use exif-terminator

vendor/github.com/superseriousbusiness/exif-terminator/terminator.go

@@ -0,0 +1,116 @@
+/*
+   exif-terminator
+   Copyright (C) 2022 SuperSeriousBusiness admin@gotosocial.org
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU Affero General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU Affero General Public License for more details.
+
+   You should have received a copy of the GNU Affero General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+package terminator
+
+import (
+	"bufio"
+	"bytes"
+	"errors"
+	"fmt"
+	"io"
+
+	jpegstructure "github.com/dsoprea/go-jpeg-image-structure/v2"
+	pngstructure "github.com/dsoprea/go-png-image-structure/v2"
+)
+
+func Terminate(in io.Reader, fileSize int, mediaType string) (io.Reader, error) {
+	// to avoid keeping too much stuff in memory we want to pipe data directly
+	pipeReader, pipeWriter := io.Pipe()
+
+	// we don't know ahead of time how long segments might be: they could be as large as
+	// the file itself, so unfortunately we need to allocate a buffer here that'scanner as large
+	// as the file
+	scanner := bufio.NewScanner(in)
+	scanner.Buffer([]byte{}, fileSize)
+	var err error
+
+	switch mediaType {
+	case "image/jpeg", "jpeg", "jpg":
+		err = terminateJpeg(scanner, pipeWriter)
+	case "image/png", "png":
+		// for pngs we need to skip the header bytes, so read them in
+		// and check we're really dealing with a png here
+		header := make([]byte, len(pngstructure.PngSignature))
+		if _, headerError := in.Read(header); headerError != nil {
+			err = headerError
+			break
+		}
+
+		if !bytes.Equal(header, pngstructure.PngSignature[:]) {
+			err = errors.New("could not decode png: invalid header")
+			break
+		}
+
+		err = terminatePng(scanner, pipeWriter)
+	default:
+		err = fmt.Errorf("mediaType %s cannot be processed", mediaType)
+	}
+
+	return pipeReader, err
+}
+
+func terminateJpeg(scanner *bufio.Scanner, writer io.WriteCloser) error {
+	// jpeg visitor is where the spicy hack of streaming the de-exifed data is contained
+	v := &jpegVisitor{
+		writer: writer,
+	}
+
+	// provide the visitor to the splitter so that it triggers on every section scan
+	js := jpegstructure.NewJpegSplitter(v)
+
+	// the visitor also needs to read back the list of segments: for this it needs
+	// to know what jpeg splitter it's attached to, so give it a pointer to the splitter
+	v.js = js
+
+	// use the jpeg splitters 'split' function, which satisfies the bufio.SplitFunc interface
+	scanner.Split(js.Split)
+
+	scanAndClose(scanner, writer)
+	return nil
+}
+
+func terminatePng(scanner *bufio.Scanner, writer io.WriteCloser) error {
+	ps := pngstructure.NewPngSplitter()
+
+	v := &pngVisitor{
+		ps:               ps,
+		writer:           writer,
+		lastWrittenChunk: -1,
+	}
+
+	// use the png visitor's 'split' function, which satisfies the bufio.SplitFunc interface
+	scanner.Split(v.split)
+
+	scanAndClose(scanner, writer)
+	return nil
+}
+
+func scanAndClose(scanner *bufio.Scanner, writer io.WriteCloser) {
+	// scan asynchronously until there's nothing left to scan, and then close the writer
+	// so that the reader on the other side knows that we're done
+	//
+	// due to the nature of io.Pipe, writing won't actually work
+	// until the pipeReader starts being read by the caller, which
+	// is why we do this asynchronously
+	go func() {
+		for scanner.Scan() {
+		}
+		writer.Close()
+	}()
+}