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writer.go
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/
writer.go
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// Copyright 2011 The LevelDB-Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package table
import (
"bufio"
"bytes"
"encoding/binary"
"errors"
"fmt"
"io"
"camlistore.org/third_party/code.google.com/p/leveldb-go/leveldb/crc"
"camlistore.org/third_party/code.google.com/p/leveldb-go/leveldb/db"
"camlistore.org/third_party/code.google.com/p/snappy-go/snappy"
)
// indexEntry is a block handle and the length of the separator key.
type indexEntry struct {
bh blockHandle
keyLen int
}
// Writer is a table writer. It implements the DB interface, as documented
// in the leveldb/db package.
type Writer struct {
writer io.Writer
bufWriter *bufio.Writer
closer io.Closer
err error
// The next four fields are copied from a db.Options.
blockRestartInterval int
blockSize int
cmp db.Comparer
compression db.Compression
// A table is a series of blocks and a block's index entry contains a
// separator key between one block and the next. Thus, a finished block
// cannot be written until the first key in the next block is seen.
// pendingBH is the blockHandle of a finished block that is waiting for
// the next call to Set. If the writer is not in this state, pendingBH
// is zero.
pendingBH blockHandle
// offset is the offset (relative to the table start) of the next block
// to be written.
offset uint64
// prevKey is a copy of the key most recently passed to Set.
prevKey []byte
// indexKeys and indexEntries hold the separator keys between each block
// and the successor key for the final block. indexKeys contains the key's
// bytes concatenated together. The keyLen field of each indexEntries
// element is the length of the respective separator key.
indexKeys []byte
indexEntries []indexEntry
// The next three fields hold data for the current block:
// - buf is the accumulated uncompressed bytes,
// - nEntries is the number of entries,
// - restarts are the offsets (relative to the block start) of each
// restart point.
buf bytes.Buffer
nEntries int
restarts []uint32
// compressedBuf is the destination buffer for snappy compression. It is
// re-used over the lifetime of the writer, avoiding the allocation of a
// temporary buffer for each block.
compressedBuf []byte
// tmp is a scratch buffer, large enough to hold either footerLen bytes,
// blockTrailerLen bytes, or (5 * binary.MaxVarintLen64) bytes.
tmp [50]byte
}
// Writer implements the db.DB interface.
var _ db.DB = (*Writer)(nil)
// Get is provided to implement the DB interface, but returns an error, as a
// Writer cannot read from a table.
func (w *Writer) Get(key []byte, o *db.ReadOptions) ([]byte, error) {
return nil, errors.New("leveldb/table: cannot Get from a write-only table")
}
// Delete is provided to implement the DB interface, but returns an error, as a
// Writer can only append key/value pairs.
func (w *Writer) Delete(key []byte, o *db.WriteOptions) error {
return errors.New("leveldb/table: cannot Delete from a table")
}
// Find is provided to implement the DB interface, but returns an error, as a
// Writer cannot read from a table.
func (w *Writer) Find(key []byte, o *db.ReadOptions) db.Iterator {
return &tableIter{
err: errors.New("leveldb/table: cannot Find from a write-only table"),
}
}
// Set implements DB.Set, as documented in the leveldb/db package. For a given
// Writer, the keys passed to Set must be in increasing order.
func (w *Writer) Set(key, value []byte, o *db.WriteOptions) error {
if w.err != nil {
return w.err
}
if w.cmp.Compare(w.prevKey, key) >= 0 {
w.err = fmt.Errorf("leveldb/table: Set called in non-increasing key order: %q, %q", w.prevKey, key)
return w.err
}
w.flushPendingBH(key)
w.append(key, value, w.nEntries%w.blockRestartInterval == 0)
// If the estimated block size is sufficiently large, finish the current block.
if w.buf.Len()+4*(len(w.restarts)+1) >= w.blockSize {
bh, err := w.finishBlock()
if err != nil {
w.err = err
return w.err
}
w.pendingBH = bh
}
return nil
}
// flushPendingBH adds any pending block handle to the index entries.
func (w *Writer) flushPendingBH(key []byte) {
if w.pendingBH.length == 0 {
// A valid blockHandle must be non-zero.
