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Kafka on Pulsar implemented using Pulsar Protocol Handler

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/**
 * Copyright (c) 2019 - 2024 StreamNative, Inc.. All Rights Reserved.
 */
/**
 * Licensed 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 io.streamnative.pulsar.handlers.kop.coordinator;

import com.google.common.annotations.VisibleForTesting;
import java.util.Map;
import java.util.Optional;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Future;

public class FutureTaskCache {

    private final Map> futures = new ConcurrentHashMap<>();
    private final ExecutorService executor;

    public FutureTaskCache(ExecutorService executor) {
        this.executor = executor;
    }

    public Future submit(int key, Runnable task) {
        if (executor.isShutdown()) {
            return CompletableFuture.completedFuture(null);
        }
        synchronized (this) {
            // For the same operation, cancel the previous task to avoid taking useless time. This usually happens in
            // the following case:
            //   t1: a topic is loaded (scheduling a load task that will update the cache)
            //   t2: the topic is unloaded (cache for this topic is cleared)
            //   t3: the topic is loaded again (scheduling a load task that will update the cache)
            // Since t1 and t3 will eventually read the same latest data, cancelling the task of t1 could save the
            // unnecessary load operation.
            if (futures.containsKey(key)) {
                cancel(key);
            }
            final var future = executor.submit(task);
            futures.put(key, future);
            executor.execute(() -> futures.remove(key, future));
            return future;
        }
    }

    public void cancel(int key) {
        Optional.ofNullable(futures.remove(key)).ifPresent(__ -> __.cancel(true));
    }

    public void clear() {
        futures.keySet().forEach(this::cancel);
    }

    @VisibleForTesting
    boolean isEmpty() {
        return futures.isEmpty();
    }
}