text/template: access to methods that are not part of interface

[sorcix]

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

$ go version
go version go1.13.3 linux/amd64

Does this issue reproduce with the latest release?

Yes

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

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/vic/.cache/go-build"
GOENV="/home/vic/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/vic/Go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/lib/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build611773016=/tmp/go-build -gno-record-gcc-switches"
GOROOT/bin/go version: go version go1.13.3 linux/amd64
GOROOT/bin/go tool compile -V: compile version go1.13.3
uname -sr: Linux 5.3.6-arch1-1-ARCH
LSB Version:	1.4
Distributor ID:	Arch
Description:	Arch Linux
Release:	rolling
Codename:	n/a
/usr/lib/libc.so.6: GNU C Library (GNU libc) stable release version 2.30.

What did you do?

I executed a text/template.Template with an interface as data.

Example code

package main

import (
	"os"
	"text/template"
)

type Foo struct {
	A int
}

func (f Foo) B() int {
	return 2
}

func (f Foo) C() int {
	return 3
}

type Bar interface {
	C() int
}

func main() {
	foo := Foo{A: 1}
	test(foo)
}

func test(bar Bar) {
	tpl := template.Must(template.New("template").Parse("{{ .A }} {{ .B }} {{ .C }}"))
	tpl.Execute(os.Stdout, bar)
}

On play: https://play.golang.org/p/YgMyBPie8kw

What did you expect to see?

My template is using functions that don't exist in the interface, thus I was expecting an error.

What did you see instead?

The template is able to execute functions on the underlying type, exposing functionality I don't want to expose.

I believe the text/template package actually behaved as I would expect in the past. I think the functionality was changed in 167a712.

[mvdan]

Have you been able to verify if old versions of Go behave differently here?

If so, could you use a git bisect to see what commit changed the logic? That can be a good starting point to see if the change was on purpose or not.

[sorcix]

I've found this question on Stack Overflow that indicates that it was different. The code listed in the answer there was removed in 167a712, and checked for an empty interface{} vs an interface that had methods, right? I hope to have time next weekend to try out some older Go versions.

Apart from how older versions behaved, it's weird to explicitly pass an interface as data to a template and see it being ignored and the underlying type used instead. It could even be a security issue, as templates have access to more than was intended. (The html/template documentation states "this package assumes that template authors are trusted", but the text/template documentation does not.)