use exif-terminator

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

@@ -0,0 +1,93 @@
+/*
+   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 (
+	"encoding/binary"
+	"io"
+
+	pngstructure "github.com/dsoprea/go-png-image-structure/v2"
+)
+
+type pngVisitor struct {
+	ps               *pngstructure.PngSplitter
+	writer           io.Writer
+	lastWrittenChunk int
+}
+
+func (v *pngVisitor) split(data []byte, atEOF bool) (int, []byte, error) {
+	// execute the ps split function to read in data
+	advance, token, err := v.ps.Split(data, atEOF)
+	if err != nil {
+		return advance, token, err
+	}
+
+	// if we haven't written anything at all yet, then write the png header back into the writer first
+	if v.lastWrittenChunk == -1 {
+		if _, err := v.writer.Write(pngstructure.PngSignature[:]); err != nil {
+			return advance, token, err
+		}
+	}
+
+	// check if the splitter has any new chunks in it that we haven't written yet
+	chunkSlice := v.ps.Chunks()
+	chunks := chunkSlice.Chunks()
+	for i, chunk := range chunks {
+		// look through all the chunks in the splitter
+		if i > v.lastWrittenChunk {
+			// we've got a chunk we haven't written yet! write it...
+			if err := v.writeChunk(chunk); err != nil {
+				return advance, token, err
+			}
+			// then remove the data
+			chunk.Data = chunk.Data[:0]
+			// and update
+			v.lastWrittenChunk = i
+		}
+	}
+
+	return advance, token, err
+}
+
+func (v *pngVisitor) writeChunk(chunk *pngstructure.Chunk) error {
+	if err := binary.Write(v.writer, binary.BigEndian, chunk.Length); err != nil {
+		return err
+	}
+
+	if _, err := v.writer.Write([]byte(chunk.Type)); err != nil {
+		return err
+	}
+
+	if chunk.Type == pngstructure.EXifChunkType {
+		blank := make([]byte, len(chunk.Data))
+		if _, err := v.writer.Write(blank); err != nil {
+			return err
+		}
+	} else {
+		if _, err := v.writer.Write(chunk.Data); err != nil {
+			return err
+		}
+	}
+
+	if err := binary.Write(v.writer, binary.BigEndian, chunk.Crc); err != nil {
+		return err
+	}
+
+	return nil
+}