[BEAM-5600] Add unimplemented split API to Runner side SDF libraries.

This creates an unsupported API to the RemoteBundle and a default split handler that throws.

runners/java-fn-execution/src/main/java/org/apache/beam/runners/fnexecution/control/BundleSplitHandler.java

@@ -0,0 +1,46 @@
+/*
+ * 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.fnexecution.control;
+
+import org.apache.beam.model.fnexecution.v1.BeamFnApi.ProcessBundleSplitResponse;
+
+/**
+ * A handler which is invoked whenever an active bundle is split. The active bundle will continue to
+ * keep processing until it is complete.
+ *
+ * <p>The returned split response contains a description of work that has been performed containing
+ * a {@code primary} portion that the SDK is responsible for processing and a {@code residual} which
+ * the runner is responsible for scheduling for future processing. See <a
+ * href="https://s.apache.org/beam-breaking-fusion">breaking the fusion barrier</a> for further
+ * details.
+ */
+public interface BundleSplitHandler {
+  void split(ProcessBundleSplitResponse splitResponse);
+
+  /** Returns a bundle split handler that throws on any split response. */
+  static BundleSplitHandler unsupported() {
+    return new BundleSplitHandler() {
+      @Override
+      public void split(ProcessBundleSplitResponse splitResponse) {
+        throw new UnsupportedOperationException(
+            String.format(
+                "%s does not support splitting.", BundleSplitHandler.class.getSimpleName()));
+      }
+    };
+  };
+}

runners/java-fn-execution/src/main/java/org/apache/beam/runners/fnexecution/control/DefaultJobBundleFactory.java

@@ -399,6 +399,11 @@ public Map<String, FnDataReceiver> getInputReceivers() {
           return bundle.getInputReceivers();
         }
 
+        @Override
+        public void split(double fractionOfRemainder) {
+          bundle.split(fractionOfRemainder);
+        }
+
         @Override
         public void close() throws Exception {
           bundle.close();

runners/java-fn-execution/src/main/java/org/apache/beam/runners/fnexecution/control/RemoteBundle.java

@@ -38,6 +38,15 @@ public interface RemoteBundle extends AutoCloseable {
    */
   Map<String, FnDataReceiver> getInputReceivers();
 
+  /**
+   * Ask the remote bundle to split its current processing based upon its knowledge of remaining
+   * work. A fraction of 0, is equivalent to asking the SDK to checkpoint.
+   *
+   * <p>This method will return after the request has been issued. Any splits will be forwarded to
+   * the {@link BundleSplitHandler}.
+   */
+  void split(double fractionOfRemainder);
+
   /**
    * Closes this bundle. This causes the input {@link FnDataReceiver} to be closed (future calls to
    * that {@link FnDataReceiver} will throw an exception), and causes the {@link RemoteBundle} to