[Drain] Support extensible element metadata propagation in ReduceFnRunner

runners/core-java/build.gradle

@@ -42,6 +42,7 @@ dependencies {
   implementation project(path: ":model:pipeline", configuration: "shadow")
   implementation project(path: ":sdks:java:core", configuration: "shadow")
   implementation project(path: ":model:job-management", configuration: "shadow")
+  implementation project(path: ":model:fn-execution", configuration: "shadow")
   implementation library.java.vendored_guava_32_1_2_jre
   implementation library.java.joda_time
   implementation library.java.vendored_grpc_1_69_0

runners/core-java/src/main/java/org/apache/beam/runners/core/CombinedMetadata.java

@@ -0,0 +1,87 @@
+/*
+ * 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.core;
+
+import com.google.auto.value.AutoValue;
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.OutputStream;
+import org.apache.beam.model.fnexecution.v1.BeamFnApi;
+import org.apache.beam.sdk.coders.AtomicCoder;
+import org.apache.beam.sdk.values.CausedByDrain;
+
+/**
+ * Encapsulates metadata that propagates with elements in the pipeline.
+ *
+ * <p>This metadata is sent along with elements. It currently includes fields like {@link
+ * CausedByDrain}, and is designed to be extensible to support future metadata fields such as
+ * OpenTelemetry context or CDC (Change Data Capture) kind.
+ *
+ * <p>The purpose of this class is to group targeted metadata fields together. This makes it easier
+ * to define combination strategies (e.g., when accumulating state in {@code ReduceFnRunner}) when
+ * multiple elements are merged or grouped, without having to extend method signatures or state
+ * handling for every new metadata field.
+ */
+@AutoValue
+public abstract class CombinedMetadata {
+  public abstract CausedByDrain causedByDrain();
+
+  public static CombinedMetadata create(CausedByDrain causedByDrain) {
+    return new AutoValue_CombinedMetadata(causedByDrain);
+  }
+
+  public static CombinedMetadata createDefault() {
+    return create(CausedByDrain.NORMAL);
+  }
+
+  public static class Coder extends AtomicCoder<CombinedMetadata> {
+    private static final Coder INSTANCE = new Coder();
+
+    public static Coder of() {
+      return INSTANCE;
+    }
+
+    @Override
+    public void encode(CombinedMetadata value, OutputStream outStream) throws IOException {
+      BeamFnApi.Elements.ElementMetadata proto =
+          BeamFnApi.Elements.ElementMetadata.newBuilder()
+              .setDrain(
+                  value.causedByDrain() == CausedByDrain.CAUSED_BY_DRAIN
+                      ? BeamFnApi.Elements.DrainMode.Enum.DRAINING
+                      : BeamFnApi.Elements.DrainMode.Enum.NOT_DRAINING)
+              .build();
+      proto.writeDelimitedTo(outStream);
+    }
+
+    @Override
+    public CombinedMetadata decode(InputStream inStream) throws IOException {
+      BeamFnApi.Elements.ElementMetadata proto =
+          BeamFnApi.Elements.ElementMetadata.parseDelimitedFrom(inStream);
+      if (proto == null) {
+        return CombinedMetadata.createDefault();
+      }
+
+      CausedByDrain causedByDrain =
+          proto.getDrain() == BeamFnApi.Elements.DrainMode.Enum.DRAINING
+              ? CausedByDrain.CAUSED_BY_DRAIN
+              : CausedByDrain.NORMAL;
+
+      return CombinedMetadata.create(causedByDrain);
+    }
+  }
+}

runners/core-java/src/main/java/org/apache/beam/runners/core/CombinedMetadataCombiner.java

@@ -0,0 +1,78 @@
+/*
+ * 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.core;
+
+import org.apache.beam.sdk.transforms.Combine.CombineFn;
+import org.apache.beam.sdk.values.CausedByDrain;
+
+/** Combiner for CombinedMetadata. */
+class CombinedMetadataCombiner
+    extends CombineFn<CombinedMetadata, CombinedMetadata, CombinedMetadata> {
+  private static final CombinedMetadataCombiner INSTANCE = new CombinedMetadataCombiner();
+
+  public static CombinedMetadataCombiner of() {
+    return INSTANCE;
+  }
+
+  @Override
+  public CombinedMetadata createAccumulator() {
+    return CombinedMetadata.create(CausedByDrainCombiner.of().createAccumulator());
+  }
+
+  @Override
+  public CombinedMetadata addInput(CombinedMetadata accumulator, CombinedMetadata input) {
+    return CombinedMetadata.create(
+        CausedByDrainCombiner.of().addInput(accumulator.causedByDrain(), input.causedByDrain()));
+  }
+
+  @Override
+  public CombinedMetadata mergeAccumulators(Iterable<CombinedMetadata> accumulators) {
+    CombinedMetadata result = createAccumulator();
+    for (CombinedMetadata accum : accumulators) {
+      result = addInput(result, accum);
+    }
+    return result;
+  }
+
+  @Override
+  public CombinedMetadata extractOutput(CombinedMetadata accumulator) {
+    return accumulator;
+  }
+
+  /** Combiner for CausedByDrain metadata. */
+  static class CausedByDrainCombiner implements MetadataCombiner<CausedByDrain> {
+    private static final CausedByDrainCombiner INSTANCE = new CausedByDrainCombiner();
+
+    public static CausedByDrainCombiner of() {
+      return INSTANCE;
+    }
+
+    @Override
+    public CausedByDrain createAccumulator() {
+      return CausedByDrain.NORMAL;
+    }
+
+    @Override
+    public CausedByDrain addInput(CausedByDrain current, CausedByDrain input) {
+      if (current == CausedByDrain.CAUSED_BY_DRAIN || input == CausedByDrain.CAUSED_BY_DRAIN) {
+        return CausedByDrain.CAUSED_BY_DRAIN;
+      }
+      return CausedByDrain.NORMAL;
+    }
+  }
+}

runners/core-java/src/main/java/org/apache/beam/runners/core/LateDataDroppingDoFnRunner.java

@@ -145,7 +145,13 @@ public <K, InputT> Iterable<WindowedValue<InputT>> filter(
           } else {
             nonLateElements.add(
                 WindowedValues.of(
-                    element.getValue(), element.getTimestamp(), window, element.getPaneInfo()));
+                    element.getValue(),
+                    element.getTimestamp(),
+                    window,
+                    element.getPaneInfo(),
+                    element.getRecordId(),
+                    element.getRecordOffset(),
+                    element.causedByDrain()));
           }
         }
       }

runners/core-java/src/main/java/org/apache/beam/runners/core/MetadataCombiner.java

@@ -0,0 +1,25 @@
+/*
+ * 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.core;
+
+/** Interface for combining pipeline metadata. */
+interface MetadataCombiner<T> {
+  T createAccumulator();
+
+  T addInput(T accumulator, T input);
+}