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+/**
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+ * Licensed to the Apache Software Foundation (ASF) under one
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+ * or more contributor license agreements. See the NOTICE file
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+ * distributed with this work for additional information
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+ * regarding copyright ownership. The ASF licenses this file
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+ * to you under the Apache License, Version 2.0 (the
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+ * "License"); you may not use this file except in compliance
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+ * with the License. You may obtain a copy of the License at
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+ *
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+ * http://www.apache.org/licenses/LICENSE-2.0
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+ *
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+ * Unless required by applicable law or agreed to in writing, software
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+ * distributed under the License is distributed on an "AS IS" BASIS,
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+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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+ * See the License for the specific language governing permissions and
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+ * limitations under the License.
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+ */
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+
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+package org.apache.hadoop.mapreduce.lib.input;
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+
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+import java.io.IOException;
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+import java.io.InputStream;
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+
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+import org.apache.hadoop.classification.InterfaceAudience;
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+import org.apache.hadoop.classification.InterfaceStability;
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+import org.apache.hadoop.conf.Configuration;
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+import org.apache.hadoop.io.Text;
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+import org.apache.hadoop.io.compress.SplitCompressionInputStream;
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+
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+/**
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+ * Line reader for compressed splits
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+ *
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+ * Reading records from a compressed split is tricky, as the
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+ * LineRecordReader is using the reported compressed input stream
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+ * position directly to determine when a split has ended. In addition the
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+ * compressed input stream is usually faking the actual byte position, often
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+ * updating it only after the first compressed block after the split is
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+ * accessed.
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+ *
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+ * Depending upon where the last compressed block of the split ends relative
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+ * to the record delimiters it can be easy to accidentally drop the last
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+ * record or duplicate the last record between this split and the next.
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+ *
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+ * Split end scenarios:
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+ *
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+ * 1) Last block of split ends in the middle of a record
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+ * Nothing special that needs to be done here, since the compressed input
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+ * stream will report a position after the split end once the record
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+ * is fully read. The consumer of the next split will discard the
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+ * partial record at the start of the split normally, and no data is lost
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+ * or duplicated between the splits.
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+ *
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+ * 2) Last block of split ends in the middle of a delimiter
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+ * The line reader will continue to consume bytes into the next block to
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+ * locate the end of the delimiter. If a custom delimiter is being used
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+ * then the next record must be read by this split or it will be dropped.
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+ * The consumer of the next split will not recognize the partial
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+ * delimiter at the beginning of its split and will discard it along with
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+ * the next record.
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+ *
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+ * However for the default delimiter processing there is a special case
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+ * because CR, LF, and CRLF are all valid record delimiters. If the
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+ * block ends with a CR then the reader must peek at the next byte to see
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+ * if it is an LF and therefore part of the same record delimiter.
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+ * Peeking at the next byte is an access to the next block and triggers
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+ * the stream to report the end of the split. There are two cases based
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+ * on the next byte:
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+ *
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+ * A) The next byte is LF
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+ * The split needs to end after the current record is returned. The
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+ * consumer of the next split will discard the first record, which
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+ * is degenerate since LF is itself a delimiter, and start consuming
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+ * records after that byte. If the current split tries to read
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+ * another record then the record will be duplicated between splits.
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+ *
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+ * B) The next byte is not LF
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+ * The current record will be returned but the stream will report
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+ * the split has ended due to the peek into the next block. If the
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+ * next record is not read then it will be lost, as the consumer of
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+ * the next split will discard it before processing subsequent
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+ * records. Therefore the next record beyond the reported split end
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+ * must be consumed by this split to avoid data loss.
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+ *
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+ * 3) Last block of split ends at the beginning of a delimiter
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+ * This is equivalent to case 1, as the reader will consume bytes into
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+ * the next block and trigger the end of the split. No further records
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+ * should be read as the consumer of the next split will discard the
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+ * (degenerate) record at the beginning of its split.
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+ *
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+ * 4) Last block of split ends at the end of a delimiter
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+ * Nothing special needs to be done here. The reader will not start
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+ * examining the bytes into the next block until the next record is read,
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+ * so the stream will not report the end of the split just yet. Once the
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+ * next record is read then the next block will be accessed and the
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+ * stream will indicate the end of the split. The consumer of the next
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+ * split will correctly discard the first record of its split, and no
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+ * data is lost or duplicated.
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+ *
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+ * If the default delimiter is used and the block ends at a CR then this
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+ * is treated as case 2 since the reader does not yet know without
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+ * looking at subsequent bytes whether the delimiter has ended.
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+ *
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+ * NOTE: It is assumed that compressed input streams *never* return bytes from
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+ * multiple compressed blocks from a single read. Failure to do so will
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+ * violate the buffering performed by this class, as it will access
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+ * bytes into the next block after the split before returning all of the
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+ * records from the previous block.
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+ */
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+@InterfaceAudience.Private
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+@InterfaceStability.Unstable
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+public class CompressedSplitLineReader extends SplitLineReader {
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+
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+ SplitCompressionInputStream scin;
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+ private boolean usingCRLF;
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+ private boolean needAdditionalRecord = false;
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+ private boolean finished = false;
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+
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+ public CompressedSplitLineReader(SplitCompressionInputStream in,
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+ Configuration conf,
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+ byte[] recordDelimiterBytes)
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+ throws IOException {
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+ super(in, conf, recordDelimiterBytes);
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+ scin = in;
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+ usingCRLF = (recordDelimiterBytes == null);
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+ }
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+
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+ @Override
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+ protected int fillBuffer(InputStream in, byte[] buffer, boolean inDelimiter)
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+ throws IOException {
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+ int bytesRead = in.read(buffer);
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+
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+ // If the split ended in the middle of a record delimiter then we need
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+ // to read one additional record, as the consumer of the next split will
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+ // not recognize the partial delimiter as a record.
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+ // However if using the default delimiter and the next character is a
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+ // linefeed then next split will treat it as a delimiter all by itself
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+ // and the additional record read should not be performed.
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+ if (inDelimiter && bytesRead > 0) {
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+ if (usingCRLF) {
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+ needAdditionalRecord = (buffer[0] != '\n');
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+ } else {
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+ needAdditionalRecord = true;
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+ }
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+ }
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+ return bytesRead;
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+ }
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+
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+ @Override
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+ public int readLine(Text str, int maxLineLength, int maxBytesToConsume)
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+ throws IOException {
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+ int bytesRead = 0;
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+ if (!finished) {
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+ // only allow at most one more record to be read after the stream
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+ // reports the split ended
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+ if (scin.getPos() > scin.getAdjustedEnd()) {
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+ finished = true;
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+ }
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+
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+ bytesRead = super.readLine(str, maxLineLength, maxBytesToConsume);
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+ }
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+ return bytesRead;
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+ }
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+
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+ @Override
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+ public boolean needAdditionalRecordAfterSplit() {
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+ return !finished && needAdditionalRecord;
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+ }
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+}
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Jason Darrell Lowe