/*
 * Copyright 2004-2019 H2 Group. Multiple-Licensed under the MPL 2.0,
 * and the EPL 1.0 (http://h2database.com/html/license.html).
 * Initial Developer: H2 Group
 */
package org.h2.store;

import java.io.IOException;
import java.io.InputStream;
import java.io.Reader;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import org.h2.api.ErrorCode;
import org.h2.engine.Database;
import org.h2.jdbc.JdbcConnection;
import org.h2.message.DbException;
import org.h2.tools.CompressTool;
import org.h2.util.IOUtils;
import org.h2.util.MathUtils;
import org.h2.util.Utils;
import org.h2.value.Value;
import org.h2.value.ValueLobDb;

This class stores LOB objects in the database, in tables. This is the
back-end i.e. the server side of the LOB storage.

Using the system session

Why do we use the system session to store the data? Some LOB operations can
take a very long time. If we did them on a normal session, we would be
locking the LOB tables for long periods of time, which is extremely
detrimental to the rest of the system. Perhaps when we shift to the MVStore
engine, we can revisit this design decision (using the StreamStore, that is,
no connection at all).

Locking

Normally, the locking order in H2 is: first lock the Session object, then
lock the Database object. However, in the case of the LOB data, we are using
the system session to store the data. If we locked the normal way, we see
deadlocks caused by the following pattern:
 Thread 1:
    locks normal session
    locks database
    waiting to lock system session
 Thread 2:
     locks system session
     waiting to lock database.

So, in this class alone, we do two things: we have our very own dedicated
session, the LOB session, and we take the locks in this order: first the
Database object, and then the LOB session. Since we own the LOB session,
no-one else can lock on it, and we are safe.
/**
 * This class stores LOB objects in the database, in tables. This is the
 * back-end i.e. the server side of the LOB storage.
 * <p>
 * Using the system session
 * <p>
 * Why do we use the system session to store the data? Some LOB operations can
 * take a very long time. If we did them on a normal session, we would be
 * locking the LOB tables for long periods of time, which is extremely
 * detrimental to the rest of the system. Perhaps when we shift to the MVStore
 * engine, we can revisit this design decision (using the StreamStore, that is,
 * no connection at all).
 * <p>
 * Locking
 * <p>
 * Normally, the locking order in H2 is: first lock the Session object, then
 * lock the Database object. However, in the case of the LOB data, we are using
 * the system session to store the data. If we locked the normal way, we see
 * deadlocks caused by the following pattern:
 *
 * <pre>
 *  Thread 1:
 *     locks normal session
 *     locks database
 *     waiting to lock system session
 *  Thread 2:
 *      locks system session
 *      waiting to lock database.
 * </pre>
 *
 * So, in this class alone, we do two things: we have our very own dedicated
 * session, the LOB session, and we take the locks in this order: first the
 * Database object, and then the LOB session. Since we own the LOB session,
 * no-one else can lock on it, and we are safe.
 */
public class LobStorageBackend implements LobStorageInterface {

    
The name of the lob data table. If this table exists, then lob storage is
used.
/**
     * The name of the lob data table. If this table exists, then lob storage is
     * used.
     */
    public static final String LOB_DATA_TABLE = "LOB_DATA";

    private static final String LOB_SCHEMA = "INFORMATION_SCHEMA";
    private static final String LOBS = LOB_SCHEMA + ".LOBS";
    private static final String LOB_MAP = LOB_SCHEMA + ".LOB_MAP";
    private static final String LOB_DATA = LOB_SCHEMA + "." + LOB_DATA_TABLE;

    
The size of the chunks we use when storing LOBs inside the database file.
/**
     * The size of the chunks we use when storing LOBs inside the database file.
     */
    private static final int BLOCK_LENGTH = 20_000;

    
The size of cache for lob block hashes. Each entry needs 2 longs (16
bytes), therefore, the size 4096 means 64 KB.
/**
     * The size of cache for lob block hashes. Each entry needs 2 longs (16
     * bytes), therefore, the size 4096 means 64 KB.
     */
    private static final int HASH_CACHE_SIZE = 4 * 1024;

