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tailfile.go
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tailfile.go
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// Package tailfile provides helper functions to read the nth lines of any
// ReadSeeker.
package tailfile // import "github.com/docker/docker/pkg/tailfile"
import (
"bufio"
"bytes"
"context"
"errors"
"io"
"os"
)
const blockSize = 1024
var eol = []byte("\n")
// ErrNonPositiveLinesNumber is an error returned if the lines number was negative.
var ErrNonPositiveLinesNumber = errors.New("The number of lines to extract from the file must be positive")
// TailFile returns last n lines of the passed in file.
func TailFile(f *os.File, n int) ([][]byte, error) {
size, err := f.Seek(0, io.SeekEnd)
if err != nil {
return nil, err
}
rAt := io.NewSectionReader(f, 0, size)
r, nLines, err := NewTailReader(context.Background(), rAt, n)
if err != nil {
return nil, err
}
buf := make([][]byte, 0, nLines)
scanner := bufio.NewScanner(r)
for scanner.Scan() {
buf = append(buf, scanner.Bytes())
}
return buf, nil
}
// SizeReaderAt is an interface used to get a ReaderAt as well as the size of the underlying reader.
// Note that the size of the underlying reader should not change when using this interface.
type SizeReaderAt interface {
io.ReaderAt
Size() int64
}
// NewTailReader scopes the passed in reader to just the last N lines passed in
func NewTailReader(ctx context.Context, r SizeReaderAt, reqLines int) (io.Reader, int, error) {
return NewTailReaderWithDelimiter(ctx, r, reqLines, eol)
}
// NewTailReaderWithDelimiter scopes the passed in reader to just the last N lines passed in
// In this case a "line" is defined by the passed in delimiter.
//
// Delimiter lengths should be generally small, no more than 12 bytes
func NewTailReaderWithDelimiter(ctx context.Context, r SizeReaderAt, reqLines int, delimiter []byte) (io.Reader, int, error) {
if reqLines < 1 {
return nil, 0, ErrNonPositiveLinesNumber
}
if len(delimiter) == 0 {
return nil, 0, errors.New("must provide a delimiter")
}
var (
size = r.Size()
tailStart int64
tailEnd = size
found int
)
if int64(len(delimiter)) >= size {
return bytes.NewReader(nil), 0, nil
}
scanner := newScanner(r, delimiter)
for scanner.Scan(ctx) {
if err := scanner.Err(); err != nil {
return nil, 0, scanner.Err()
}
found++
if found == 1 {
tailEnd = scanner.End()
}
if found == reqLines {
break
}
}
tailStart = scanner.Start(ctx)
if found == 0 {
return bytes.NewReader(nil), 0, nil
}
if found < reqLines && tailStart != 0 {
tailStart = 0
}
return io.NewSectionReader(r, tailStart, tailEnd-tailStart), found, nil
}
func newScanner(r SizeReaderAt, delim []byte) *scanner {
size := r.Size()
readSize := blockSize
if readSize > int(size) {
readSize = int(size)
}
// silly case...
if len(delim) >= readSize/2 {
readSize = len(delim)*2 + 2
}
return &scanner{
r: r,
pos: size,
buf: make([]byte, readSize),
delim: delim,
}
}
type scanner struct {
r SizeReaderAt
pos int64
buf []byte
delim []byte
err error
idx int
}
func (s *scanner) Start(ctx context.Context) int64 {
if s.idx > 0 {
idx := bytes.LastIndex(s.buf[:s.idx], s.delim)
if idx >= 0 {
return s.pos + int64(idx) + int64(len(s.delim))
}
}
// slow path
buf := make([]byte, len(s.buf))
copy(buf, s.buf)
readAhead := &scanner{
r: s.r,
pos: s.pos,
delim: s.delim,
idx: s.idx,
buf: buf,
}
if !readAhead.Scan(ctx) {
return 0
}
return readAhead.End()
}
func (s *scanner) End() int64 {
return s.pos + int64(s.idx) + int64(len(s.delim))
}
func (s *scanner) Err() error {
return s.err
}
func (s *scanner) Scan(ctx context.Context) bool {
if s.err != nil {
return false
}
for {
select {
case <-ctx.Done():
s.err = ctx.Err()
return false
default:
}
idx := s.idx - len(s.delim)
if idx < 0 {
readSize := int(s.pos)
if readSize > len(s.buf) {
readSize = len(s.buf)
}
if readSize < len(s.delim) {
return false
}
offset := s.pos - int64(readSize)
n, err := s.r.ReadAt(s.buf[:readSize], offset)
if err != nil && err != io.EOF {
s.err = err
return false
}
s.pos -= int64(n)
idx = n
}
s.idx = bytes.LastIndex(s.buf[:idx], s.delim)
if s.idx >= 0 {
return true
}
if len(s.delim) > 1 && s.pos > 0 {
// in this case, there may be a partial delimiter at the front of the buffer, so set the position forward
// up to the maximum size partial that could be there so it can be read again in the next iteration with any
// potential remainder.
// An example where delimiter is `####`:
// [##asdfqwerty]
// ^
// This resets the position to where the arrow is pointing.
// It could actually check if a partial exists and at the front, but that is pretty similar to the indexing
// code above though a bit more complex since each byte has to be checked (`len(delimiter)-1`) factorial).
// It's much simpler and cleaner to just re-read `len(delimiter)-1` bytes again.
s.pos += int64(len(s.delim)) - 1
}
}
}