=promise Optimize toCompletableFuture implantation if it's already completed.

Author: He-Pin

subprojects/parseq/src/main/java/com/linkedin/parseq/promise/Promise.java

@@ -136,15 +136,23 @@ public interface Promise<P> {
    * @return CompletionStage
    */
   default CompletionStage<P> toCompletionStage() {
-    final CompletableFuture<P> future = new CompletableFuture<>();
-    addListener(p -> {
-      if (!p.isFailed()) {
-        future.complete(p.get());
-      }
-      else {
-        future.completeExceptionally(p.getError());
-      }
-    });
-    return future;
+    if (isDone()) {
+      return CompletableFuture.completedFuture(get());
+    } else if (isFailed()) {
+      final CompletableFuture<P> future = new CompletableFuture<>();
+      future.completeExceptionally(getError());
+      return future;
+    } else {
+        final CompletableFuture<P> future = new CompletableFuture<>();
+        addListener(p -> {
+            if (!p.isFailed()) {
+                future.complete(p.get());
+            }
+            else {
+                future.completeExceptionally(p.getError());
+            }
+        });
+        return future;
+    }
   }
 }