encoding/json: detect circular data structures when encoding #10769

package main

import (
    "html/template"
    "io/ioutil"
)

func main() {
    t, err := template.New("foo").Parse(data)
    if err != nil {
        return
    }

    a := &A{}
    a.B1 = &B{a}
    ping := &Ping{a}

    t.Execute(ioutil.Discard, ping)
}

var data = "<script>{{ .Pong }}</script>"

type Ping struct {
    Pong *A
}

type A struct {
    B1 *B
}

type B struct {
    A1 *A
}

runtime: goroutine stack exceeds 1000000000-byte limit
fatal error: stack overflow

runtime stack:
runtime.throw(0x5e7490, 0xe)
    /home/bep/dev/golang/go/src/runtime/panic.go:543 +0x96
runtime.newstack()
    /home/bep/dev/golang/go/src/runtime/stack1.go:744 +0xb3f
runtime.morestack()
    /home/bep/dev/golang/go/src/runtime/asm_amd64.s:330 +0x82

goroutine 1 [stack growth]:
encoding/json.(*structEncoder).encode(0xc208012e40, 0xc208095810, 0x585560, 0xc208028038, 0xd9, 0x0)
    /home/bep/dev/golang/go/src/encoding/json/encode.go:569 fp=0xc2280fa280 sp=0xc2280fa278
encoding/json.(*structEncoder).(encoding/json.encode)-fm(0xc208095810, 0x585560, 0xc208028038, 0xd9, 0x5bf900)
    /home/bep/dev/golang/go/src/encoding/json/encode.go:598 +0x58 fp=0xc2280fa2b8 sp=0xc2280fa280
encoding/json.typeEncoder.func1(0xc208095810, 0x585560, 0xc208028038, 0xd9, 0xc208028000)
    /home/bep/dev/golang/go/src/encoding/json/encode.go:336 +0x6d fp=0xc2280fa2f0 sp=0xc2280fa2b8
encoding/json.(*ptrEncoder).encode(0xc208028088, 0xc208095810, 0x531e60, 0xc208028040, 0xd6, 0x4c5200)
    /home/bep/dev/golang/go/src/encoding/json/encode.go:706 +0xed fp=0xc2280fa340 sp=0xc2280fa2f0
encoding/json.(*ptrEncoder).(encoding/json.encode)-fm(0xc208095810, 0x531e60, 0xc208028040, 0xd6, 0x0)
    /home/bep/dev/golang/go/src/encoding/json/encode.go:711 +0x58 fp=0xc2280fa378 sp=0xc2280fa340
encoding/json.(*structEncoder).encode(0xc208012f30, 0xc208095810, 0x585620, 0xc208028040, 0xd9, 0x4aad00)
    /home/bep/dev/golang/go/src/encoding/json/encode.go:584 +0x2c3 fp=0xc2280fa520 sp=0xc2280fa378
encoding/json.(*structEncoder).(encoding/json.encode)-fm(0xc208095810, 0x585620, 0xc208028040, 0xd9, 0xc208028000)
    /home/bep/dev/golang/go/src/encoding/json/encode.go:598 +0x58 fp=0xc2280fa558 sp=0xc2280fa520
encoding/json.(*ptrEncoder).encode(0xc208028090, 0xc208095810, 0x531ec0, 0xc208028038, 0xd6, 0x4c5200)
    /home/bep/dev/g