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wasm callback loop handling: TypeError: "target argument of Reflect.get must be an object, got (void 0)" #28448

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iivvoo opened this issue Oct 28, 2018 · 2 comments
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wasm callback loop handling: TypeError: "target argument of Reflect.get must be an object, got (void 0)" #28448

iivvoo opened this issue Oct 28, 2018 · 2 comments
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@iivvoo
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iivvoo commented Oct 28, 2018

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

go version go1.11.1 linux/amd64

Does this issue reproduce with the latest release?

This is the latest (right?)

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

GOARCH="amd64"
GOBIN=""
GOCACHE="/home/ivo/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/ivo/m3r/projects/WASMBug"
GOPROXY=""
GORACE=""
GOROOT="/opt/go1.11.1"
GOTMPDIR=""
GOTOOLDIR="/opt/go1.11.1/pkg/tool/linux_amd64"
GCCGO="gccgo"
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-build637835953=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Play with wasm. Specifically, with registering callbacks. The most simple setup already results in a panic after the first callback.

main.go:

package main

import "syscall/js"


func bla(i []js.Value) {
	println("Hello World")
}

func registerCallbacks() {
	js.Global().Set("bla", js.NewCallback(bla))
}

func main() {
	println("WASM Go Initialized")
	registerCallbacks()
	js.Global().Get("document").Call("getElementById", "blaButton").Call("click")
	select {}
}

index.html

<html>

<head>
	<meta charset="utf-8">
	<title>Go wasm</title>
</head>

<body>

	<script src="wasm_exec.js"></script>

	<script>
		if (!WebAssembly.instantiateStreaming) { // polyfill
			WebAssembly.instantiateStreaming = async (resp, importObject) => {
				const source = await (await resp).arrayBuffer();
				return await WebAssembly.instantiate(source, importObject);
			};
		}

		const go = new Go();
		let mod, inst;
		WebAssembly.instantiateStreaming(fetch("lib.wasm"), go.importObject).then(async (result) => {
			mod = result.module;
			inst = result.instance;
			await go.run(inst)
		});

	</script>

	<button onClick="bla();" id="blaButton">Bla</button>

</body>

</html>

What did you expect to see?

Hello World on the console (which is shown) without a crash.

What did you see instead?

On firefox (but similar stacktraces on chromium)

call to closed callback wasm_exec.js:293:28
  syscall/js.valueCall http://localhost:8080/wasm_exec.js:293:28
  syscall_js.valueCall http://localhost:8080/lib.wasm:712567:2147484590
  syscall_js.Value.Call http://localhost:8080/lib.wasm:703432:2147484575
  syscall_js.callbackLoop http://localhost:8080/lib.wasm:693712:2147484563
  _rt0_wasm_js http://localhost:8080/lib.wasm:658064:2147484445
  run http://localhost:8080/wasm_exec.js:414:5

<-- this is debugging statements inserted by me. It appears to reflect.Get(undefined, "id")

obj undefined wasm_exec.js:260:5
key id wasm_exec.js:261:5

TypeError: "`target` argument of Reflect.get must be an object, got (void 0)"
	valueGet http://localhost:8080/wasm_exec.js:264:19
	valueGet http://localhost:8080/lib.wasm:wasm-function[938]:0xadeeb
	Get http://localhost:8080/lib.wasm:wasm-function[921]:0xab447
	callbackLoop http://localhost:8080/lib.wasm:wasm-function[915]:0xa932b
	_rt0_wasm_js http://localhost:8080/lib.wasm:wasm-function[797]:0xa0a90
	run http://localhost:8080/wasm_exec.js:414:5
wasm_exec.js:268:7 

panic: syscall/js: call of Value.Int on object wasm_exec.js:47:6


goroutine 4 [running]: wasm_exec.js:47:6
syscall/js.Value.float(0x7ff800000000001c, 0x194ab, 0x9, 0x7ff800000000001c) wasm_exec.js:47:6
	/opt/go1.11.1/src/syscall/js/js.go:326 +0x35 wasm_exec.js:47:6
syscall/js.Value.Int(0x7ff800000000001c, 0x190ac) wasm_exec.js:47:6
	/opt/go1.11.1/src/syscall/js/js.go:338 +0x2 wasm_exec.js:47:6
syscall/js.callbackLoop() wasm_exec.js:47:6
	/opt/go1.11.1/src/syscall/js/callback.go:102 +0xc wasm_exec.js:47:6
created by syscall/js.NewCallback.func1 wasm_exec.js:47:6
	/opt/go1.11.1/src/syscall/js/callback.go:40 +0x2

I've attempted to debug this myself. It seems that in go/src/syscall/js/callback.js,

cb := pendingCallbacks.Call("shift")

returns a (ref) Value 0 (not Undefined()) if the array is empty. If I add multiple similar shifts, I keep getting a 0 value:

Global().Get("console").Call("info", "cb is " + strconv.FormatUint(uint64(cb.ref ), 10))

It almost feels asif Call("shift") on an empty array gives some 0 value in stead of undefined like it does in js.

The id := uint32(cb.Get("id").Int()) a few lines later is what's eventually resulting in the panic "call of Value.Int on object wasm_exec.js:47:6"

I can't imagine such a trivial example not working so I must be doing something wrong but I'm unable to find it.
@agnivade
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You most probably have an incorrect wasm_exec.js. Your wasm_exec must match with the Go compiler version. I just tried with the wasm_exec from 1.11 and it worked.

@iivvoo
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iivvoo commented Oct 28, 2018

That was it. The tutorial wrongly suggests to download wasm_exec.js (I thinks I've seen it elsewhere as well), and the only other google hit I found on this issue (based on the chrome error "TypeError: Reflect.get called on non-object") suggested to download the latest version.

Perhaps wasm_exec should somehow check the go version it's running against?

I'll notify the tutorial author

@iivvoo iivvoo closed this as completed Oct 28, 2018
@golang golang locked and limited conversation to collaborators Oct 28, 2019
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