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Access to the Neo4j graph database through Java

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/*
 * Copyright (c) "Neo4j"
 * Neo4j Sweden AB [http://neo4j.com]
 *
 * This file is part of Neo4j.
 *
 * 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 org.neo4j.driver.internal.async;

import static org.neo4j.driver.internal.util.Futures.completedWithNull;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Objects;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.CompletionStage;
import org.neo4j.driver.internal.FailableCursor;

public class ResultCursorsHolder {
    private final List> cursorStages =
            Collections.synchronizedList(new ArrayList<>());

    public void add(CompletionStage cursorStage) {
        Objects.requireNonNull(cursorStage);
        cursorStages.add(cursorStage);
    }

    CompletionStage retrieveNotConsumedError() {
        CompletableFuture[] failures = retrieveAllFailures();

        return CompletableFuture.allOf(failures).thenApply(ignore -> findFirstFailure(failures));
    }

    @SuppressWarnings("unchecked")
    private CompletableFuture[] retrieveAllFailures() {
        return cursorStages.stream()
                .map(ResultCursorsHolder::retrieveFailure)
                .map(CompletionStage::toCompletableFuture)
                .toArray(CompletableFuture[]::new);
    }

    private static Throwable findFirstFailure(CompletableFuture[] completedFailureFutures) {
        // all given futures should be completed, it is thus safe to get their values

        for (CompletableFuture failureFuture : completedFailureFutures) {
            Throwable failure = failureFuture.getNow(null); // does not block
            if (failure != null) {
                return failure;
            }
        }
        return null;
    }

    private static CompletionStage retrieveFailure(CompletionStage cursorStage) {
        return cursorStage
                .exceptionally(cursor -> null)
                .thenCompose(cursor -> cursor == null ? completedWithNull() : cursor.discardAllFailureAsync());
    }
}