Please answer these questions before submitting your issue. Thanks!

What did you do?

1. Save https://play.golang.org/p/6d9xGk8S6f and run it locally to produce bad.gif — 2.5Mb GIF file 1000x1000px and 1000 frames.

2. Save https://play.golang.org/p/SSLx-1wd9N, compile and run it providing bad.gif as first argument, optionally getting its rusage (i.e. with time(1)):

    command time -l ./main bad.gif
   

What did you expect to see?

Decent memory usage, since gif.Decode is documented to only return image for the first frame:

Decode reads a GIF image from r and returns the first embedded image as an image.Image.

What did you see instead?

High memory usage because gif.decoder.decode saves all frames in memory despite being called from gif.Decode that only needs the first frame:

On macOS time -l shows it used more than 800 Mb RSS:

 917983232  maximum resident set size

I have a straightforward optimization for this by making gif.decoder.decode() only store the first frame to gif.decoder.image slice when appropriate (gif.Decode call) while still decoding file to the end so any malformed input is properly handled. Please let me know if this approach is ok and I'll submit a CL.

Memory usage reported with the modifications applied is close to what I'd expect to see for a single image:

  60616704  maximum resident set size

System details

go version go1.9.1 darwin/amd64
GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/tmp/go:/Users/artyom/go"
GORACE=""
GOROOT="/Users/artyom/Library/go"
GOTOOLDIR="/Users/artyom/Library/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/lb/3rk8rqs53czgb4v35w_342xc0000gn/T/go-build415198232=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOROOT/bin/go version: go version go1.9.1 darwin/amd64
GOROOT/bin/go tool compile -V: compile version go1.9.1
uname -v: Darwin Kernel Version 17.0.0: Thu Aug 24 21:48:19 PDT 2017; root:xnu-4570.1.46~2/RELEASE_X86_64
ProductName:	Mac OS X
ProductVersion:	10.13
BuildVersion:	17A405
lldb --version: lldb-900.0.45
  Swift-4.0