    JdbcConnection conn;
    final Database database;

    private final HashMap<String, PreparedStatement> prepared = new HashMap<>();
    private long nextBlock;
    private final CompressTool compress = CompressTool.getInstance();
    private long[] hashBlocks;

    private boolean init;

    public LobStorageBackend(Database database) {
        this.database = database;
    }

    @Override
    public void init() {
        if (init) {
            return;
        }
        synchronized (database) {
            // have to check this again or we might miss an update on another
            // thread
            if (init) {
                return;
            }
            init = true;
            conn = database.getLobConnectionForRegularUse();
            JdbcConnection initConn = database.getLobConnectionForInit();
            try {
                Statement stat = initConn.createStatement();
                // stat.execute("SET UNDO_LOG 0");
                // stat.execute("SET REDO_LOG_BINARY 0");
                boolean create = true;
                PreparedStatement prep = initConn.prepareStatement(
                        "SELECT ZERO() FROM INFORMATION_SCHEMA.COLUMNS WHERE " +
                        "TABLE_SCHEMA=? AND TABLE_NAME=? AND COLUMN_NAME=?");
                prep.setString(1, "INFORMATION_SCHEMA");
                prep.setString(2, "LOB_MAP");
                prep.setString(3, "POS");
                ResultSet rs;
                rs = prep.executeQuery();
                if (rs.next()) {
                    prep = initConn.prepareStatement(
                            "SELECT ZERO() FROM INFORMATION_SCHEMA.TABLES WHERE " +
                            "TABLE_SCHEMA=? AND TABLE_NAME=?");
                    prep.setString(1, "INFORMATION_SCHEMA");
                    prep.setString(2, "LOB_DATA");
                    rs = prep.executeQuery();
                    if (rs.next()) {
                        create = false;
                    }
                }
                if (create) {
                    stat.execute("CREATE CACHED TABLE IF NOT EXISTS " + LOBS +
                            "(ID BIGINT PRIMARY KEY, BYTE_COUNT BIGINT, `TABLE` INT) HIDDEN");
                    stat.execute("CREATE INDEX IF NOT EXISTS " +
                            "INFORMATION_SCHEMA.INDEX_LOB_TABLE ON " +
                            LOBS + "(`TABLE`)");
                    stat.execute("CREATE CACHED TABLE IF NOT EXISTS " + LOB_MAP +
                            "(LOB BIGINT, SEQ INT, POS BIGINT, HASH INT, " +
                            "BLOCK BIGINT, PRIMARY KEY(LOB, SEQ)) HIDDEN");
                    stat.execute("ALTER TABLE " + LOB_MAP +
                            " RENAME TO " + LOB_MAP + " HIDDEN");
                    stat.execute("ALTER TABLE " + LOB_MAP +
                            " ADD IF NOT EXISTS POS BIGINT BEFORE HASH");
                    // TODO the column name OFFSET was used in version 1.3.156,
                    // so this can be remove in a later version
                    stat.execute("ALTER TABLE " + LOB_MAP +
                            " DROP COLUMN IF EXISTS \"OFFSET\"");
                    stat.execute("CREATE INDEX IF NOT EXISTS " +
                            "INFORMATION_SCHEMA.INDEX_LOB_MAP_DATA_LOB ON " +
                            LOB_MAP + "(BLOCK, LOB)");
                    stat.execute("CREATE CACHED TABLE IF NOT EXISTS " +
                            LOB_DATA +
                            "(BLOCK BIGINT PRIMARY KEY, COMPRESSED INT, DATA BINARY) HIDDEN");
                }
                rs = stat.executeQuery("SELECT MAX(BLOCK) FROM " + LOB_DATA);
                rs.next();
                nextBlock = rs.getLong(1) + 1;
                stat.close();
            } catch (SQLException e) {
                throw DbException.convert(e);
            }
        }
    }

    private long getNextLobId() throws SQLException {
        String sql = "SELECT MAX(LOB) FROM " + LOB_MAP;
        PreparedStatement prep = prepare(sql);
        ResultSet rs = prep.executeQuery();
        rs.next();
        long x = rs.getLong(1) + 1;
        reuse(sql, prep);
        sql = "SELECT MAX(ID) FROM " + LOBS;
        prep = prepare(sql);
        rs = prep.executeQuery();
        rs.next();
        x = Math.max(x, rs.getLong(1) + 1);
        reuse(sql, prep);
        return x;
    }

