image/png: DecodeConfig gives incorrect result for paletted images with transparency

[depp]

The color model returned from png.DecodeConfig is different from the one returned from png.Decode, because it incorrectly omits the alpha channel data.

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

$ go version
go version go1.19 darwin/amd64

Does this issue reproduce with the latest release?

Yes.

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

go env Output

GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/depp/Library/Caches/go-build"
GOENV="/Users/depp/Library/Application Support/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/depp/Projects/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/depp/Projects/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/Cellar/go/1.19/libexec"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.19/libexec/pkg/tool/darwin_amd64"
GOVCS=""
GOVERSION="go1.19"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/dev/null"
GOWORK=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -arch x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/1l/g8_2q7j95rndy_wpjnd5b1h00000gn/T/go-build1338501950=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

See go.dev/play snippet

When run, prints

Color 0 (png.DecodeConfig): {0 0 0 255}
Color 0 (png.Decode): {0 0 0 0}

Program exited.

What did you expect to see?

Same color model, no matter whether I used DecodeConfig or Decode

What did you see instead?

Palette entry 0 has alpha = 255 (incorrect) if returned by DecodeConfig, alpha = 0 (correct) if returned by Decode.

[ianlancetaylor]

CC @nigeltao

[gopherbot]

Change https://go.dev/cl/424917 mentions this issue: image/png: have DecodeConfig read tRNS chunks