rclone/lib/readers/repeatable.go

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package readers
import (
"io"
"sync"
"github.com/pkg/errors"
)
// A RepeatableReader implements the io.ReadSeeker it allow to seek cached data
// back and forth within the reader but will only read data from the internal Reader as necessary
// and will play nicely with the Account and io.LimitedReader to reflect current speed
type RepeatableReader struct {
mu sync.Mutex // protect against concurrent use
in io.Reader // Input reader
i int64 // current reading index
b []byte // internal cache buffer
}
var _ io.ReadSeeker = (*RepeatableReader)(nil)
// Seek implements the io.Seeker interface.
// If seek position is passed the cache buffer length the function will return
// the maximum offset that can be used and "fs.RepeatableReader.Seek: offset is unavailable" Error
func (r *RepeatableReader) Seek(offset int64, whence int) (int64, error) {
r.mu.Lock()
defer r.mu.Unlock()
var abs int64
cacheLen := int64(len(r.b))
switch whence {
case io.SeekStart:
abs = offset
case io.SeekCurrent:
abs = r.i + offset
case io.SeekEnd:
abs = cacheLen + offset
default:
return 0, errors.New("fs.RepeatableReader.Seek: invalid whence")
}
if abs < 0 {
return 0, errors.New("fs.RepeatableReader.Seek: negative position")
}
if abs > cacheLen {
return offset - (abs - cacheLen), errors.New("fs.RepeatableReader.Seek: offset is unavailable")
}
r.i = abs
return abs, nil
}
// Read data from original Reader into bytes
// Data is either served from the underlying Reader or from cache if was already read
func (r *RepeatableReader) Read(b []byte) (n int, err error) {
r.mu.Lock()
defer r.mu.Unlock()
cacheLen := int64(len(r.b))
if r.i == cacheLen {
n, err = r.in.Read(b)
if n > 0 {
r.b = append(r.b, b[:n]...)
}
} else {
n = copy(b, r.b[r.i:])
}
r.i += int64(n)
return n, err
}
// NewRepeatableReader create new repeatable reader from Reader r
func NewRepeatableReader(r io.Reader) *RepeatableReader {
return &RepeatableReader{in: r}
}
// NewRepeatableReaderSized create new repeatable reader from Reader r
// with an initial buffer of size.
func NewRepeatableReaderSized(r io.Reader, size int) *RepeatableReader {
return &RepeatableReader{
in: r,
b: make([]byte, 0, size),
}
}
// NewRepeatableLimitReader create new repeatable reader from Reader r
// with an initial buffer of size wrapped in an io.LimitReader to read
// only size.
func NewRepeatableLimitReader(r io.Reader, size int) *RepeatableReader {
return NewRepeatableReaderSized(io.LimitReader(r, int64(size)), size)
}
// NewRepeatableReaderBuffer create new repeatable reader from Reader r
// using the buffer passed in.
func NewRepeatableReaderBuffer(r io.Reader, buf []byte) *RepeatableReader {
return &RepeatableReader{
in: r,
b: buf[:0],
}
}
// NewRepeatableLimitReaderBuffer create new repeatable reader from
// Reader r and buf wrapped in an io.LimitReader to read only size.
func NewRepeatableLimitReaderBuffer(r io.Reader, buf []byte, size int64) *RepeatableReader {
return NewRepeatableReaderBuffer(io.LimitReader(r, size), buf)
}