text/template: Panic when an undefined variable was accessed

[YoitoFes]

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

$ go version
go version go1.18.4 windows/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
set GO111MODULE=
set GOARCH=amd64
set GOBIN=
set GOCACHE=C:\Users\Naoto\AppData\Local\go-build
set GOENV=C:\Users\Naoto\AppData\Roaming\go\env
set GOEXE=.exe
set GOEXPERIMENT=
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOINSECURE=
set GOMODCACHE=E:\Users\Naoto\go\pkg\mod
set GONOPROXY=
set GONOSUMDB=
set GOOS=windows
set GOPATH=E:\Users\Naoto\go
set GOPRIVATE=
set GOPROXY=https://proxy.golang.org,direct
set GOROOT=C:\Program Files\Go
set GOSUMDB=sum.golang.org
set GOTMPDIR=
set GOTOOLDIR=C:\Program Files\Go\pkg\tool\windows_amd64
set GOVCS=
set GOVERSION=go1.18.4
set GCCGO=gccgo
set GOAMD64=v1
set AR=ar
set CC=gcc
set CXX=g++
set CGO_ENABLED=1
set GOMOD=E:\Documents\golang-test\go.mod
set GOWORK=
set CGO_CFLAGS=-g -O2
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-g -O2
set CGO_FFLAGS=-g -O2
set CGO_LDFLAGS=-g -O2
set PKG_CONFIG=pkg-config
set GOGCCFLAGS=-m64 -mthreads -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=D:\AppData\Local\Temp\go-build3068758331=/tmp/go-build -gno-record-gcc-switches

What did you do?

Accessed undefined variable in a template as follows:

Runnable program

package main

import (
	"fmt"
	"os"
	"text/template"
)

func main() {
	tmpl, _ := template.New("test_template").Parse("Hello {{$val}}")
	err := tmpl.Execute(os.Stdout, "test")
	if err != nil {
		fmt.Println(err)
	}
}

go.dev/play link

• https://go.dev/play/p/R34PJT0waeO

What did you expect to see?

Got non-nil err

What did you see instead?

Got a panic

panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x10 pc=0x4e0d8c]

goroutine 1 [running]:
text/template.errRecover(0xc00004de98)
	/usr/local/go-faketime/src/text/template/exec.go:164 +0x15b
panic({0x501d20, 0x5f65c0})
	/usr/local/go-faketime/src/runtime/panic.go:838 +0x207
text/template.(*Template).execute(0x0, {0x547168?, 0xc00000e018?}, {0x4fb840?, 0x5468a0?})
	/usr/local/go-faketime/src/text/template/exec.go:215 +0x22c
text/template.(*Template).Execute(...)
	/usr/local/go-faketime/src/text/template/exec.go:201
main.main()
	/tmp/sandbox2346080729/prog.go:13 +0x11c

[seankhliao]

Don't ignore errors.

Unlike many projects, the Go project does not use GitHub Issues for general discussion or asking questions. GitHub Issues are used for tracking bugs and proposals only.

For questions please refer to https://github.com/golang/go/wiki/Questions

[YoitoFes]

@seankhliao

Don't ignore errors.

tmpl.Execute(os.Stdout, "test") panics instead of returning error. IMO tmpl.Execute() should return error.

[ianlancetaylor]

@YoitoFes You are ignoring the error from template.New. Don't do that.

[ianlancetaylor]

It is not a goal for the standard library to never panic. It will panic in some cases when called with invalid arguments.