    @Override
    public void removeAllForTable(int tableId) {
        init();
        try {
            String sql = "SELECT ID FROM " + LOBS + " WHERE `TABLE` = ?";
            PreparedStatement prep = prepare(sql);
            prep.setInt(1, tableId);
            ResultSet rs = prep.executeQuery();
            while (rs.next()) {
                removeLob(rs.getLong(1));
            }
            reuse(sql, prep);
        } catch (SQLException e) {
            throw DbException.convert(e);
        }
        if (tableId == LobStorageFrontend.TABLE_ID_SESSION_VARIABLE) {
            removeAllForTable(LobStorageFrontend.TABLE_TEMP);
            removeAllForTable(LobStorageFrontend.TABLE_RESULT);
        }
    }

    
Read a block of data from the given LOB.
Params:
block – the block number
Returns:
the block (expanded if stored compressed)/**
     * Read a block of data from the given LOB.
     *
     * @param block the block number
     * @return the block (expanded if stored compressed)
     */
    byte[] readBlock(long block) throws SQLException {
        // see locking discussion at the top
        assertNotHolds(conn.getSession());
        synchronized (database) {
            synchronized (conn.getSession()) {
                String sql = "SELECT COMPRESSED, DATA FROM " +
                        LOB_DATA + " WHERE BLOCK = ?";
                PreparedStatement prep = prepare(sql);
                prep.setLong(1, block);
                ResultSet rs = prep.executeQuery();
                if (!rs.next()) {
                    throw DbException.getJdbcSQLException(ErrorCode.IO_EXCEPTION_1,
                            "Missing lob entry, block: " + block);
                }
                int compressed = rs.getInt(1);
                byte[] buffer = rs.getBytes(2);
                if (compressed != 0) {
                    buffer = compress.expand(buffer);
                }
                reuse(sql, prep);
                return buffer;
            }
        }
    }

    
Create a prepared statement, or re-use an existing one.
Params:
sql – the SQL statement
Returns:
the prepared statement/**
     * Create a prepared statement, or re-use an existing one.
     *
     * @param sql the SQL statement
     * @return the prepared statement
     */
    PreparedStatement prepare(String sql) throws SQLException {
        assert Thread.holdsLock(database);
        PreparedStatement prep = prepared.remove(sql);
        if (prep == null) {
            prep = conn.prepareStatement(sql);
        }
        return prep;
    }

    
Allow to re-use the prepared statement.
Params:
sql – the SQL statement
prep – the prepared statement/**
     * Allow to re-use the prepared statement.
     *
     * @param sql the SQL statement
     * @param prep the prepared statement
     */
    void reuse(String sql, PreparedStatement prep) {
        assert Thread.holdsLock(database);
        prepared.put(sql, prep);
    }

    @Override
    public void removeLob(ValueLobDb lob) {
        removeLob(lob.getLobId());
    }

    private void removeLob(long lobId) {
        try {
            // see locking discussion at the top
            assertNotHolds(conn.getSession());
            synchronized (database) {
                synchronized (conn.getSession()) {
                    String sql = "SELECT BLOCK, HASH FROM " + LOB_MAP + " D WHERE D.LOB = ? " +
                            "AND NOT EXISTS(SELECT 1 FROM " + LOB_MAP + " O " +
                            "WHERE O.BLOCK = D.BLOCK AND O.LOB <> ?)";
                    PreparedStatement prep = prepare(sql);
                    prep.setLong(1, lobId);
                    prep.setLong(2, lobId);
                    ResultSet rs = prep.executeQuery();
                    ArrayList<Long> blocks = Utils.newSmallArrayList();
                    while (rs.next()) {
                        blocks.add(rs.getLong(1));
                        int hash = rs.getInt(2);
                        setHashCacheBlock(hash, -1);
                    }
                    reuse(sql, prep);

