Пример #1
0
        public /*internal*/ Document Doc(int n, FieldSelector fieldSelector, IState state)
        {
            SeekIndex(n, state);
            long position = indexStream.ReadLong(state);

            fieldsStream.Seek(position, state);

            var doc       = new Document();
            int numFields = fieldsStream.ReadVInt(state);

            for (int i = 0; i < numFields; i++)
            {
                int                 fieldNumber = fieldsStream.ReadVInt(state);
                FieldInfo           fi          = fieldInfos.FieldInfo(fieldNumber);
                FieldSelectorResult acceptField = fieldSelector == null?FieldSelectorResult.LOAD:fieldSelector.Accept(fi.name);

                byte bits = fieldsStream.ReadByte(state);
                System.Diagnostics.Debug.Assert(bits <= FieldsWriter.FIELD_IS_COMPRESSED + FieldsWriter.FIELD_IS_TOKENIZED + FieldsWriter.FIELD_IS_BINARY);

                bool compressed = (bits & FieldsWriter.FIELD_IS_COMPRESSED) != 0;
                System.Diagnostics.Debug.Assert(
                    (!compressed || (format < FieldsWriter.FORMAT_LUCENE_3_0_NO_COMPRESSED_FIELDS)),
                    "compressed fields are only allowed in indexes of version <= 2.9");
                bool tokenize = (bits & FieldsWriter.FIELD_IS_TOKENIZED) != 0;
                bool binary   = (bits & FieldsWriter.FIELD_IS_BINARY) != 0;
                //TODO: Find an alternative approach here if this list continues to grow beyond the
                //list of 5 or 6 currently here.  See Lucene 762 for discussion
                if (acceptField.Equals(FieldSelectorResult.LOAD))
                {
                    AddField(doc, fi, binary, compressed, tokenize, state);
                }
                else if (acceptField.Equals(FieldSelectorResult.LOAD_AND_BREAK))
                {
                    AddField(doc, fi, binary, compressed, tokenize, state);
                    break;                     //Get out of this loop
                }
                else if (acceptField.Equals(FieldSelectorResult.LAZY_LOAD))
                {
                    AddFieldLazy(doc, fi, binary, compressed, tokenize, state);
                }
                else if (acceptField.Equals(FieldSelectorResult.SIZE))
                {
                    SkipField(binary, compressed, AddFieldSize(doc, fi, binary, compressed, state), state);
                }
                else if (acceptField.Equals(FieldSelectorResult.SIZE_AND_BREAK))
                {
                    AddFieldSize(doc, fi, binary, compressed, state);
                    break;
                }
                else
                {
                    SkipField(binary, compressed, state);
                }
            }

            return(doc);
        }
Пример #2
0
        /// <summary>Retrieve the length (in bytes) of the tvd and tvf
        /// entries for the next numDocs starting with
        /// startDocID.  This is used for bulk copying when
        /// merging segments, if the field numbers are
        /// congruent.  Once this returns, the tvf &amp; tvd streams
        /// are seeked to the startDocID.
        /// </summary>
        internal void  RawDocs(int[] tvdLengths, int[] tvfLengths, int startDocID, int numDocs, IState state)
        {
            if (tvx == null)
            {
                for (int i = 0; i < tvdLengths.Length; i++)
                {
                    tvdLengths[i] = 0;
                }
                for (int i = 0; i < tvfLengths.Length; i++)
                {
                    tvfLengths[i] = 0;
                }
                return;
            }

            // SegmentMerger calls canReadRawDocs() first and should
            // not call us if that returns false.
            if (format < FORMAT_VERSION2)
            {
                throw new System.SystemException("cannot read raw docs with older term vector formats");
            }

