Add SolaceCheckpointMark for handling Solace checkpointing

sdks/java/io/solace/build.gradle

@@ -34,4 +34,5 @@ dependencies {
     implementation project(path: ":sdks:java:core", configuration: "shadow")
     implementation library.java.joda_time
     implementation library.java.solace
+    implementation project(":sdks:java:extensions:avro")
 }

sdks/java/io/solace/src/main/java/org/apache/beam/sdk/io/solace/read/SolaceCheckpointMark.java

@@ -0,0 +1,94 @@
+/*
+ * 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.sdk.io.solace.read;
+
+import com.solacesystems.jcsmp.BytesXMLMessage;
+import java.util.ArrayDeque;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Objects;
+import java.util.concurrent.atomic.AtomicBoolean;
+import org.apache.beam.sdk.coders.DefaultCoder;
+import org.apache.beam.sdk.extensions.avro.coders.AvroCoder;
+import org.apache.beam.sdk.io.UnboundedSource;
+import org.checkerframework.checker.nullness.qual.Nullable;
+
+/**
+ * Checkpoint for an unbounded Solace source. Consists of the Solace messages waiting to be
+ * acknowledged.
+ */
+@DefaultCoder(AvroCoder.class)
+class SolaceCheckpointMark implements UnboundedSource.CheckpointMark {
+  private transient AtomicBoolean activeReader;
+  // BytesXMLMessage is not serializable so if a job restarts from the checkpoint, we cannot retry
+  // these messages here. We relay on Solace's retry mechanism.
+  private transient ArrayDeque<BytesXMLMessage> ackQueue;
+
+  @SuppressWarnings("initialization") // Avro will set the fields by breaking abstraction
+  private SolaceCheckpointMark() {}
+
+  /**
+   * Creates a new {@link SolaceCheckpointMark}.
+   *
+   * @param activeReader {@link AtomicBoolean} indicating if the related reader is active. The
+   *     reader creating the messages has to be active to acknowledge the messages.
+   * @param ackQueue {@link List} of {@link BytesXMLMessage} to be acknowledged.
+   */
+  SolaceCheckpointMark(AtomicBoolean activeReader, List<BytesXMLMessage> ackQueue) {
+    this.activeReader = activeReader;
+    this.ackQueue = new ArrayDeque<>(ackQueue);
+  }
+
+  @Override
+  public void finalizeCheckpoint() {
+    if (activeReader == null || !activeReader.get() || ackQueue == null) {
+      return;
+    }
+
+    while (!ackQueue.isEmpty()) {
+      BytesXMLMessage msg = ackQueue.poll();
+      if (msg != null) {
+        msg.ackMessage();
+      }
+    }
+  }
+
+  @Override
+  public boolean equals(@Nullable Object o) {
+    if (o == null) {
+      return false;
+    }
+    if (this == o) {
+      return true;
+    }
+    if (!(o instanceof SolaceCheckpointMark)) {
+      return false;
+    }
+    SolaceCheckpointMark that = (SolaceCheckpointMark) o;
+    // Needed to convert to ArrayList because ArrayDeque.equals checks only for reference, not
+    // content.
+    ArrayList<BytesXMLMessage> ackList = new ArrayList<>(ackQueue);
+    ArrayList<BytesXMLMessage> thatAckList = new ArrayList<>(that.ackQueue);
+    return Objects.equals(activeReader, that.activeReader) && Objects.equals(ackList, thatAckList);
+  }
+
+  @Override
+  public int hashCode() {
+    return Objects.hash(activeReader, ackQueue);
+  }
+}