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/*
 * 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.flink.runtime.rest.handler;

import org.apache.flink.api.common.time.Time;
import org.apache.flink.runtime.concurrent.FutureUtils;
import org.apache.flink.runtime.rest.AbstractHandler;
import org.apache.flink.runtime.rest.handler.util.HandlerUtils;
import org.apache.flink.runtime.rest.messages.ErrorResponseBody;
import org.apache.flink.runtime.rest.messages.MessageHeaders;
import org.apache.flink.runtime.rest.messages.MessageParameters;
import org.apache.flink.runtime.rest.messages.RequestBody;
import org.apache.flink.runtime.rest.messages.ResponseBody;
import org.apache.flink.runtime.webmonitor.RestfulGateway;
import org.apache.flink.runtime.webmonitor.retriever.GatewayRetriever;
import org.apache.flink.util.ExceptionUtils;
import org.apache.flink.util.Preconditions;

import org.apache.flink.shaded.netty4.io.netty.channel.ChannelHandler;
import org.apache.flink.shaded.netty4.io.netty.channel.ChannelHandlerContext;
import org.apache.flink.shaded.netty4.io.netty.handler.codec.http.HttpRequest;
import org.apache.flink.shaded.netty4.io.netty.handler.codec.http.HttpResponseStatus;

import javax.annotation.Nonnull;

import java.util.Map;
import java.util.concurrent.CompletableFuture;

/**
 * Super class for netty-based handlers that work with {@link RequestBody}s and {@link ResponseBody}s.
 *
 * Subclasses must be thread-safe.
 *
 * @param  type of incoming requests
 * @param 
 type of outgoing responses
 */
@ChannelHandler.Sharable
public abstract class AbstractRestHandler extends AbstractHandler {

	private final MessageHeaders messageHeaders;

	protected AbstractRestHandler(
			CompletableFuture localRestAddress,
			GatewayRetriever leaderRetriever,
			Time timeout,
			Map responseHeaders,
			MessageHeaders messageHeaders) {
		super(localRestAddress, leaderRetriever, timeout, responseHeaders, messageHeaders);
		this.messageHeaders = Preconditions.checkNotNull(messageHeaders);
	}

	public MessageHeaders getMessageHeaders() {
		return messageHeaders;
	}

	@Override
	protected CompletableFuture respondToRequest(ChannelHandlerContext ctx, HttpRequest httpRequest, HandlerRequest handlerRequest, T gateway) {
		CompletableFuture
 response;

		try {
			response = handleRequest(handlerRequest, gateway);
		} catch (RestHandlerException e) {
			response = FutureUtils.completedExceptionally(e);
		}

		return response.whenComplete((P resp, Throwable throwable) -> {
			if (throwable != null) {

				Throwable error = ExceptionUtils.stripCompletionException(throwable);

				if (error instanceof RestHandlerException) {
					final RestHandlerException rhe = (RestHandlerException) error;

					processRestHandlerException(ctx, httpRequest, rhe);
				} else {
					log.error("Implementation error: Unhandled exception.", error);
					HandlerUtils.sendErrorResponse(
						ctx,
						httpRequest,
						new ErrorResponseBody("Internal server error."),
						HttpResponseStatus.INTERNAL_SERVER_ERROR,
						responseHeaders);
				}
			} else {
				HandlerUtils.sendResponse(
					ctx,
					httpRequest,
					resp,
					messageHeaders.getResponseStatusCode(),
					responseHeaders);
			}
		}).thenApply(ignored -> null);
	}

	private void processRestHandlerException(ChannelHandlerContext ctx, HttpRequest httpRequest, RestHandlerException rhe) {
		log.error("Exception occurred in REST handler.", rhe);

		HandlerUtils.sendErrorResponse(
			ctx,
			httpRequest,
			new ErrorResponseBody(rhe.getMessage()),
			rhe.getHttpResponseStatus(),
			responseHeaders);
	}

	/**
	 * This method is called for every incoming request and returns a {@link CompletableFuture} containing a the response.
	 *
	 * 
Implementations may decide whether to throw {@link RestHandlerException}s or fail the returned
	 * {@link CompletableFuture} with a {@link RestHandlerException}.
	 *
	 * 
Failing the future with another exception type or throwing unchecked exceptions is regarded as an
	 * implementation error as it does not allow us to provide a meaningful HTTP status code. In this case a
	 * {@link HttpResponseStatus#INTERNAL_SERVER_ERROR} will be returned.
	 *
	 * @param request request that should be handled
	 * @param gateway leader gateway
	 * @return future containing a handler response
	 * @throws RestHandlerException if the handling failed
	 */
	protected abstract CompletableFuture handleRequest(@Nonnull HandlerRequest request, @Nonnull T gateway) throws RestHandlerException;
}