            SeekTvx(startDocID, state);

            long tvdPosition = tvx.ReadLong(state);

            tvd.Seek(tvdPosition, state);

            long tvfPosition = tvx.ReadLong(state);

            tvf.Seek(tvfPosition, state);

            long lastTvdPosition = tvdPosition;
            long lastTvfPosition = tvfPosition;

            int count = 0;

            while (count < numDocs)
            {
                int docID = docStoreOffset + startDocID + count + 1;
                System.Diagnostics.Debug.Assert(docID <= numTotalDocs);
                if (docID < numTotalDocs)
                {
                    tvdPosition = tvx.ReadLong(state);
                    tvfPosition = tvx.ReadLong(state);
                }
                else
                {
                    tvdPosition = tvd.Length(state);
                    tvfPosition = tvf.Length(state);
                    System.Diagnostics.Debug.Assert(count == numDocs - 1);
                }
                tvdLengths[count] = (int)(tvdPosition - lastTvdPosition);
                tvfLengths[count] = (int)(tvfPosition - lastTvfPosition);
                count++;
                lastTvdPosition = tvdPosition;
                lastTvfPosition = tvfPosition;
            }
        }
Пример #3
0
        public CompoundFileReader(Directory dir, System.String name, int readBufferSize, IState state)
        {
            directory           = dir;
            fileName            = name;
            this.readBufferSize = readBufferSize;

            bool success = false;

            try
            {
                stream = dir.OpenInput(name, readBufferSize, state);

                // read the directory and init files
                int       count = stream.ReadVInt(state);
                FileEntry entry = null;
                for (int i = 0; i < count; i++)
                {
                    long          offset = stream.ReadLong(state);
                    System.String id     = stream.ReadString(state);

                    if (entry != null)
                    {
                        // set length of the previous entry
                        entry.length = offset - entry.offset;
                    }

                    entry = new FileEntry {
                        offset = offset
                    };
                    entries[id] = entry;
                }

                // set the length of the final entry
                if (entry != null)
                {
                    entry.length = stream.Length(state) - entry.offset;
                }

                success = true;
            }
            finally
            {
                if (!success && (stream != null))
                {
                    try
                    {
                        stream.Close();
                    }
                    catch (System.IO.IOException)
                    {
                    }
                }
            }
        }
        private BinaryEntry ReadBinaryEntry(IndexInput meta)
        {
            var entry = new BinaryEntry();
            entry.offset = meta.ReadLong();
            entry.numBytes = meta.ReadInt();
            entry.count = meta.ReadInt();
            entry.missingOffset = meta.ReadLong();
            if (entry.missingOffset != -1)
            {
                entry.missingBytes = meta.ReadLong();
            }
            else
            {
                entry.missingBytes = 0;
            }

            return entry;
        }
        private static NumericEntry ReadNumericEntry(IndexInput meta)
        {
            var entry = new NumericEntry { offset = meta.ReadLong(), count = meta.ReadInt(), missingOffset = meta.ReadLong() };
            if (entry.missingOffset != -1)
            {
                entry.missingBytes = meta.ReadLong();
            }
            else
            {
                entry.missingBytes = 0;
            }
            entry.byteWidth = meta.ReadByte();

            return entry;
        }
Пример #6
0
 protected override void SeekDir(IndexInput input, long dirOffset)
 {
     input.Seek(input.Length() - sizeof(long)/8);
     long offset = input.ReadLong();
     input.Seek(offset);
 }
Пример #7
0
        internal SegmentTermEnum(IndexInput i, FieldInfos fis, bool isi, IState state)
        {
            input         = i;
            fieldInfos    = fis;
            isIndex       = isi;
            maxSkipLevels = 1;             // use single-level skip lists for formats > -3

            int firstInt = input.ReadInt(state);

            if (firstInt >= 0)
            {
                // original-format file, without explicit format version number
                format = 0;
                size   = firstInt;

                // back-compatible settings
                indexInterval = 128;
                skipInterval  = System.Int32.MaxValue;                // switch off skipTo optimization
            }
            else
            {
                // we have a format version number
                format = firstInt;

