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reader.go
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reader.go
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// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this
// file, You can obtain one at http://mozilla.org/MPL/2.0/.
package biglog
import (
"errors"
"fmt"
"io"
"sync"
"sync/atomic"
)
// ErrInvalidReader is returned on read with nil pointers
var ErrInvalidReader = errors.New("biglog: invalid reader - use NewReader")
// Reader keeps the state among separate concurrent reads
// Readers handle segment transitions transparently
type Reader struct {
mu sync.Mutex
bl *BigLog
seg *segment
dFO int64
}
// NewReader returns a Reader that will start reading from a given offset
// the reader implements the io.ReaderCloser interface
func NewReader(bl *BigLog, from int64) (r *Reader, ret int64, err error) {
seg, RO, err := bl.locateOffset(from)
if err != nil {
return nil, -1, err
}
ret = from
l, err := seg.Lookup(RO)
if err == ErrEmbeddedOffset {
ret = from - int64(RO-l.fRO)
} else if err != nil {
return nil, -1, err
}
r = &Reader{
dFO: l.dFO,
bl: bl,
}
r.setSegment(seg)
bl.addReader(r)
return r, ret, err
}
// Read reads bytes from the big log, offset is auto-incremented in every read
func (r *Reader) Read(b []byte) (n int, err error) {
r.mu.Lock()
defer r.mu.Unlock()
if r == nil || r.seg == nil {
return 0, ErrInvalidReader
}
var sumn int
var seg *segment
for {
n, err = r.seg.ReadAt(b[sumn:], r.dFO)
r.dFO += int64(n)
if err != io.EOF {
break
}
seg = r.nextSeg()
if seg == nil {
err = io.EOF
break
}
r.setSegment(seg)
r.dFO = headerSize
sumn += n
}
n += sumn
return n, err
}
// we need to scan all segments every time since
// the slice could have changed since the last read
func (r *Reader) nextSeg() (seg *segment) {
segs := r.bl.segments()
i := indexOfSegment(segs, r.seg.baseOffset)
if i < 0 || i == len(segs)-1 {
return nil
}
return segs[i+1]
}
// Close frees up the segments and renders the reader unusable
// returns nil error to satisfy io.Closer
func (r *Reader) Close() error {
atomic.AddInt32(r.seg.readers, -1)
r.bl.removeReader(r)
r = nil
return nil
}
// Seek implements the io.Seeker interface for a reader.
// Only whence=1 (relative) is not supported since the data reader has no
// knowledge of where it is once it starts reading the biglog.
func (r *Reader) Seek(offset int64, whence int) (ret int64, err error) {
r.mu.Lock()
defer r.mu.Unlock()
switch whence {
case 0:
case 2:
offset += r.bl.Latest()
default:
panic(fmt.Sprintf("unsupported index reader whence = %d", whence))
}
if r == nil || r.seg == nil {
return -1, ErrInvalidReader
}
seg, RO, err := r.bl.locateOffset(offset)
if err != nil {
return -1, err
}
ret = offset
l, err := seg.Lookup(RO)
if err == ErrEmbeddedOffset {
ret = offset - int64(RO-l.fRO)
} else if err != nil {
return -1, err
}
r.setSegment(seg)
r.dFO = l.dFO
return ret, err
}
func (r *Reader) setSegment(seg *segment) {
if r.seg != nil {
atomic.AddInt32(r.seg.readers, -1)
}
atomic.AddInt32(seg.readers, 1)
r.seg = seg
}