image/png: decoding images with truecolor transparency returns wrong error

[artyom]

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

go version go1.7.1 darwin/amd64

What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/tmp/gopath:/Users/artyom/gopath"
GORACE=""
GOROOT="/Users/artyom/Library/go"
GOTOOLDIR="/Users/artyom/Library/go/pkg/tool/darwin_amd64"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/lb/3rk8rqs53czgb4v35w_342xc0000gn/T/go-build425021710=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"

What did you do?

If possible, provide a recipe for reproducing the error.
A complete runnable program is good.
A link on play.golang.org is best.

1. Save linked original png file or its attached copy as /tmp/bad.png.
2. Save and run https://play.golang.org/p/7KawpAoMqX

What did you expect to see?

Either no error message or png.UnsupportedError text.

What did you see instead?

png: invalid format: chunk out of order

---

Initially I thought that file was incorrect but that doesn't seem to be the case:

$ pngcheck -v -p /tmp/bad.png 
File: /tmp/bad.png (3616 bytes)
  chunk IHDR at offset 0x0000c, length 13
    128 x 128 image, 24-bit RGB, non-interlaced
  chunk tRNS at offset 0x00025, length 6
    red = 0x00ff, green = 0x00ff, blue = 0x00ff
  chunk IDAT at offset 0x00037, length 3541
    zlib: deflated, 32K window, superfast compression
  chunk IEND at offset 0x00e18, length 0
No errors detected in /tmp/bad.png (4 chunks, 92.6% compression).

It seems to have tRNS chunk but no PLTE chunk, as it's a truecolor transparent file, which is valid per spec. Ordering seem to be valid as well.

Applying the following patch:

diff --git i/src/image/png/reader.go w/src/image/png/reader.go
index 2dd5ed8..6159323 100644
--- i/src/image/png/reader.go
+++ w/src/image/png/reader.go
@@ -709,7 +709,7 @@ func (d *decoder) parseChunk() error {
        d.stage = dsSeenPLTE
        return d.parsePLTE(length)
    case "tRNS":
-       if d.stage != dsSeenPLTE {
+       if d.stage != dsSeenPLTE && d.stage != dsSeenIHDR {
            return chunkOrderError
        }
        d.stage = dsSeentRNS

results in error reported as png: unsupported feature: truecolor transparency which seems to be the correct message for this case.

/cc @nigeltao

[gopherbot]

CL https://golang.org/cl/32139 mentions this issue.