                // check that it is a format we can understand
                if (format < TermInfosWriter.FORMAT_CURRENT)
                {
                    throw new CorruptIndexException("Unknown format version:" + format + " expected " + TermInfosWriter.FORMAT_CURRENT + " or higher");
                }

                size = input.ReadLong(state);                 // read the size

                if (format == -1)
                {
                    if (!isIndex)
                    {
                        indexInterval        = input.ReadInt(state);
                        formatM1SkipInterval = input.ReadInt(state);
                    }
                    // switch off skipTo optimization for file format prior to 1.4rc2 in order to avoid a bug in
                    // skipTo implementation of these versions
                    skipInterval = System.Int32.MaxValue;
                }
                else
                {
                    indexInterval = input.ReadInt(state);
                    skipInterval  = input.ReadInt(state);
                    if (format <= TermInfosWriter.FORMAT)
                    {
                        // this new format introduces multi-level skipping
                        maxSkipLevels = input.ReadInt(state);
                    }
                }
                System.Diagnostics.Debug.Assert(indexInterval > 0, "indexInterval=" + indexInterval + " is negative; must be > 0");
                System.Diagnostics.Debug.Assert(skipInterval > 0, "skipInterval=" + skipInterval + " is negative; must be > 0");
            }
            if (format > TermInfosWriter.FORMAT_VERSION_UTF8_LENGTH_IN_BYTES)
            {
                termBuffer.SetPreUTF8Strings();
                scanBuffer.SetPreUTF8Strings();
                prevBuffer.SetPreUTF8Strings();
            }
        }
Пример #8
0
 public override long ReadLong()
 {
     EnsureOpen();
     return(@delegate.ReadLong());
 }
        private void ReadFields(IndexInput meta, FieldInfos infos)
        {
            int fieldNumber = meta.ReadVInt();
            while (fieldNumber != -1)
            {
                // check should be: infos.fieldInfo(fieldNumber) != null, which incorporates negative check
                // but docvalues updates are currently buggy here (loading extra stuff, etc): LUCENE-5616
                if (fieldNumber < 0)
                {
                    // trickier to validate more: because we re-use for norms, because we use multiple entries
                    // for "composite" types like sortedset, etc.
                    throw new CorruptIndexException("Invalid field number: " + fieldNumber + ", input=" + meta);
                }
                int fieldType = meta.ReadByte();
                if (fieldType == NUMBER)
                {
                    var entry = new NumericEntry {Offset = meta.ReadLong(), Format = (sbyte)meta.ReadByte()};
                    switch (entry.Format)
                    {
                        case DELTA_COMPRESSED:
                        case TABLE_COMPRESSED:
                        case GCD_COMPRESSED:
                        case UNCOMPRESSED:
                            break;

                        default:
                            throw new CorruptIndexException("Unknown format: " + entry.Format + ", input=" + meta);
                    }
                    if (entry.Format != UNCOMPRESSED)
                    {
                        entry.PackedIntsVersion = meta.ReadVInt();
                    }
                    Numerics[fieldNumber] = entry;
                }
                else if (fieldType == BYTES)
                {
                    BinaryEntry entry = new BinaryEntry();
                    entry.Offset = meta.ReadLong();
                    entry.NumBytes = meta.ReadLong();
                    entry.MinLength = meta.ReadVInt();
                    entry.MaxLength = meta.ReadVInt();
                    if (entry.MinLength != entry.MaxLength)
                    {
                        entry.PackedIntsVersion = meta.ReadVInt();
                        entry.BlockSize = meta.ReadVInt();
                    }
                    Binaries[fieldNumber] = entry;
                }
                else if (fieldType == FST)
                {
                    FSTEntry entry = new FSTEntry();
                    entry.Offset = meta.ReadLong();
                    entry.NumOrds = meta.ReadVLong();
                    Fsts[fieldNumber] = entry;
                }
                else
                {
                    throw new CorruptIndexException("invalid entry type: " + fieldType + ", input=" + meta);
                }
                fieldNumber = meta.ReadVInt();
            }
        }
Пример #10
0
        private static IDictionary <string, FileEntry> ReadLegacyEntries(IndexInput stream, int firstInt)
        {
            IDictionary <string, FileEntry> entries = new Dictionary <string, FileEntry>();
            int  count;
            bool stripSegmentName;

            if (firstInt < CompoundFileWriter.FORMAT_PRE_VERSION)
            {
                if (firstInt < CompoundFileWriter.FORMAT_NO_SEGMENT_PREFIX)
                {
                    throw new CorruptIndexException("Incompatible format version: " + firstInt + " expected >= " + CompoundFileWriter.FORMAT_NO_SEGMENT_PREFIX + " (resource: " + stream + ")");
                }
                // It's a post-3.1 index, read the count.
                count            = stream.ReadVInt();
                stripSegmentName = false;
            }
            else
            {
                count            = firstInt;
                stripSegmentName = true;
            }

