Fix doFinalReduction to remove keys when post-barrier steps produce nothing

CHANGELOG.asciidoc

@@ -31,6 +31,7 @@ This release also includes changes from prior 3.7.x releases.
 * Bumped to Groovy 4.0.32 which adds support for parsing Java 25 bytecode.
 * Bumped Hadoop to 3.4.3 (and Kerby to 2.0.3) to enable `hadoop-gremlin` to build and run on Java 25.
 * Add missing `Configuring` interface to `GraphStepPlaceholder` and `VertexStepPlaceholder`
+* Fixed bug in `group()` value traversal where keys were retained with stale barrier state instead of being filtered when steps following a `Barrier` in the second `by()` produced no output (e.g. `by(values("age").fold().unfold())` or `by(__.out().fold().count(local).is(P.gt(0)))` for vertices with no out-edges).
 
 [[release-3-8-1]]
 === TinkerPop 3.8.1 (Release Date: April 1, 2026)

gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/Grouping.java

@@ -31,6 +31,7 @@
 import org.apache.tinkerpop.gremlin.process.traversal.step.sideEffect.GroupSideEffectStep;
 import org.apache.tinkerpop.gremlin.process.traversal.step.util.ProfileStep;
 
+import java.util.ArrayList;
 import java.util.List;
 import java.util.Map;
 
@@ -106,12 +107,17 @@ public default Traversal.Admin<S, V> convertValueTraversal(final Traversal.Admin
     public default Map<K, V> doFinalReduction(final Map<K, Object> map, final Traversal.Admin<S, V> valueTraversal) {
         final Barrier barrierStep = determineBarrierStep(valueTraversal);
         if (barrierStep != null) {
+            final List<K> keysToRemove = new ArrayList<>();
             for (final K key : map.keySet()) {
                 valueTraversal.reset();
                 barrierStep.addBarrier(map.get(key));
-                if (valueTraversal.hasNext())
+                if (valueTraversal.hasNext()) {
                     map.put(key, valueTraversal.next());
+                } else {
+                    keysToRemove.add(key);
+                }
             }
+            keysToRemove.forEach(map::remove);
         }
         return (Map<K, V>) map;
     }

gremlin-test/src/main/resources/org/apache/tinkerpop/gremlin/test/features/sideEffect/Group.feature

@@ -259,6 +259,28 @@ Feature: Step - group()
       | result |
       | m[{"v[peter]":"v[lop]"}] |
 
+  Scenario: g_V_group_byXvaluesXnameXX_byXvaluesXageX_fold_unfoldX
+    Given the modern graph
+    And the traversal of
+      """
+      g.V().group().by(values("name")).by(values("age").fold().unfold())
+      """
+    When iterated to list
+    Then the result should be unordered
+      | result |
+      | m[{"marko":"d[29].i", "vadas":"d[27].i", "josh":"d[32].i", "peter":"d[35].i"}] |
+
+  Scenario: g_V_group_byXvaluesXnameXX_byXout_fold_countXlocalX_isXgtX0XXX
+    Given the modern graph
+    And the traversal of
+      """
+      g.V().group().by(values("name")).by(__.out().fold().count(local).is(P.gt(0)))
+      """
+    When iterated to list
+    Then the result should be unordered
+      | result |
+      | m[{"marko":"d[3].l", "josh":"d[2].l", "peter":"d[1].l"}] |
+
   Scenario: g_V_hasXperson_name_withinXvadas_peterXX_group_by_byXout_order_countX
     Given the modern graph
     And the traversal of