go-exif/v3/data_layer.go
Dustin Oprea 312218d7b1 Use encapsulated data/reader rather than bytes
Given a stream of data, it is possible to determine the beginning of
EXIF data but not the end. Therefore, either an image-aware
implementation must know how to parse an image and extract the EXIF
data or a brute-force search implementation (one of which is provided
by this project) must find the start anchor and then return all bytes
from that to the end of the file.

We have been made aware of some use-cases where a brute-force search
might be unavoidable due to trust or stability issues with the image
structure. This leads to large allocations. This can be avoided by
accomodating support that will allow for both a byte-slice or an
`io.ReadSeeker`. Since the EXIF structure is typically not read-
intensive (a couple of kilobytes if no thumbnail is present), this
should have a minimal performance impact.

Closes #42
2020-07-11 12:12:48 -04:00

51 lines
1.1 KiB
Go

package exif
import (
"io"
"github.com/dsoprea/go-logging"
"github.com/dsoprea/go-utility/filesystem"
)
type ExifBlobSeeker interface {
GetReadSeeker(initialOffset int64) (rs io.ReadSeeker, err error)
}
// ExifReadSeeker knows how to retrieve data from the EXIF blob relative to the
// beginning of the blob (so, absolute position (0) is the first byte of the
// EXIF data).
type ExifReadSeeker struct {
rs io.ReadSeeker
}
func NewExifReadSeeker(rs io.ReadSeeker) *ExifReadSeeker {
return &ExifReadSeeker{
rs: rs,
}
}
func NewExifReadSeekerWithBytes(exifData []byte) *ExifReadSeeker {
sb := rifs.NewSeekableBufferWithBytes(exifData)
edbs := NewExifReadSeeker(sb)
return edbs
}
// Fork creates a new ReadSeeker instead that wraps a BouncebackReader to
// maintain its own position in the stream.
func (edbs *ExifReadSeeker) GetReadSeeker(initialOffset int64) (rs io.ReadSeeker, err error) {
defer func() {
if state := recover(); state != nil {
err = log.Wrap(state.(error))
}
}()
br, err := rifs.NewBouncebackReader(edbs.rs)
log.PanicIf(err)
_, err = br.Seek(initialOffset, io.SeekStart)
log.PanicIf(err)
return br, nil
}