            // read the directory and init files
            long      streamLength = stream.Length();
            FileEntry entry        = null;

            for (int i = 0; i < count; i++)
            {
                long offset = stream.ReadLong();
                if (offset < 0 || offset > streamLength)
                {
                    throw new CorruptIndexException("Invalid CFS entry offset: " + offset + " (resource: " + stream + ")");
                }
                string id = stream.ReadString();

                if (stripSegmentName)
                {
                    // Fix the id to not include the segment names. this is relevant for
                    // pre-3.1 indexes.
                    id = IndexFileNames.StripSegmentName(id);
                }

                if (entry != null)
                {
                    // set length of the previous entry
                    entry.Length = offset - entry.Offset;
                }

                entry        = new FileEntry();
                entry.Offset = offset;

                FileEntry previous = entries[id] = entry;
                if (previous != null)
                {
                    throw new CorruptIndexException("Duplicate cfs entry id=" + id + " in CFS: " + stream);
                }
            }

            // set the length of the final entry
            if (entry != null)
            {
                entry.Length = streamLength - entry.Offset;
            }

            return(entries);
        }
Пример #11
0
        private static IDictionary<string, FileEntry> ReadLegacyEntries(IndexInput stream, int firstInt)
        {
            IDictionary<string, FileEntry> entries = new Dictionary<string, FileEntry>();
            int count;
            bool stripSegmentName;
            if (firstInt < CompoundFileWriter.FORMAT_PRE_VERSION)
            {
                if (firstInt < CompoundFileWriter.FORMAT_NO_SEGMENT_PREFIX)
                {
                    throw new CorruptIndexException("Incompatible format version: " + firstInt + " expected >= " + CompoundFileWriter.FORMAT_NO_SEGMENT_PREFIX + " (resource: " + stream + ")");
                }
                // It's a post-3.1 index, read the count.
                count = stream.ReadVInt();
                stripSegmentName = false;
            }
            else
            {
                count = firstInt;
                stripSegmentName = true;
            }

            // read the directory and init files
            long streamLength = stream.Length();
            FileEntry entry = null;
            for (int i = 0; i < count; i++)
            {
                long offset = stream.ReadLong();
                if (offset < 0 || offset > streamLength)
                {
                    throw new CorruptIndexException("Invalid CFS entry offset: " + offset + " (resource: " + stream + ")");
                }
                string id = stream.ReadString();

                if (stripSegmentName)
                {
                    // Fix the id to not include the segment names. this is relevant for
                    // pre-3.1 indexes.
                    id = IndexFileNames.StripSegmentName(id);
                }

                if (entry != null)
                {
                    // set length of the previous entry
                    entry.Length = offset - entry.Offset;
                }

                entry = new FileEntry();
                entry.Offset = offset;

                FileEntry previous = entries[id] = entry;
                if (previous != null)
                {
                    throw new CorruptIndexException("Duplicate cfs entry id=" + id + " in CFS: " + stream);
                }
            }

            // set the length of the final entry
            if (entry != null)
            {
                entry.Length = streamLength - entry.Offset;
            }

