class IndexedContext implements SelfScopedContext {
   final Object[] store;
 
   IndexedContext(Object[] store) {

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

@@ -2,12 +2,9 @@
 
 /** {@link ContextContinuation} that has no effect on execution units. */
 final class NoopContextContinuation implements ContextContinuation, ContextScope {
-  static final NoopContextContinuation ROOT_CONTINUATION =
-      new NoopContextContinuation(Context.root());
-
   private final Context context;
 
-  private NoopContextContinuation(Context context) {
+  NoopContextContinuation(Context context) {
     this.context = context;
   }
 

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

@@ -2,15 +2,9 @@
 
 /** {@link ContextScope} that has no effect on execution units. */
 final class NoopContextScope implements ContextScope {
-  static final NoopContextScope ROOT_SCOPE = new NoopContextScope(Context.root());
-
-  static NoopContextScope create(Context context) {
-    return context == Context.root() ? ROOT_SCOPE : new NoopContextScope(context);
-  }
-
   private final Context context;
 
-  private NoopContextScope(Context context) {
+  NoopContextScope(Context context) {
     this.context = context;
   }
 

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

@@ -0,0 +1,17 @@
+package datadog.context;
+
+/** Context that acts as its own unattached scope. */
+interface SelfScopedContext extends Context, ContextScope {
+  @Override
+  default ContextScope asScope() {
+    return this; // acts as no-op scope, avoiding allocation
+  }
+
+  @Override
+  default Context context() {
+    return this;
+  }
+
+  @Override
+  default void close() {}
+}

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

@@ -7,7 +7,7 @@
 import javax.annotation.Nullable;
 
 /** {@link Context} containing a single value. */
-final class SingletonContext implements Context {
+final class SingletonContext implements SelfScopedContext {
   final int index;
   final Object value;
 

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

@@ -8,6 +8,9 @@
 final class ThreadLocalContextManager implements ContextManager {
   static final ThreadLocalContextManager INSTANCE = new ThreadLocalContextManager();
 
+  private static final NoopContextContinuation ROOT_CONTINUATION =
+      new NoopContextContinuation(Context.root());
+
   private static final ThreadLocal<ContextHolder> CONTEXT_HOLDER =
       ThreadLocal.withInitial(ContextHolder::new);
 
@@ -34,11 +37,7 @@ ContextScope doAttach(Context context, @Nullable ContextContinuationImpl continu
         continuation.releaseOnScopeClose();
         return continuation; // acts as no-op scope, avoiding allocation
       }
-      NoopContextScope noopScope = holder.noopScope;
-      if (context != noopScope.context()) {
-        noopScope = holder.noopScope = NoopContextScope.create(context);
-      }
-      return noopScope;
+      return context.asScope(); // convert to scope without attaching
     }
 
     ContextListener[] ls = listeners;
@@ -73,7 +72,7 @@ public Context swap(Context context) {
   @Override
   public ContextContinuation capture(Context context) {
     if (context == Context.root()) {
-      return NoopContextContinuation.ROOT_CONTINUATION;
+      return ROOT_CONTINUATION;
     } else {
       return new ContextContinuationImpl(context);
     }
@@ -170,7 +169,6 @@ public void close() {
         ContextListener[] ls = INSTANCE.listeners;
         notifyDetach(context, ls);
         holder.current = previous;
-        holder.noopScope = NoopContextScope.ROOT_SCOPE;
         notifyAttach(previous, ls);
         closed = true;
       }
@@ -295,7 +293,5 @@ public void close() {}
 
   private static final class ContextHolder {
     Context current = Context.root();
-
-    NoopContextScope noopScope = NoopContextScope.ROOT_SCOPE;
   }
 }

components/context/src/test/java/datadog/context/ContextProvidersForkedTest.java

@@ -73,7 +73,7 @@ public Context current() {
 
           @Override
           public ContextScope attach(Context context) {
-            return NoopContextScope.ROOT_SCOPE;
+            return new NoopContextScope(root());
           }
 
           @Override
@@ -83,7 +83,7 @@ public Context swap(Context context) {
 
           @Override
           public ContextContinuation capture(Context context) {
-            return NoopContextContinuation.ROOT_CONTINUATION;
+            return new NoopContextContinuation(root());
           }
 
           @Override