// In particular, it must have a non-zero length.
return
}
n0 := len(w.indexKeys)
w.indexKeys = w.cmp.AppendSeparator(w.indexKeys, w.prevKey, key)
n1 := len(w.indexKeys)
w.indexEntries = append(w.indexEntries, indexEntry{w.pendingBH, n1 - n0})
w.pendingBH = blockHandle{}
}
// append appends a key/value pair, which may also be a restart point.
func (w *Writer) append(key, value []byte, restart bool) {
nShared := 0
if restart {
w.restarts = append(w.restarts, uint32(w.buf.Len()))
} else {
nShared = db.SharedPrefixLen(w.prevKey, key)
}
w.prevKey = append(w.prevKey[:0], key...)
w.nEntries++
n := binary.PutUvarint(w.tmp[0:], uint64(nShared))
n += binary.PutUvarint(w.tmp[n:], uint64(len(key)-nShared))
n += binary.PutUvarint(w.tmp[n:], uint64(len(value)))
w.buf.Write(w.tmp[:n])
w.buf.Write(key[nShared:])
w.buf.Write(value)
}
// finishBlock finishes the current block and returns its block handle, which is
// its offset and length in the table.
func (w *Writer) finishBlock() (blockHandle, error) {
// Write the restart points to the buffer.
if w.nEntries == 0 {
// Every block must have at least one restart point.
w.restarts = w.restarts[:1]
w.restarts[0] = 0
}
tmp4 := w.tmp[:4]
for _, x := range w.restarts {
binary.LittleEndian.PutUint32(tmp4, x)
w.buf.Write(tmp4)
}
binary.LittleEndian.PutUint32(tmp4, uint32(len(w.restarts)))
w.buf.Write(tmp4)
// Compress the buffer, discarding the result if the improvement
// isn't at least 12.5%.
b := w.buf.Bytes()
w.tmp[0] = noCompressionBlockType
if w.compression == db.SnappyCompression {
compressed, err := snappy.Encode(w.compressedBuf, b)
if err != nil {
return blockHandle{}, err
}
w.compressedBuf = compressed[:cap(compressed)]
if len(compressed) < len(b)-len(b)/8 {
w.tmp[0] = snappyCompressionBlockType
b = compressed
}
}
// Calculate the checksum.
checksum := crc.New(b).Update(w.tmp[:1]).Value()
binary.LittleEndian.PutUint32(w.tmp[1:5], checksum)
// Write the bytes to the file.
if _, err := w.writer.Write(b); err != nil {
return blockHandle{}, err
}
if _, err := w.writer.Write(w.tmp[:5]); err != nil {
return blockHandle{}, err
}
bh := blockHandle{w.offset, uint64(len(b))}
w.offset += uint64(len(b)) + blockTrailerLen
// Reset the per-block state.
w.buf.Reset()
w.nEntries = 0
w.restarts = w.restarts[:0]
return bh, nil
}
// Close implements DB.Close, as documented in the leveldb/db package.
func (w *Writer) Close() (err error) {
defer func() {
if w.closer == nil {
return
}
err1 := w.closer.Close()
if err == nil {
err = err1
}
w.closer = nil
}()
if w.err != nil {
return w.err
}
// Finish the last data block, or force an empty data block if there
// aren't any data blocks at all.
if w.nEntries > 0 || len(w.indexEntries) == 0 {
bh, err := w.finishBlock()
if err != nil {
w.err = err
return w.err
}
w.pendingBH = bh
w.flushPendingBH(nil)
}
// Write the (empty) metaindex block.
metaindexBlockHandle, err := w.finishBlock()
if err != nil {
w.err = err
return w.err
}
// Write the index block.
// writer.append uses w.tmp[:3*binary.MaxVarintLen64].
i0, tmp := 0, w.tmp[3*binary.MaxVarintLen64:5*binary.MaxVarintLen64]
for _, ie := range w.indexEntries {
n := encodeBlockHandle(tmp, ie.bh)
i1 := i0 + ie.keyLen
w.append(w.indexKeys[i0:i1], tmp[:n], true)
i0 = i1
}
indexBlockHandle, err := w.finishBlock()
if err != nil {
w.err = err
return w.err
}
// Write the table footer.
footer := w.tmp[:footerLen]
for i := range footer {
footer[i] = 0
}
n := encodeBlockHandle(footer, metaindexBlockHandle)
encodeBlockHandle(footer[n:], indexBlockHandle)
copy(footer[footerLen-len(magic):], magic)
if _, err := w.writer.Write(footer); err != nil {
w.err = err
return w.err
}
// Flush the buffer.
if w.bufWriter != nil {
if err := w.bufWriter.Flush(); err != nil {
w.err = err
return err
}
}
// Make any future calls to Set or Close return an error.
w.err = errors.New("leveldb/table: writer is closed")
return nil
}
// NewWriter returns a new table writer for the file. Closing the writer will
// close the file.
func NewWriter(f File, o *db.Options) *Writer {
w := &Writer{
closer: f,
blockRestartInterval: o.GetBlockRestartInterval(),
blockSize: o.GetBlockSize(),
cmp: o.GetComparer(),
compression: o.GetCompression(),
prevKey: make([]byte, 0, 256),
restarts: make([]uint32, 0, 256),
}
if f == nil {
w.err = errors.New("leveldb/table: nil file")
return w
}
// If f does not have a Flush method, do our own buffering.
type flusher interface {
Flush() error
}
if _, ok := f.(flusher); ok {
w.writer = f
} else {
w.bufWriter = bufio.NewWriter(f)
w.writer = w.bufWriter
}
return w
}