                    sql = "DELETE FROM " + LOB_MAP + " WHERE LOB = ?";
                    prep = prepare(sql);
                    prep.setLong(1, lobId);
                    prep.execute();
                    reuse(sql, prep);

                    sql = "DELETE FROM " + LOB_DATA + " WHERE BLOCK = ?";
                    prep = prepare(sql);
                    for (long block : blocks) {
                        prep.setLong(1, block);
                        prep.execute();
                    }
                    reuse(sql, prep);

                    sql = "DELETE FROM " + LOBS + " WHERE ID = ?";
                    prep = prepare(sql);
                    prep.setLong(1, lobId);
                    prep.execute();
                    reuse(sql, prep);
                }
            }
        } catch (SQLException e) {
            throw DbException.convert(e);
        }
    }

    @Override
    public InputStream getInputStream(ValueLobDb lob, byte[] hmac,
            long byteCount) throws IOException {
        try {
            init();
            assertNotHolds(conn.getSession());
            // see locking discussion at the top
            synchronized (database) {
                synchronized (conn.getSession()) {
                    long lobId = lob.getLobId();
                    return new LobInputStream(lobId, byteCount);
                }
            }
        } catch (SQLException e) {
            throw DbException.convertToIOException(e);
        }
    }

    private ValueLobDb addLob(InputStream in, long maxLength, int type,
            CountingReaderInputStream countingReaderForClob) {
        try {
            byte[] buff = new byte[BLOCK_LENGTH];
            if (maxLength < 0) {
                maxLength = Long.MAX_VALUE;
            }
            long length = 0;
            long lobId = -1;
            int maxLengthInPlaceLob = database.getMaxLengthInplaceLob();
            String compressAlgorithm = database.getLobCompressionAlgorithm(type);
            try {
                byte[] small = null;
                for (int seq = 0; maxLength > 0; seq++) {
                    int len = (int) Math.min(BLOCK_LENGTH, maxLength);
                    len = IOUtils.readFully(in, buff, len);
                    if (len <= 0) {
                        break;
                    }
                    maxLength -= len;
                    // if we had a short read, trim the buffer
                    byte[] b;
                    if (len != buff.length) {
                        b = Arrays.copyOf(buff, len);
                    } else {
                        b = buff;
                    }
                    if (seq == 0 && b.length < BLOCK_LENGTH &&
                            b.length <= maxLengthInPlaceLob) {
                        small = b;
                        break;
                    }
                    assertNotHolds(conn.getSession());
                    // see locking discussion at the top
                    synchronized (database) {
                        synchronized (conn.getSession()) {
                            if (seq == 0) {
                                lobId = getNextLobId();
                            }
                            storeBlock(lobId, seq, length, b, compressAlgorithm);
                        }
                    }
                    length += len;
                }
                if (lobId == -1 && small == null) {
                    // zero length
                    small = new byte[0];
                }
                if (small != null) {
                    // For a BLOB, precision is length in bytes.
                    // For a CLOB, precision is length in chars
                    long precision = countingReaderForClob == null ?
                            small.length : countingReaderForClob.getLength();
                    return ValueLobDb.createSmallLob(type, small, precision);
                }
                // For a BLOB, precision is length in bytes.
                // For a CLOB, precision is length in chars
                long precision = countingReaderForClob == null ?
                        length : countingReaderForClob.getLength();
                return registerLob(type, lobId,
                        LobStorageFrontend.TABLE_TEMP, length, precision);
            } catch (IOException e) {
                if (lobId != -1) {
                    removeLob(lobId);
                }
                throw DbException.convertIOException(e, null);
            }
        } catch (SQLException e) {
            throw DbException.convert(e);
        }
    }