            return entries;
        }
Пример #12
0
        /// <summary> Construct a new SegmentInfo instance by reading a
        /// previously saved SegmentInfo from input.
        ///
        /// </summary>
        /// <param name="dir">directory to load from
        /// </param>
        /// <param name="format">format of the segments info file
        /// </param>
        /// <param name="input">input handle to read segment info from
        /// </param>
        internal SegmentInfo(Directory dir, int format, IndexInput input, IState state)
        {
            this.dir = dir;
            name     = input.ReadString(state);
            docCount = input.ReadInt(state);
            if (format <= SegmentInfos.FORMAT_LOCKLESS)
            {
                delGen = input.ReadLong(state);
                if (format <= SegmentInfos.FORMAT_SHARED_DOC_STORE)
                {
                    docStoreOffset = input.ReadInt(state);
                    if (docStoreOffset != -1)
                    {
                        docStoreSegment        = input.ReadString(state);
                        docStoreIsCompoundFile = (1 == input.ReadByte(state));
                    }
                    else
                    {
                        docStoreSegment        = name;
                        docStoreIsCompoundFile = false;
                    }
                }
                else
                {
                    docStoreOffset         = -1;
                    docStoreSegment        = name;
                    docStoreIsCompoundFile = false;
                }
                if (format <= SegmentInfos.FORMAT_SINGLE_NORM_FILE)
                {
                    hasSingleNormFile = (1 == input.ReadByte(state));
                }
                else
                {
                    hasSingleNormFile = false;
                }
                int numNormGen = input.ReadInt(state);
                if (numNormGen == NO)
                {
                    normGen = null;
                }
                else
                {
                    normGen = new long[numNormGen];
                    for (int j = 0; j < numNormGen; j++)
                    {
                        normGen[j] = input.ReadLong(state);
                    }
                }
                isCompoundFile = (sbyte)input.ReadByte(state);
                preLockless    = (isCompoundFile == CHECK_DIR);
                if (format <= SegmentInfos.FORMAT_DEL_COUNT)
                {
                    delCount = input.ReadInt(state);
                    System.Diagnostics.Debug.Assert(delCount <= docCount);
                }
                else
                {
                    delCount = -1;
                }
                if (format <= SegmentInfos.FORMAT_HAS_PROX)
                {
                    hasProx = input.ReadByte(state) == 1;
                }
                else
                {
                    hasProx = true;
                }

                if (format <= SegmentInfos.FORMAT_DIAGNOSTICS)
                {
                    diagnostics = input.ReadStringStringMap(state);
                }
                else
                {
                    diagnostics = new Dictionary <string, string>();
                }
            }
            else
            {
                delGen                 = CHECK_DIR;
                normGen                = null;
                isCompoundFile         = (sbyte)(CHECK_DIR);
                preLockless            = true;
                hasSingleNormFile      = false;
                docStoreOffset         = -1;
                docStoreIsCompoundFile = false;
                docStoreSegment        = null;
                delCount               = -1;
                hasProx                = true;
                diagnostics            = new Dictionary <string, string>();
            }
        }
Пример #13
0
 /// <summary>
 /// Returns (but does not validate) the checksum previously written by <seealso cref="#checkFooter"/>. </summary>
 /// <returns> actual checksum value </returns>
 /// <exception cref="IOException"> if the footer is invalid </exception>
 public static long RetrieveChecksum(IndexInput @in)
 {
     @in.Seek(@in.Length() - FooterLength());
     ValidateFooter(@in);
     return @in.ReadLong();
 }
Пример #14
0
 private void ReadFields(IndexInput meta, FieldInfos infos)
 {
     int fieldNumber = meta.ReadVInt();
     while (fieldNumber != -1)
     {
         int fieldType = meta.ReadByte();
         if (fieldType == NUMBER)
         {
             var entry = new NumericEntry {offset = meta.ReadLong(), missingOffset = meta.ReadLong()};
             if (entry.missingOffset != -1)
             {
                 entry.missingBytes = meta.ReadLong();
             }
             else
             {
                 entry.missingBytes = 0;
             }
             entry.format = meta.ReadByte();
             switch (entry.format)
             {
                 case DELTA_COMPRESSED:
                 case TABLE_COMPRESSED:
                 case GCD_COMPRESSED:
                 case UNCOMPRESSED:
                     break;
                 default:
                     throw new CorruptIndexException("Unknown format: " + entry.format + ", input=" + meta);
             }
             if (entry.format != UNCOMPRESSED)
             {
                 entry.packedIntsVersion = meta.ReadVInt();
             }
             numerics[fieldNumber] = entry;
         }
         else if (fieldType == BYTES)
         {
             var entry = new BinaryEntry
             {
                 offset = meta.ReadLong(),
                 numBytes = meta.ReadLong(),
                 missingOffset = meta.ReadLong()
             };
             if (entry.missingOffset != -1)
             {
                 entry.missingBytes = meta.ReadLong();
             }
             else
             {
                 entry.missingBytes = 0;
             }
             entry.minLength = meta.ReadVInt();
             entry.maxLength = meta.ReadVInt();
             if (entry.minLength != entry.maxLength)
             {
                 entry.packedIntsVersion = meta.ReadVInt();
                 entry.blockSize = meta.ReadVInt();
             }
             binaries[fieldNumber] = entry;
         }
         else if (fieldType == FST)
         {
             var entry = new FSTEntry {offset = meta.ReadLong(), numOrds = meta.ReadVLong()};
             fsts[fieldNumber] = entry;
         }
         else
         {
             throw new CorruptIndexException("invalid entry type: " + fieldType + ", input=" + meta);
         }
         fieldNumber = meta.ReadVInt();
     }
 }
Пример #15
0
            public System.Object Run(IndexCommit commit, IState state)
            {
                if (commit != null)
                {
                    if (directory != commit.Directory)
                    {
                        throw new System.IO.IOException("the specified commit does not match the specified Directory");
                    }
                    return(DoBody(commit.SegmentsFileName, state));
                }

