disable multiplexing on VoidCoder

runners/google-cloud-dataflow-java/src/main/java/org/apache/beam/runners/dataflow/DataflowRunner.java

@@ -119,7 +119,6 @@
 import org.apache.beam.sdk.transforms.Combine.GroupedValues;
 import org.apache.beam.sdk.transforms.Create;
 import org.apache.beam.sdk.transforms.DoFn;
-import org.apache.beam.sdk.transforms.GroupByKey;
 import org.apache.beam.sdk.transforms.GroupIntoBatches;
 import org.apache.beam.sdk.transforms.Impulse;
 import org.apache.beam.sdk.transforms.PTransform;
@@ -805,7 +804,7 @@ private List<PTransformOverride> getOverrides(boolean streaming) {
           options, StateMultiplexingGroupByKey.EXPERIMENT_ENABLE_GBK_STATE_MULTIPLEXING)) {
         overridesBuilder.add(
             PTransformOverride.of(
-                PTransformMatchers.classEqualTo(GroupByKey.class),
+                StateMultiplexingGroupByKeyTransformMatcher.getInstance(),
                 new StateMultiplexingGroupByKeyOverrideFactory<>(options)));
       }
       // For update compatibility, always use a Read for Create in streaming mode.

runners/google-cloud-dataflow-java/src/main/java/org/apache/beam/runners/dataflow/StateMultiplexingGroupByKeyTransformMatcher.java

@@ -0,0 +1,62 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.beam.runners.dataflow;
+
+import org.apache.beam.sdk.coders.Coder;
+import org.apache.beam.sdk.coders.KvCoder;
+import org.apache.beam.sdk.coders.VoidCoder;
+import org.apache.beam.sdk.runners.AppliedPTransform;
+import org.apache.beam.sdk.runners.PTransformMatcher;
+import org.apache.beam.sdk.transforms.GroupByKey;
+import org.apache.beam.sdk.values.PCollection;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+class StateMultiplexingGroupByKeyTransformMatcher implements PTransformMatcher {
+
+  private static final Logger LOG =
+      LoggerFactory.getLogger(StateMultiplexingGroupByKeyTransformMatcher.class);
+  private static final StateMultiplexingGroupByKeyTransformMatcher INSTANCE =
+      new StateMultiplexingGroupByKeyTransformMatcher();
+
+  @Override
+  public boolean matches(AppliedPTransform<?, ?, ?> application) {
+    LOG.info(application.getFullName());
+    if (!(application.getTransform() instanceof GroupByKey)) {
+      LOG.info(application.getFullName() + " returning false");
+      return false;
+    }
+    for (PCollection<?> pCollection : application.getMainInputs().values()) {
+      LOG.info(application.getFullName() + " " + pCollection.getCoder());
+      Coder<?> coder = pCollection.getCoder();
+      if (!(coder instanceof KvCoder)) {
+        return false;
+      }
+      // Don't enable multiplexing on Void keys
+      if (((KvCoder<?, ?>) coder).getKeyCoder() instanceof VoidCoder) {
+        return false;
+      }
+    }
+    LOG.info(application.getFullName() + " returning true");
+    return true;
+  }
+
+  public static StateMultiplexingGroupByKeyTransformMatcher getInstance() {
+    return INSTANCE;
+  }
+}

runners/google-cloud-dataflow-java/src/main/java/org/apache/beam/runners/dataflow/internal/StateMultiplexingGroupByKey.java

@@ -201,6 +201,7 @@ public KV<K, Iterable<V>> apply(KV<ByteString, Iterable<V>> kv) {
                     new SimpleFunction<KV<ByteString, V>, KV<Integer, V>>() {
                       @Override
                       public KV<Integer, V> apply(KV<ByteString, V> value) {
+                        // should we use a different hash code?
                         return KV.of(value.getKey().hashCode() % numVirtualKeys, value.getValue());
                       }
                     }))