    private ValueLobDb registerLob(int type, long lobId, int tableId,
            long byteCount, long precision) throws SQLException {
        assertNotHolds(conn.getSession());
        // see locking discussion at the top
        synchronized (database) {
            synchronized (conn.getSession()) {
                String sql = "INSERT INTO " + LOBS +
                        "(ID, BYTE_COUNT, `TABLE`) VALUES(?, ?, ?)";
                PreparedStatement prep = prepare(sql);
                prep.setLong(1, lobId);
                prep.setLong(2, byteCount);
                prep.setInt(3, tableId);
                prep.execute();
                reuse(sql, prep);
                return ValueLobDb.create(type,
                        database, tableId, lobId, null, precision);
            }
        }
    }

    @Override
    public boolean isReadOnly() {
        return database.isReadOnly();
    }

    @Override
    public ValueLobDb copyLob(ValueLobDb old, int tableId, long length) {
        int type = old.getValueType();
        long oldLobId = old.getLobId();
        assertNotHolds(conn.getSession());
        // see locking discussion at the top
        synchronized (database) {
            synchronized (conn.getSession()) {
                try {
                    init();
                    ValueLobDb v = null;
                    if (!old.isRecoveryReference()) {
                        long lobId = getNextLobId();
                        String sql = "INSERT INTO " + LOB_MAP +
                                "(LOB, SEQ, POS, HASH, BLOCK) " +
                                "SELECT ?, SEQ, POS, HASH, BLOCK FROM " +
                                LOB_MAP + " WHERE LOB = ?";
                        PreparedStatement prep = prepare(sql);
                        prep.setLong(1, lobId);
                        prep.setLong(2, oldLobId);
                        prep.executeUpdate();
                        reuse(sql, prep);

                        sql = "INSERT INTO " + LOBS +
                                "(ID, BYTE_COUNT, `TABLE`) " +
                                "SELECT ?, BYTE_COUNT, ? FROM " + LOBS +
                                " WHERE ID = ?";
                        prep = prepare(sql);
                        prep.setLong(1, lobId);
                        prep.setLong(2, tableId);
                        prep.setLong(3, oldLobId);
                        prep.executeUpdate();
                        reuse(sql, prep);

                        v = ValueLobDb.create(type, database, tableId, lobId, null, length);
                    } else {
                        // Recovery process, no need to copy LOB using normal
                        // infrastructure
                        v = ValueLobDb.create(type, database, tableId, oldLobId, null, length);
                    }
                    return v;
                } catch (SQLException e) {
                    throw DbException.convert(e);
                }
            }
        }
    }

    private long getHashCacheBlock(int hash) {
        if (HASH_CACHE_SIZE > 0) {
            initHashCache();
            int index = hash & (HASH_CACHE_SIZE - 1);
            long oldHash = hashBlocks[index];
            if (oldHash == hash) {
                return hashBlocks[index + HASH_CACHE_SIZE];
            }
        }
        return -1;
    }

    private void setHashCacheBlock(int hash, long block) {
        if (HASH_CACHE_SIZE > 0) {
            initHashCache();
            int index = hash & (HASH_CACHE_SIZE - 1);
            hashBlocks[index] = hash;
            hashBlocks[index + HASH_CACHE_SIZE] = block;
        }
    }

    private void initHashCache() {
        if (hashBlocks == null) {
            hashBlocks = new long[HASH_CACHE_SIZE * 2];
        }
    }