                System.String segmentFileName   = null;
                long          lastGen           = -1;
                long          gen               = 0;
                int           genLookaheadCount = 0;

                System.IO.IOException exc = null;
                bool retry = false;

                int method = 0;

                // Loop until we succeed in calling doBody() without
                // hitting an IOException.  An IOException most likely
                // means a commit was in process and has finished, in
                // the time it took us to load the now-old infos files
                // (and segments files).  It's also possible it's a
                // true error (corrupt index).  To distinguish these,
                // on each retry we must see "forward progress" on
                // which generation we are trying to load.  If we
                // don't, then the original error is real and we throw
                // it.

                // We have three methods for determining the current
                // generation.  We try the first two in parallel, and
                // fall back to the third when necessary.

                while (true)
                {
                    if (0 == method)
                    {
                        // Method 1: list the directory and use the highest
                        // segments_N file.  This method works well as long
                        // as there is no stale caching on the directory
                        // contents (NOTE: NFS clients often have such stale
                        // caching):
                        System.String[] files = null;

                        long genA = -1;

                        files = directory.ListAll(state);

                        if (files != null)
                        {
                            genA = Lucene.Net.Index.SegmentInfos.GetCurrentSegmentGeneration(files);
                        }

                        Lucene.Net.Index.SegmentInfos.Message("directory listing genA=" + genA);

                        // Method 2: open segments.gen and read its
                        // contents.  Then we take the larger of the two
                        // gens.  This way, if either approach is hitting
                        // a stale cache (NFS) we have a better chance of
                        // getting the right generation.
                        long genB = -1;
                        for (int i = 0; i < Lucene.Net.Index.SegmentInfos.defaultGenFileRetryCount; i++)
                        {
                            IndexInput genInput = null;
                            try
                            {
                                genInput = directory.OpenInput(IndexFileNames.SEGMENTS_GEN, state);
                            }
                            catch (System.IO.FileNotFoundException e)
                            {
                                Lucene.Net.Index.SegmentInfos.Message("segments.gen open: FileNotFoundException " + e);
                                break;
                            }
                            catch (System.IO.IOException e)
                            {
                                Lucene.Net.Index.SegmentInfos.Message("segments.gen open: IOException " + e);
                            }

