text/template: `call` with no arguments after a pipe crashes the program

[ozelis]

Go version

go version go1.23.2 linux/amd64

Output of go env in your module/workspace:

GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='/home/ao/.cache/go-build'
GOENV='/home/ao/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMODCACHE='/home/ao/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/ao/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/lib/go'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/lib/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.23.2'
GODEBUG=''
GOTELEMETRY='local'
GOTELEMETRYDIR='/home/ao/.config/go/telemetry'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='gcc'
CXX='g++'
CGO_ENABLED='1'
GOMOD='/dev/null'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build785295111=/tmp/go-build -gno-record-gcc-switches'

What did you do?

https://go.dev/play/p/RapayQaYsZw

package main

import "fmt"
import "text/template"
import "bytes"

func main() {
    tmpl, err := template.New("foo").Parse("{{ 1 | call }}")

    if err != nil {
      fmt.Println(err)
    } else {
      var buf bytes.Buffer
      tmpl.Execute(&buf, "whatever")
      fmt.Println(&buf)
    }
}

What did you see happen?

panic: runtime error: index out of range [0] with length 0 [recovered]
	panic: runtime error: index out of range [0] with length 0

goroutine 1 [running]:
text/template.errRecover(0xc00010ae48)
	/usr/lib/go/src/text/template/exec.go:170 +0x15a
panic({0x539c80?, 0xc00001a150?})
	/usr/lib/go/src/runtime/panic.go:785 +0x132
text/template.(*state).evalCall(0xc00010ae58, {0x51f500?, 0x57ffa0?, 0xc00010aac0?}, {0x525b40?, 0x553530?, 0x1?}, 0x1, {0x582240, 0xc000102270}, ...)
	/usr/lib/go/src/text/template/exec.go:806 +0xb65
text/template.(*state).evalFunction(0xc00010ae58, {0x51f500?, 0x57ffa0?, 0xc00010e1a0?}, 0xf?, {0x582240, 0xc000102270}, {0xc0000140d0, 0x1, 0x1}, ...)
	/usr/lib/go/src/text/template/exec.go:623 +0x1ff
text/template.(*state).evalCommand(0xc00010ae58, {0x51f500?, 0x57ffa0?, 0x0?}, 0xc000102270, {0x51f740?, 0x65b4e8?, 0x469485?})
	/usr/lib/go/src/text/template/exec.go:510 +0x1ce
text/template.(*state).evalPipeline(0xc000125e58, {0x51f500?, 0x57ffa0?, 0xc000102360?}, 0xc00002e1e0)
	/usr/lib/go/src/text/template/exec.go:479 +0x125
text/template.(*state).walk(0xc000125e58, {0x51f500?, 0x57ffa0?, 0x7e0d9f96b108?}, {0x582360?, 0xc0001022d0})
	/usr/lib/go/src/text/template/exec.go:267 +0x225
text/template.(*state).walk(0xc000125e58, {0x51f500?, 0x57ffa0?, 0xc00010aea0?}, {0x5822d0?, 0xc000102210?})
	/usr/lib/go/src/text/template/exec.go:280 +0x2e5
text/template.(*Template).execute(0xc0001040c0, {0x580b58?, 0xc000102330?}, {0x51f500?, 0x57ffa0?})
	/usr/lib/go/src/text/template/exec.go:224 +0x275
text/template.(*Template).Execute(...)
	/usr/lib/go/src/text/template/exec.go:207
main.main()
	/home/ao/proj/go_test/test.go:14 +0x185

What did you expect to see?

Previously it would report a normal error calling call: non-function of type int, but after changes in recent
commit c506f03

// ...
	if isBuiltin && name == "call" {
		calleeName := args[0].String()
		argv = append([]reflect.Value{reflect.ValueOf(calleeName)}, argv...)
		fun = reflect.ValueOf(call)
	}
// ...

it tries to access args[0] which in this case is empty and crashes the whole program.

[gabyhelp]

Related Issues

• text/template: panics encountered during function calls are hard to treat and debug #28242 (closed)
• html/template: an infinite recursion in Template.Execute() #53241 (closed)
• text/template: panic: Call using zero Value argument #10634 (closed)
• text/template: Call using string as type int #10800 (closed)
• text/template: panics on method on nil interface value #30143 (closed)
• text/template: panic when referencing field of parenthesized argument #21171 (closed)
• go/types: panic: unreachable #17716 (closed)
• html/template: slice bounds out of range #10799 (closed)
• text/template: Call using complex128 as type string #10946 (closed)
• text/template: does not recover from panics where it should #2663 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[seankhliao]

cc @Zxilly

[gopherbot]

Change https://go.dev/cl/628095 mentions this issue: text/template: avoid index-out-of-range panic when accessing args[0]

[gopherbot]

Change https://go.dev/cl/628096 mentions this issue: text/template: don't crash piping to call with no arguments