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Git aware HTTP server implementation.
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/*
 * Copyright (C) 2010, Google Inc. and others
 *
 * This program and the accompanying materials are made available under the
 * terms of the Eclipse Distribution License v. 1.0 which is available at
 * https://www.eclipse.org/org/documents/edl-v10.php.
 *
 * SPDX-License-Identifier: BSD-3-Clause
 */

package org.eclipse.jgit.http.server;

import static org.eclipse.jgit.http.server.ServletUtils.acceptsGzipEncoding;
import static org.eclipse.jgit.util.HttpSupport.ENCODING_GZIP;
import static org.eclipse.jgit.util.HttpSupport.HDR_CONTENT_ENCODING;

import java.io.IOException;
import java.io.OutputStream;
import java.util.zip.GZIPOutputStream;

import jakarta.servlet.http.HttpServletRequest;
import jakarta.servlet.http.HttpServletResponse;

import org.eclipse.jgit.util.TemporaryBuffer;

/**
 * Buffers a response, trying to gzip it if the user agent supports that.
 * 
 * If the response overflows the buffer, gzip is skipped and the response is
 * streamed to the client as its produced, most likely using HTTP/1.1 chunked
 * encoding. This is useful for servlets that produce mixed-mode content, where
 * smaller payloads are primarily pure text that compresses well, while much
 * larger payloads are heavily compressed binary data. {@link UploadPackServlet}
 * is one such servlet.
 */
class SmartOutputStream extends TemporaryBuffer {
	private static final int LIMIT = 32 * 1024;

	private final HttpServletRequest req;
	private final HttpServletResponse rsp;
	private boolean compressStream;
	private boolean startedOutput;

	SmartOutputStream(final HttpServletRequest req,
			final HttpServletResponse rsp,
			boolean compressStream) {
		super(LIMIT);
		this.req = req;
		this.rsp = rsp;
		this.compressStream = compressStream;
	}

	@Override
	protected OutputStream overflow() throws IOException {
		startedOutput = true;

		OutputStream out = rsp.getOutputStream();
		if (compressStream && acceptsGzipEncoding(req)) {
			rsp.setHeader(HDR_CONTENT_ENCODING, ENCODING_GZIP);
			out = new GZIPOutputStream(out);
		}
		return out;
	}

	@Override
	public void close() throws IOException {
		super.close();

		if (!startedOutput) {
			// If output hasn't started yet, the entire thing fit into our
			// buffer. Try to use a proper Content-Length header, and also
			// deflate the response with gzip if it will be smaller.
			if (256 < this.length() && acceptsGzipEncoding(req)) {
				TemporaryBuffer gzbuf = new TemporaryBuffer.Heap(LIMIT);
				try {
					try (GZIPOutputStream gzip = new GZIPOutputStream(gzbuf)) {
						this.writeTo(gzip, null);
					}
					if (gzbuf.length() < this.length()) {
						rsp.setHeader(HDR_CONTENT_ENCODING, ENCODING_GZIP);
						writeResponse(gzbuf);
						return;
					}
				} catch (IOException err) {
					// Most likely caused by overflowing the buffer, meaning
					// its larger if it were compressed. Discard compressed
					// copy and use the original.
				}
			}
			writeResponse(this);
		}
	}

	private void writeResponse(TemporaryBuffer out) throws IOException {
		// The Content-Length cannot overflow when cast to an int, our
		// hardcoded LIMIT constant above assures us we wouldn't store
		// more than 2 GiB of content in memory.
		rsp.setContentLength((int) out.length());
		try (OutputStream os = rsp.getOutputStream()) {
			out.writeTo(os, null);
			os.flush();
		}
	}
}