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   /*
    * Copyright 2012 The Netty Project
    *
    * The Netty Project 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:
    *
    *   https://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.netty.handler.codec;
  
  import io.netty.buffer.ByteBuf;
  import io.netty.channel.ChannelHandlerContext;
  import io.netty.util.ByteProcessor;
  
  import java.util.List;
  
  /**
   * A decoder that splits the received {@link ByteBuf}s on line endings.
   * <p>
   * Both {@code "\n"} and {@code "\r\n"} are handled.
   * <p>
   * The byte stream is expected to be in UTF-8 character encoding or ASCII. The current implementation
   * uses direct {@code byte} to {@code char} cast and then compares that {@code char} to a few low range
   * ASCII characters like {@code '\n'} or {@code '\r'}. UTF-8 is not using low range [0..0x7F]
   * byte values for multibyte codepoint representations therefore fully supported by this implementation.
   * <p>
   * For a more general delimiter-based decoder, see {@link DelimiterBasedFrameDecoder}.
   * <p>
   * Users should be aware that used as is, the lenient approach on lone {@code '\n} might result on a parser
   * diffenrencial on line based protocols requiring the use of {@code "\r\n"} delimiters like SMTP and can
   * result in attacks similar to
   * <a href="https://sec-consult.com/blog/detail/smtp-smuggling-spoofing-e-mails-worldwide/">SMTP smuggling</a>.
   * Validating afterward the end of line pattern can be a possible mitigation.
   */
  public class LineBasedFrameDecoder extends ByteToMessageDecoder {
  
      /** Maximum length of a frame we're willing to decode.  */
      private final int maxLength;
      /** Whether or not to throw an exception as soon as we exceed maxLength. */
      private final boolean failFast;
      private final boolean stripDelimiter;
  
      /** True if we're discarding input because we're already over maxLength.  */
      private boolean discarding;
      private int discardedBytes;
  
      /** Last scan position. */
      private int offset;
  
      /**
       * Creates a new decoder.
       * @param maxLength  the maximum length of the decoded frame.
       *                   A {@link TooLongFrameException} is thrown if
       *                   the length of the frame exceeds this value.
       */
      public LineBasedFrameDecoder(final int maxLength) {
          this(maxLength, true, false);
      }
  
      /**
       * Creates a new decoder.
       * @param maxLength  the maximum length of the decoded frame.
       *                   A {@link TooLongFrameException} is thrown if
       *                   the length of the frame exceeds this value.
       * @param stripDelimiter  whether the decoded frame should strip out the
       *                        delimiter or not
       * @param failFast  If <tt>true</tt>, a {@link TooLongFrameException} is
       *                  thrown as soon as the decoder notices the length of the
       *                  frame will exceed <tt>maxFrameLength</tt> regardless of
       *                  whether the entire frame has been read.
       *                  If <tt>false</tt>, a {@link TooLongFrameException} is
       *                  thrown after the entire frame that exceeds
       *                  <tt>maxFrameLength</tt> has been read.
       */
      public LineBasedFrameDecoder(final int maxLength, final boolean stripDelimiter, final boolean failFast) {
          this.maxLength = maxLength;
          this.failFast = failFast;
          this.stripDelimiter = stripDelimiter;
      }
  
      @Override
      protected final void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) throws Exception {
          Object decoded = decode(ctx, in);
          if (decoded != null) {
              out.add(decoded);
          }
      }
  
      /**
       * Create a frame out of the {@link ByteBuf} and return it.
       *
       * @param   ctx             the {@link ChannelHandlerContext} which this {@link ByteToMessageDecoder} belongs to
      * @param   buffer          the {@link ByteBuf} from which to read data
      * @return  frame           the {@link ByteBuf} which represent the frame or {@code null} if no frame could
      *                          be created.
      */
     protected Object decode(ChannelHandlerContext ctx, ByteBuf buffer) throws Exception {
         final int eol = findEndOfLine(buffer);
         if (!discarding) {
             if (eol >= 0) {
                 final ByteBuf frame;
                 final int length = eol - buffer.readerIndex();
                 final int delimLength = buffer.getByte(eol) == '\r'? 2 : 1;
 
                 if (length > maxLength) {
                     buffer.readerIndex(eol + delimLength);
                     fail(ctx, length);
                     return null;
                 }
 
                 if (stripDelimiter) {
                     frame = buffer.readRetainedSlice(length);
                     buffer.skipBytes(delimLength);
                 } else {
                     frame = buffer.readRetainedSlice(length + delimLength);
                 }
 
                 return frame;
             } else {
                 final int length = buffer.readableBytes();
                 if (length > maxLength) {
                     discardedBytes = length;
                     buffer.readerIndex(buffer.writerIndex());
                     discarding = true;
                     offset = 0;
                     if (failFast) {
                         fail(ctx, "over " + discardedBytes);
                     }
                 }
                 return null;
             }
         } else {
             if (eol >= 0) {
                 final int length = discardedBytes + eol - buffer.readerIndex();
                 final int delimLength = buffer.getByte(eol) == '\r'? 2 : 1;
                 buffer.readerIndex(eol + delimLength);
                 discardedBytes = 0;
                 discarding = false;
                 if (!failFast) {
                     fail(ctx, length);
                 }
             } else {
                 discardedBytes += buffer.readableBytes();
                 buffer.readerIndex(buffer.writerIndex());
                 // We skip everything in the buffer, we need to set the offset to 0 again.
                 offset = 0;
             }
             return null;
         }
     }
 
     private void fail(final ChannelHandlerContext ctx, int length) {
         fail(ctx, String.valueOf(length));
     }
 
     private void fail(final ChannelHandlerContext ctx, String length) {
         ctx.fireExceptionCaught(
                 new TooLongFrameException(
                         "frame length (" + length + ") exceeds the allowed maximum (" + maxLength + ')'));
     }
 
     /**
      * Returns the index in the buffer of the end of line found.
      * Returns -1 if no end of line was found in the buffer.
      */
     private int findEndOfLine(final ByteBuf buffer) {
         int totalLength = buffer.readableBytes();
         int i = buffer.indexOf(buffer.readerIndex() + offset,
                                buffer.readerIndex() + totalLength, (byte) '\n');
         if (i >= 0) {
             offset = 0;
             if (i > 0 && buffer.getByte(i - 1) == '\r') {
                 i--;
             }
         } else {
             offset = totalLength;
         }
         return i;
     }
 }