                            if (genInput != null)
                            {
                                try
                                {
                                    int version = genInput.ReadInt(state);
                                    if (version == Lucene.Net.Index.SegmentInfos.FORMAT_LOCKLESS)
                                    {
                                        long gen0 = genInput.ReadLong(state);
                                        long gen1 = genInput.ReadLong(state);
                                        Lucene.Net.Index.SegmentInfos.Message("fallback check: " + gen0 + "; " + gen1);
                                        if (gen0 == gen1)
                                        {
                                            // The file is consistent.
                                            genB = gen0;
                                            break;
                                        }
                                    }
                                }
                                catch (System.IO.IOException)
                                {
                                    // will retry
                                }
                                finally
                                {
                                    genInput.Close();
                                }
                            }

                            System.Threading.Thread.Sleep(new TimeSpan((System.Int64) 10000 * Lucene.Net.Index.SegmentInfos.defaultGenFileRetryPauseMsec));
                        }

                        Lucene.Net.Index.SegmentInfos.Message(IndexFileNames.SEGMENTS_GEN + " check: genB=" + genB);

                        // Pick the larger of the two gen's:
                        if (genA > genB)
                        {
                            gen = genA;
                        }
                        else
                        {
                            gen = genB;
                        }

                        if (gen == -1)
                        {
                            throw new System.IO.FileNotFoundException("no segments* file found in " + directory + ": files:" + string.Join(" ", files));
                        }
                    }

                    // Third method (fallback if first & second methods
                    // are not reliable): since both directory cache and
                    // file contents cache seem to be stale, just
                    // advance the generation.
                    if (1 == method || (0 == method && lastGen == gen && retry))
                    {
                        method = 1;

                        if (genLookaheadCount < Lucene.Net.Index.SegmentInfos.defaultGenLookaheadCount)
                        {
                            gen++;
                            genLookaheadCount++;
                            Lucene.Net.Index.SegmentInfos.Message("look ahead increment gen to " + gen);
                        }
                    }

                    if (lastGen == gen)
                    {
                        // This means we're about to try the same
                        // segments_N last tried.  This is allowed,
                        // exactly once, because writer could have been in
                        // the process of writing segments_N last time.

                        if (retry)
                        {
                            // OK, we've tried the same segments_N file
                            // twice in a row, so this must be a real
                            // error.  We throw the original exception we
                            // got.
                            throw exc;
                        }

                        retry = true;
                    }
                    else if (0 == method)
                    {
                        // Segment file has advanced since our last loop, so
                        // reset retry:
                        retry = false;
                    }

                    lastGen = gen;

                    segmentFileName = IndexFileNames.FileNameFromGeneration(IndexFileNames.SEGMENTS, "", gen);

                    try
                    {
                        System.Object v = DoBody(segmentFileName, state);
                        Lucene.Net.Index.SegmentInfos.Message("success on " + segmentFileName);

                        return(v);
                    }
                    catch (System.IO.IOException err)
                    {
                        // Save the original root cause:
                        if (exc == null)
                        {
                            exc = err;
                        }

                        Lucene.Net.Index.SegmentInfos.Message("primary Exception on '" + segmentFileName + "': " + err + "'; will retry: retry=" + retry + "; gen = " + gen);

                        if (!retry && gen > 1)
                        {
                            // This is our first time trying this segments
                            // file (because retry is false), and, there is
                            // possibly a segments_(N-1) (because gen > 1).
                            // So, check if the segments_(N-1) exists and
                            // try it if so:
                            System.String prevSegmentFileName = IndexFileNames.FileNameFromGeneration(IndexFileNames.SEGMENTS, "", gen - 1);

                            bool prevExists;
                            prevExists = directory.FileExists(prevSegmentFileName, state);

                            if (prevExists)
                            {
                                Lucene.Net.Index.SegmentInfos.Message("fallback to prior segment file '" + prevSegmentFileName + "'");
                                try
                                {
                                    System.Object v = DoBody(prevSegmentFileName, state);
                                    if (exc != null)
                                    {
                                        Lucene.Net.Index.SegmentInfos.Message("success on fallback " + prevSegmentFileName);
                                    }
                                    return(v);
                                }
                                catch (System.IO.IOException err2)
                                {
                                    Lucene.Net.Index.SegmentInfos.Message("secondary Exception on '" + prevSegmentFileName + "': " + err2 + "'; will retry");
                                }
                            }
                        }
                    }
                }
            }