gotosocial/internal/media/png-stripper.go

/*
   GoToSocial
   Copyright (C) 2021-2023 GoToSocial Authors 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 media

/*
   The code in this file is taken from the following source:
   https://github.com/google/wuffs/blob/414a011491ff513b86d8694c5d71800f3cb5a715/script/strip-png-ancillary-chunks.go

   It presents a workaround for this issue: https://github.com/golang/go/issues/43382

   The license for the copied code is reproduced below:

      Copyright 2021 The Wuffs Authors.

      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at

         https://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
      limitations under the License.
*/

// strip-png-ancillary-chunks.go copies PNG data from stdin to stdout, removing
// any ancillary chunks.
//
// Specification-compliant PNG decoders are required to honor critical chunks
// but may ignore ancillary (non-critical) chunks. Stripping out ancillary
// chunks before decoding should mean that different PNG decoders will agree on
// the decoded output regardless of which ancillary chunk types they choose to
// honor. Specifically, some PNG decoders may implement color and gamma
// correction but not all do.
//
// This program will strip out all ancillary chunks, but it should be
// straightforward to copy-paste-and-modify it to strip out only certain chunk
// types (e.g. only "tRNS" transparency chunks).
//
// --------
//
// A PNG file consists of an 8-byte magic identifier and then a series of
// chunks. Each chunk is:
//
//  - a 4-byte uint32 payload length N.
//  - a 4-byte chunk type (e.g. "gAMA" for gamma correction metadata).
//  - an N-byte payload.
//  - a 4-byte CRC-32 checksum of the previous (N + 4) bytes, including the
//    chunk type but excluding the payload length.
//
// Chunk types consist of 4 ASCII letters. The upper-case / lower-case bit of
// the first letter denote critical or ancillary chunks: "IDAT" and "PLTE" are
// critical, "gAMA" and "tEXt" are ancillary. See
// https://www.w3.org/TR/2003/REC-PNG-20031110/#5Chunk-naming-conventions
//
// --------

import (
	"encoding/binary"
	"io"
)

// chunkTypeAncillaryBit is whether the first byte of a big-endian uint32 chunk
// type (the first of four ASCII letters) is lower-case.
const chunkTypeAncillaryBit = 0x20000000

// pngAncillaryChunkStripper wraps another io.Reader to strip ancillary chunks,
// if the data is in the PNG file format. If the data isn't PNG, it is passed
// through unmodified.
type pngAncillaryChunkStripper struct {
	// Reader is the wrapped io.Reader.
	Reader io.Reader

	// stickyErr is the first error returned from the wrapped io.Reader.
	stickyErr error

	// buffer[rIndex:wIndex] holds data read from the wrapped io.Reader that
	// wasn't passed through yet.
	buffer [8]byte
	rIndex int
	wIndex int

	// pending and discard is the number of remaining bytes for (and whether to
	// discard or pass through) the current chunk-in-progress.
	pending int64
	discard bool

	// notPNG is set true if the data stream doesn't start with the 8-byte PNG
	// magic identifier. If true, the wrapped io.Reader's data (including the
	// first up-to-8 bytes) is passed through without modification.
	notPNG bool

	// seenMagic is whether we've seen the 8-byte PNG magic identifier.
	seenMagic bool
}

// Read implements io.Reader.
func (r *pngAncillaryChunkStripper) Read(p []byte) (int, error) {
	for {
		// If the wrapped io.Reader returned a non-nil error, drain r.buffer
		// (what data we have) and return that error (if fully drained).
		if r.stickyErr != nil {
			n := copy(p, r.buffer[r.rIndex:r.wIndex])
			r.rIndex += n
			if r.rIndex < r.wIndex {
				return n, nil
			}
			return n, r.stickyErr
		}

		// Handle trivial requests, including draining our buffer.
		if len(p) == 0 {
			return 0, nil
		} else if r.rIndex < r.wIndex {
			n := copy(p, r.buffer[r.rIndex:r.wIndex])
			r.rIndex += n
			return n, nil
		}

		// From here onwards, our buffer is drained: r.rIndex == r.wIndex.

		// Handle non-PNG input.
		if r.notPNG {
			return r.Reader.Read(p)
		}

		// Continue processing any PNG chunk that's in progress, whether
		// discarding it or passing it through.
		for r.pending > 0 {
			if int64(len(p)) > r.pending {
				p = p[:r.pending]
			}
			n, err := r.Reader.Read(p)
			r.pending -= int64(n)
			r.stickyErr = err
			if r.discard {
				continue
			}
			return n, err
		}

		// We're either expecting the 8-byte PNG magic identifier or the 4-byte
		// PNG chunk length + 4-byte PNG chunk type. Either way, read 8 bytes.
		r.rIndex = 0
		r.wIndex, r.stickyErr = io.ReadFull(r.Reader, r.buffer[:8])
		if r.stickyErr != nil {
			// Undo io.ReadFull converting io.EOF to io.ErrUnexpectedEOF.
			if r.stickyErr == io.ErrUnexpectedEOF {
				r.stickyErr = io.EOF
			}
			continue
		}

		// Process those 8 bytes, either:
		//  - a PNG chunk (if we've already seen the PNG magic identifier),
		//  - the PNG magic identifier itself (if the input is a PNG) or
		//  - something else (if it's not a PNG).
		//nolint:gocritic
		if r.seenMagic {
			// The number of pending bytes is equal to (N + 4) because of the 4
			// byte trailer, a checksum.
			r.pending = int64(binary.BigEndian.Uint32(r.buffer[:4])) + 4
			chunkType := binary.BigEndian.Uint32(r.buffer[4:])
			r.discard = (chunkType & chunkTypeAncillaryBit) != 0
			if r.discard {
				r.rIndex = r.wIndex
			}
		} else if string(r.buffer[:8]) == "\x89PNG\x0D\x0A\x1A\x0A" {
			r.seenMagic = true
		} else {
			r.notPNG = true
		}
	}
}