    
Store a block in the LOB storage.
Params:
lobId – the lob id
seq – the sequence number
pos – the position within the lob
b – the data
compressAlgorithm – the compression algorithm (may be null)/**
     * Store a block in the LOB storage.
     *
     * @param lobId the lob id
     * @param seq the sequence number
     * @param pos the position within the lob
     * @param b the data
     * @param compressAlgorithm the compression algorithm (may be null)
     */
    void storeBlock(long lobId, int seq, long pos, byte[] b,
            String compressAlgorithm) throws SQLException {
        long block;
        boolean blockExists = false;
        if (compressAlgorithm != null) {
            b = compress.compress(b, compressAlgorithm);
        }
        int hash = Arrays.hashCode(b);
        assertHoldsLock(conn.getSession());
        assertHoldsLock(database);
        block = getHashCacheBlock(hash);
        if (block != -1) {
            String sql =  "SELECT COMPRESSED, DATA FROM " + LOB_DATA +
                    " WHERE BLOCK = ?";
            PreparedStatement prep = prepare(sql);
            prep.setLong(1, block);
            ResultSet rs = prep.executeQuery();
            if (rs.next()) {
                boolean compressed = rs.getInt(1) != 0;
                byte[] compare = rs.getBytes(2);
                if (compressed == (compressAlgorithm != null) && Arrays.equals(b, compare)) {
                    blockExists = true;
                }
            }
            reuse(sql, prep);
        }
        if (!blockExists) {
            block = nextBlock++;
            setHashCacheBlock(hash, block);
            String sql = "INSERT INTO " + LOB_DATA +
                    "(BLOCK, COMPRESSED, DATA) VALUES(?, ?, ?)";
            PreparedStatement prep = prepare(sql);
            prep.setLong(1, block);
            prep.setInt(2, compressAlgorithm == null ? 0 : 1);
            prep.setBytes(3, b);
            prep.execute();
            reuse(sql, prep);
        }
        String sql = "INSERT INTO " + LOB_MAP +
                "(LOB, SEQ, POS, HASH, BLOCK) VALUES(?, ?, ?, ?, ?)";
        PreparedStatement prep = prepare(sql);
        prep.setLong(1, lobId);
        prep.setInt(2, seq);
        prep.setLong(3, pos);
        prep.setLong(4, hash);
        prep.setLong(5, block);
        prep.execute();
        reuse(sql, prep);
    }

    @Override
    public Value createBlob(InputStream in, long maxLength) {
        init();
        return addLob(in, maxLength, Value.BLOB, null);
    }

    @Override
    public Value createClob(Reader reader, long maxLength) {
        init();
        long max = maxLength == -1 ? Long.MAX_VALUE : maxLength;
        CountingReaderInputStream in = new CountingReaderInputStream(reader, max);
        return addLob(in, Long.MAX_VALUE, Value.CLOB, in);
    }

    private static void assertNotHolds(Object lock) {
        if (Thread.holdsLock(lock)) {
            throw DbException.throwInternalError(lock.toString());
        }
    }

    
Check whether this thread has synchronized on this object.
Params:
lock – the object/**
     * Check whether this thread has synchronized on this object.
     *
     * @param lock the object
     */
    static void assertHoldsLock(Object lock) {
        if (!Thread.holdsLock(lock)) {
            throw DbException.throwInternalError(lock.toString());
        }
    }

    
An input stream that reads from a LOB.
/**
     * An input stream that reads from a LOB.
     */
    public class LobInputStream extends InputStream {

        
Data from the LOB_MAP table. We cache this to prevent other updates
to the table that contains the LOB column from changing the data
under us.
/**
         * Data from the LOB_MAP table. We cache this to prevent other updates
         * to the table that contains the LOB column from changing the data
         * under us.
         */
        private final long[] lobMapBlocks;

        
index into the lobMapBlocks array.
/**
         * index into the lobMapBlocks array.
         */
        private int lobMapIndex;

        
The remaining bytes in the lob.
/**
         * The remaining bytes in the lob.
         */
        private long remainingBytes;

        
The temporary buffer.
/**
         * The temporary buffer.
         */
        private byte[] buffer;

