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/*
 * Copyright (c) 2017 AsyncHttpClient Project. All rights reserved.
 *
 * This program is licensed to you under the Apache License Version 2.0,
 * and you may not use this file except in compliance with the Apache License Version 2.0.
 * You may obtain a copy of the Apache License Version 2.0 at
 *     http://www.apache.org/licenses/LICENSE-2.0.
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the Apache License Version 2.0 is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the Apache License Version 2.0 for the specific language governing permissions and limitations there under.
 */
package org.asynchttpclient.extras.rxjava2.maybe;

import static java.util.Objects.requireNonNull;
import io.netty.handler.codec.http.HttpHeaders;
import io.reactivex.MaybeEmitter;
import io.reactivex.exceptions.CompositeException;
import io.reactivex.exceptions.Exceptions;

import java.util.Arrays;
import java.util.concurrent.atomic.AtomicBoolean;

import org.asynchttpclient.AsyncHandler;
import org.asynchttpclient.HttpResponseBodyPart;
import org.asynchttpclient.HttpResponseStatus;
import org.asynchttpclient.extras.rxjava2.DisposedException;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * Abstract base class that bridges events between the {@code Maybe} reactive base type and {@code AsyncHandlers}.
 *
 * When an event is received, it's first checked if the Rx stream has been disposed asynchronously. If so, request
 * processing is {@linkplain #disposed() aborted}, otherwise, the event is forwarded to the {@linkplain #delegate()
 * wrapped handler}.
 *
 * When the request is {@link AsyncHandler#onCompleted() completed}, the result produced by the wrapped instance is
 * forwarded to the {@code Maybe}: If the result is {@code null}, {@link MaybeEmitter#onComplete()} is invoked,
 * {@link MaybeEmitter#onSuccess(Object)} otherwise.
 *
 * Any errors during request processing are forwarded via {@link MaybeEmitter#onError(Throwable)}.
 *
 * @param 
 *            the result type produced by the wrapped {@code AsyncHandler} and emitted via RxJava
 */
public abstract class AbstractMaybeAsyncHandlerBridge implements AsyncHandler {

    private static final Logger LOGGER = LoggerFactory.getLogger(AbstractMaybeAsyncHandlerBridge.class);

    private static volatile DisposedException sharedDisposed;

    /**
     * The Rx callback object that receives downstream events and will be queried for its
     * {@link MaybeEmitter#isDisposed() disposed state} when Async HTTP Client callbacks are invoked.
     */
    protected final MaybeEmitter emitter;

    /**
     * Indicates if the delegate has already received a terminal event.
     */
    private final AtomicBoolean delegateTerminated = new AtomicBoolean();

    protected AbstractMaybeAsyncHandlerBridge(MaybeEmitter emitter) {
        this.emitter = requireNonNull(emitter);
    }

    @Override
    public final State onBodyPartReceived(HttpResponseBodyPart content) throws Exception {
        return emitter.isDisposed() ? disposed() : delegate().onBodyPartReceived(content);
    }

    @Override
    public final State onStatusReceived(HttpResponseStatus status) throws Exception {
        return emitter.isDisposed() ? disposed() : delegate().onStatusReceived(status);
    }

    @Override
    public final State onHeadersReceived(HttpHeaders headers) throws Exception {
        return emitter.isDisposed() ? disposed() : delegate().onHeadersReceived(headers);
    }

    @Override
    public State onTrailingHeadersReceived(HttpHeaders headers) throws Exception {
        return emitter.isDisposed() ? disposed() : delegate().onTrailingHeadersReceived(headers);
    }

    /**
     * {@inheritDoc}
     *
     * 
     * The value returned by the wrapped {@code AsyncHandler} won't be returned by this method, but emtited via RxJava.
     * 
     *
     * @return always {@code null}
     */
    @Override
    public final Void onCompleted() {
        if (delegateTerminated.getAndSet(true)) {
            return null;
        }

        final T result;
        try {
            result = delegate().onCompleted();
        } catch (final Throwable t) {
            emitOnError(t);
            return null;
        }

        if (!emitter.isDisposed()) {
            if (result == null) {
                emitter.onComplete();
            } else {
                emitter.onSuccess(result);
            }
        }

        return null;
    }

    /**
     * {@inheritDoc}
     *
     * 
     * The exception will first be propagated to the wrapped {@code AsyncHandler}, then emitted via RxJava. If the
     * invocation of the delegate itself throws an exception, both the original exception and the follow-up exception
     * will be wrapped into RxJava's {@code CompositeException} and then be emitted.
     * 
     */
    @Override
    public final void onThrowable(Throwable t) {
        if (delegateTerminated.getAndSet(true)) {
            return;
        }

        Throwable error = t;
        try {
            delegate().onThrowable(t);
        } catch (final Throwable x) {
            error = new CompositeException(Arrays.asList(t, x));
        }

        emitOnError(error);
    }

    /**
     * Called to indicate that request processing is to be aborted because the linked Rx stream has been disposed. If
     * the {@link #delegate() delegate} didn't already receive a terminal event,
     * {@code AsyncHandler#onThrowable(Throwable) onThrowable} will be called with a {@link DisposedException}.
     *
     * @return always {@link State#ABORT}
     */
    protected final AsyncHandler.State disposed() {
        if (!delegateTerminated.getAndSet(true)) {

            DisposedException disposed = sharedDisposed;
            if (disposed == null) {
                disposed = new DisposedException("Subscription has been disposed.");
                final StackTraceElement[] stackTrace = disposed.getStackTrace();
                if (stackTrace.length > 0) {
                    disposed.setStackTrace(new StackTraceElement[] { stackTrace[0] });
                }

                sharedDisposed = disposed;
            }

            delegate().onThrowable(disposed);
        }

        return State.ABORT;
    }

    /**
     * @return the wrapped {@code AsyncHandler} instance to which calls are delegated
     */
    protected abstract AsyncHandler delegate();

    private void emitOnError(Throwable error) {
        Exceptions.throwIfFatal(error);
        if (!emitter.isDisposed()) {
            emitter.onError(error);
        } else {
            LOGGER.debug("Not propagating onError after disposal: {}", error.getMessage(), error);
        }
    }
}