mime: WordDecoder#Decode panics for invalid input

[hirochachacha]

Please answer these questions before submitting your issue. Thanks!

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.

package main

import (
	"fmt"
	"io"
	"mime"
)

type charsetError string

func (e charsetError) Error() string {
	return fmt.Sprintf("charset not supported: %q", string(e))
}

var rfc2047Decoder = mime.WordDecoder{
	CharsetReader: func(charset string, input io.Reader) (io.Reader, error) {
		return nil, charsetError(charset)
	},
}

func main() {
	_, err := rfc2047Decoder.Decode("=????=")
	fmt.Println(err)
}

What did you expect to see?

error

What did you see instead?

panic

Does this issue reproduce with the latest release (go1.8)?

I think so

System details

go version devel +33f2733 Sat Mar 4 20:07:20 2017 +0900 darwin/amd64
GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/hiro/.go"
GORACE=""
GOROOT="/Users/hiro/go"
GOTOOLDIR="/Users/hiro/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/wq/dwn8hs0x7njbzty9f68y61700000gn/T/go-build675122905=/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 devel +33f2733 Sat Mar 4 20:07:20 2017 +0900 darwin/amd64
GOROOT/bin/go tool compile -V: compile version devel +33f2733 Sat Mar 4 20:07:20 2017 +0900 X:framepointer
uname -v: Darwin Kernel Version 16.4.0: Thu Dec 22 22:53:21 PST 2016; root:xnu-3789.41.3~3/RELEASE_X86_64
ProductName:	Mac OS X
ProductVersion:	10.12.3
BuildVersion:	16D32
lldb --version: lldb-360.1.70
gdb --version: GNU gdb (GDB) 7.12.1

[gopherbot]

CL https://golang.org/cl/37812 mentions this issue.

[gopherbot]

CL https://golang.org/cl/37912 mentions this issue.

[hirochachacha]

@bradfitz Could you re-open this?
previous fix is incomplete. (I'm stupid)

package main

import (
	"fmt"
	"io"
	"mime"
)

type charsetError string

func (e charsetError) Error() string {
	return fmt.Sprintf("charset not supported: %q", string(e))
}

var rfc2047Decoder = mime.WordDecoder{
	CharsetReader: func(charset string, input io.Reader) (io.Reader, error) {
		return nil, charsetError(charset)
	},
}

func main() {
	_, err := rfc2047Decoder.Decode("=?UTF-8???=")
	fmt.Println(err)
}

still panics.
Then, please review CL 37912 again.
Thanks.

[bradfitz]

Done.