        
The position within the buffer.
/**
         * The position within the buffer.
         */
        private int bufferPos;


        public LobInputStream(long lobId, long byteCount) throws SQLException {

            // we have to take the lock on the session
            // before the lock on the database to prevent ABBA deadlocks
            assertHoldsLock(conn.getSession());
            assertHoldsLock(database);

            if (byteCount == -1) {
                String sql = "SELECT BYTE_COUNT FROM " + LOBS + " WHERE ID = ?";
                PreparedStatement prep = prepare(sql);
                prep.setLong(1, lobId);
                ResultSet rs = prep.executeQuery();
                if (!rs.next()) {
                    throw DbException.getJdbcSQLException(ErrorCode.IO_EXCEPTION_1,
                            "Missing lob entry: " + lobId);
                }
                byteCount = rs.getLong(1);
                reuse(sql, prep);
            }
            this.remainingBytes = byteCount;

            String sql = "SELECT COUNT(*) FROM " + LOB_MAP + " WHERE LOB = ?";
            PreparedStatement prep = prepare(sql);
            prep.setLong(1, lobId);
            ResultSet rs = prep.executeQuery();
            rs.next();
            int lobMapCount = rs.getInt(1);
            if (lobMapCount == 0) {
                throw DbException.getJdbcSQLException(ErrorCode.IO_EXCEPTION_1,
                        "Missing lob entry: " + lobId);
            }
            reuse(sql, prep);

            this.lobMapBlocks = new long[lobMapCount];

            sql = "SELECT BLOCK FROM " + LOB_MAP + " WHERE LOB = ? ORDER BY SEQ";
            prep = prepare(sql);
            prep.setLong(1, lobId);
            rs = prep.executeQuery();
            int i = 0;
            while (rs.next()) {
                this.lobMapBlocks[i] = rs.getLong(1);
                i++;
            }
            reuse(sql, prep);
        }

        @Override
        public int read() throws IOException {
            fillBuffer();
            if (remainingBytes <= 0) {
                return -1;
            }
            remainingBytes--;
            return buffer[bufferPos++] & 255;
        }

        @Override
        public long skip(long n) throws IOException {
            if (n <= 0) {
                return 0;
            }
            long remaining = n;
            remaining -= skipSmall(remaining);
            if (remaining > BLOCK_LENGTH) {
                while (remaining > BLOCK_LENGTH) {
                    remaining -= BLOCK_LENGTH;
                    remainingBytes -= BLOCK_LENGTH;
                    lobMapIndex++;
                }
                bufferPos = 0;
                buffer = null;
            }
            fillBuffer();
            remaining -= skipSmall(remaining);
            remaining -= super.skip(remaining);
            return n - remaining;
        }

        private int skipSmall(long n) {
            if (buffer != null && bufferPos < buffer.length) {
                int x = MathUtils.convertLongToInt(Math.min(n, buffer.length - bufferPos));
                bufferPos += x;
                remainingBytes -= x;
                return x;
            }
            return 0;
        }

        @Override
        public int available() throws IOException {
            return MathUtils.convertLongToInt(remainingBytes);
        }

        @Override
        public int read(byte[] buff) throws IOException {
            return readFully(buff, 0, buff.length);
        }

        @Override
        public int read(byte[] buff, int off, int length) throws IOException {
            return readFully(buff, off, length);
        }

        private int readFully(byte[] buff, int off, int length) throws IOException {
            if (length == 0) {
                return 0;
            }
            int read = 0;
            while (length > 0) {
                fillBuffer();
                if (remainingBytes <= 0) {
                    break;
                }
                int len = (int) Math.min(length, remainingBytes);
                len = Math.min(len, buffer.length - bufferPos);
                System.arraycopy(buffer, bufferPos, buff, off, len);
                bufferPos += len;
                read += len;
                remainingBytes -= len;
                off += len;
                length -= len;
            }
            return read == 0 ? -1 : read;
        }

        private void fillBuffer() throws IOException {
            if (buffer != null && bufferPos < buffer.length) {
                return;
            }
            if (remainingBytes <= 0) {
                return;
            }
if (lobMapIndex >= lobMapBlocks.length) {
    System.out.println("halt!");
}
            try {
                buffer = readBlock(lobMapBlocks[lobMapIndex]);
                lobMapIndex++;
                bufferPos = 0;
            } catch (SQLException e) {
                throw DbException.convertToIOException(e);
            }
        }

    }

}