syscall/js: running web assembly in Wasmer is failing on bad type flag

[martinkunc]

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

$ go version
go version go1.14 darwin/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
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/mkunc/Library/Caches/go-build"
GOENV="/Users/mkunc/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/mkunc/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/mkunc/go/src/github.com/martinkunc/wagather/go.mod"
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 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/nw/2584cxnn0_545f3kzf2g8mlw0000gn/T/go-build220972923=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

I am running Golang WebAssembly in Wasmer. In Go 1.14 the application fails on panic: bad type flag, but in Go 1.13 it runs well. The application is importing "syscall/js".

What is failing:

I believe it is failing because throwing panic was added in Go 1.14:
https://github.com/golang/go/blob/master/src/syscall/js/js.go#L291

In Go 1.13.9, it by default considers the object type to be js object:
https://github.com/golang/go/blob/go1.13.9/src/syscall/js/js.go#L244

But I believe the real problem is to expect javascript runtime having fs.constants of some type,
which I believe is happening here:
https://github.com/golang/go/blob/master/src/syscall/fs_js.go#L21

Unfortunatelly this syscall variable requires the constant exists before initialization, so I cannot set it before importing "syscall/js"

My humble opinion would be that we could change constants to fsConstants function() {} so that it would be evaluated only when particular fs function would require it (and I would have chance to set it up upfront on another environment).

What did you expect to see?

Application runs fine.

What did you see instead?

Application crashes with:

panic: bad type flag

goroutine 1 [running]:
syscall/js.Value.Type(0x7ff800000000000b, 0x0, 0x0)
	/usr/local/go/src/syscall/js/js.go:291 +0x1b
syscall/js.Value.Get(0x7ff800000000000b, 0x0, 0x2873f8, 0x9, 0x7ff800000000000b, 0x0)
	/usr/local/go/src/syscall/js/js.go:298 +0x2
syscall.init()
	/usr/local/go/src/syscall/fs_js.go:21 +0xe

[agnivade]

/cc @neelance

[neelance]

The interface between src/syscall/js/js.go and misc/wasm/wasm_exec.js is internal and not stable. Wasmer can not use wasm_exec.js since it has no JavaScript runtime. So if Wasmer has support for syscall/js, then it seems to be a hack on their end which is using the internal interface. If there is any incompatibility with the new Go version, then it needs to be resolved on Wasmer's end. Please file an issue with the Wasmer project.

[martinkunc]

The problem is that syscall/js alone is presuming existence fo Nodejs Wasm constants, and it is doing it during declaration of variables. I can guess that it is current limitation, but could we postpone loading of variables from here:

https://github.com/golang/go/blob/master/src/syscall/fs_js.go#L26

I tried to prepare minimal reproducible code in repo: https://github.com/martinkunc/gowasmer

To reproduce, you can run

make build-wasm-1-14 && make build && ./wasmcmd

and then compare it with

make build-wasm-1-13 && make build && ./wasmcmd

I hope I am not using something which is only intended to work this way. I understand that WASMI is not stable, but maybe it might allow at least run at other runtimes, and I hope the change would be small one.

[neelance]

The error "bad type flag" is an internal error. It should not even be thrown if something is null or undefined.

The constants that you say are missing are in your own code: https://github.com/martinkunc/gowasmer/blob/master/wasm/bridge.go#L169
However, this "bridge" is a hack on an internal interface and apparently not compatible with the latest version of said interface.

[martinkunc]

Thank you for the explanation Richard and for pointing to my code and sorry for a confused report. I think I can close the issue. I will try to fix my hack.

Btw, thank you for all your hard work on the Wasm support :)

[BruceChoca]

@martinkunc excuse me! Has this problem beem solved? How to solve it?

[martinkunc]

@BruceChoca Unfortunatelly I didn't continued on implementation of fs constants. Instead of what I did was I let my webassembly binary compiled on go 1.13 and built my host binary in latest go.
You can look at what I prepared here: https://github.com/martinkunc/wagather
It is PoC of collecting data from OpenShift Api (list of operators), What I did was I implemented Authenticated Http Proxy on the host, which I called from the webassembly.

[BruceChoca]

@martinkunc thank u !
I found the reason, 1.14+ and 1.13+ typeflag is different. -> https://github.com/golang/go/blob/master/src/syscall/js/js.go#L280
u must modify the typeFlag to make is consistent.
modify function storeValue. -> https://github.com/martinkunc/gowasmer/blob/master/wasm/bridge.go#L352
https://github.com/martinkunc/gowasmer/blob/master/wasm/bridge.go#L420

typeFlag := 0
switch rt.Kind() {
case reflect.String:
	typeFlag = 1
case reflect.Func:
	typeFlag = 3
}

=>

switch rt.Kind() {
case reflect.String:
	typeFlag = 2
case reflect.Func:
	typeFlag = 4
case reflect.Struct:
 	typeFlag = 1
case reflect.Slice:
 	typeFlag = 1
}

u can try, hope it will solve your problem.