[chore] Move to codeberg's exif-terminator (#2536)

vendor/codeberg.org/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 (
+	"io"
+
+	pngstructure "github.com/superseriousbusiness/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 now has
+	// any new chunks in it for us.
+	chunkSlice, err := v.ps.Chunks()
+	if err != nil {
+		return advance, token, err
+	}
+
+	// Write each chunk by passing it
+	// through our custom write func,
+	// which strips out exif and fixes
+	// the CRC of each chunk.
+	chunks := chunkSlice.Chunks()
+	for i, chunk := range chunks {
+		if i <= v.lastWrittenChunk {
+			// Skip already
+			// written chunks.
+			continue
+		}
+
+		// Write this new chunk.
+		if err := v.writeChunk(chunk); err != nil {
+			return advance, token, err
+		}
+		v.lastWrittenChunk = i
+
+		// Zero data; here you
+		// go garbage collector.
+		chunk.Data = nil
+	}
+
+	return advance, token, err
+}
+
+func (v *pngVisitor) writeChunk(chunk *pngstructure.Chunk) error {
+	if chunk.Type == pngstructure.EXifChunkType {
+		// Replace exif data
+		// with zero bytes.
+		clear(chunk.Data)
+	}
+
+	// Fix CRC of each chunk.
+	chunk.UpdateCrc32()
+
+	// finally, write chunk to writer.
+	_, err := chunk.WriteTo(v.writer)
+	return err
+}