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// 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.
//go:build ignore
// +build ignore
package main
// 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
//
// --------
//
// Usage:
// go run strip-png-ancillary-chunks.go < in.png > out.png
//
// Adding a -random=123 flag should not change the output:
// go run strip-png-ancillary-chunks.go -random=123 < in.png > out.png
import (
"encoding/binary"
"flag"
"io"
"math/rand"
"os"
"os/signal"
"syscall"
)
var (
randomFlag = flag.Int64("random", 0, "If non-zero, the seed for generating smaller reads")
)
// 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
// RandomSizedReader wraps another io.Reader such that the Reader.Read []byte
// argument has randomized lengths, up to a maximum of 9 bytes.
//
// This is slower but helps manually test that PNGAncillaryChunkStripper
// produces identical output regardless of whether Read is called many times
// (with small buffers) or few times (with large buffers). Specifically, this
// exercises having Read boundaries straddle chunk boundaries.
type RandomSizedReader struct {
// Reader is the wrapped io.Reader.
Reader io.Reader
// Rand is the random number generator.
Rand *rand.Rand
}
// Read implements io.Reader.
func (r *RandomSizedReader) Read(p []byte) (int, error) {
if n := r.Rand.Intn(10); len(p) > n {
p = p[:n]
}
return r.Reader.Read(p)
}
// 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).
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
}
}
}
func main() {
// Piping to /usr/bin/head can cause SIGPIPE (in the upstream process)
// since head can exit early, leaving this process with pending output. The
// command line experience can be cleaner if this program ignores SIGPIPE.
signal.Ignore(syscall.SIGPIPE)
flag.Parse()
r := io.Reader(&PNGAncillaryChunkStripper{
Reader: os.Stdin,
})
if *randomFlag != 0 {
r = &RandomSizedReader{
Reader: r,
Rand: rand.New(rand.NewSource(*randomFlag)),
}
}
io.Copy(os.Stdout, r)
}