text/template: panic when referencing field of parenthesized argument

[mynameiswhm]

Our team encountered this problem in the wild with Helm template like this one:

{{ (index .Values "non-existent-key").config.host }}

We've fixed it with multiple arguments call to index:

{{ index .Values "non-existent-key" "config" "host" }}

But after some investigation we've narrowed it down to simple case with parentheses and empty interface{} value:

var v interface{}
t.Execute(os.Stdout, v)
...
{{ .foo }} // works fine producing "<no value>"
{{ (.).foo }} // panics

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

go version go1.8.3 darwin/amd64

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

GOARCH="amd64"
GOBIN="/Users/iesaulenko/gocode/bin"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/iesaulenko/gocode"
GORACE=""
GOROOT="/usr/local/Cellar/go/1.8.3/libexec"
GOTOOLDIR="/usr/local/Cellar/go/1.8.3/libexec/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/lq/qclr1nvn4tzf_fcnl4xq5t_0fyxkg0/T/go-build885238657=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"
PKG_CONFIG="pkg-config"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"

What did you do?

package main

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

func main() {
	t, _ := template.New("test").Parse(`{{ (.).foo }}`)
	
	var v interface{}
	err := t.Execute(os.Stdout, v)
	if err != nil {
		log.Fatalf("execution failed: %s", err)
	}
}

https://play.golang.org/p/H2lI-3Taxq

What did you expect to see?

<no value>

(which is returned if you change template to {{ .foo }})

What did you see instead?

panic: reflect: Zero(nil) [recovered]
	panic: reflect: Zero(nil)

goroutine 1 [running]:
text/template.errRecover(0xc420099f00)
	/usr/local/Cellar/go/1.8.3/libexec/src/text/template/exec.go:146 +0x62
panic(0x10f8c60, 0xc42009c040)
	/usr/local/Cellar/go/1.8.3/libexec/src/runtime/panic.go:489 +0x2cf
reflect.Zero(0x0, 0x0, 0x0, 0x0, 0x0)
	/usr/local/Cellar/go/1.8.3/libexec/src/reflect/value.go:2122 +0x134
text/template.(*state).validateType(0xc420099e80, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0)
	/usr/local/Cellar/go/1.8.3/libexec/src/text/template/exec.go:697 +0x700
text/template.(*state).evalArg(0xc420099e80, 0x0, 0x0, 0x0, 0x0, 0x0, 0x11a64e0, 0xc42008e050, 0x100dc56, 0x1205858, ...)
	/usr/local/Cellar/go/1.8.3/libexec/src/text/template/exec.go:746 +0x870
text/template.(*state).evalChainNode(0xc420099e80, 0x0, 0x0, 0x0, 0xc4200a0000, 0xc42009c020, 0x1, 0x1, 0x0, 0x0, ...)
	/usr/local/Cellar/go/1.8.3/libexec/src/text/template/exec.go:502 +0xf2
text/template.(*state).evalCommand(0xc420099e80, 0x0, 0x0, 0x0, 0xc420090000, 0x0, 0x0, 0x0, 0xc42001d300, 0x1122b3c, ...)
	/usr/local/Cellar/go/1.8.3/libexec/src/text/template/exec.go:430 +0x5f3
text/template.(*state).evalPipeline(0xc420099e80, 0x0, 0x0, 0x0, 0xc42008e000, 0x100e162, 0xc420090090, 0x30)
	/usr/local/Cellar/go/1.8.3/libexec/src/text/template/exec.go:406 +0xf2
text/template.(*state).walk(0xc420099e80, 0x0, 0x0, 0x0, 0x11a60c0, 0xc420090060)
	/usr/local/Cellar/go/1.8.3/libexec/src/text/template/exec.go:232 +0x55a
text/template.(*state).walk(0xc420099e80, 0x0, 0x0, 0x0, 0x11a63c0, 0xc420018360)
	/usr/local/Cellar/go/1.8.3/libexec/src/text/template/exec.go:240 +0x139
text/template.(*Template).execute(0xc420012300, 0x11a5280, 0xc42000c018, 0x0, 0x0, 0x0, 0x0)
	/usr/local/Cellar/go/1.8.3/libexec/src/text/template/exec.go:195 +0x20a
text/template.(*Template).Execute(0xc420012300, 0x11a5280, 0xc42000c018, 0x0, 0x0, 0x0, 0xc420018240)
	/usr/local/Cellar/go/1.8.3/libexec/src/text/template/exec.go:178 +0x53
main.main()
	/Users/iesaulenko/_go_tmp/src/go.avito.ru/do/foo/main.go:13 +0xbe
exit status 2

[odeke-em]

I can also reproduce this on go tip 3d9475c, not a regression so marking this for Go1.10.

[odeke-em]

Sorry @mynameiswhm that we didn't look at this issue during Go1.10. I'll move this to the Go1.11 milestone instead.

[mvdan]

I believe I have found the fix for this - sending a CL now.

[gopherbot]

Change https://golang.org/cl/84482 mentions this issue: text/template: never call reflect.Zero(nil)