$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/atd/.cache/go-build"
GOENV="/home/atd/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/atd/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/atd/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.18.2"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/dev/null"
GOWORK=""
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-build4134153916=/tmp/go-build -gno-record-gcc-switches"

package main

import (
	"strconv"
	"sync"
	"syscall/js"
        "log
)

// GOOS=js GOARCH=wasm go build -o  server/assets/app.wasm

var mutex = &sync.Mutex{}

// Mutable is a variable that is protected for access by multiple goroutines 
// (we may want to access it asynchronously)
// It is modified in response to user actions in the browser.
// As such, it models the UI tree that would get modified by user initiated events.
// Thus, different event handlers need to take the lock before modifying it
var Mutable int 

var evtcounter = js.Global().Get("document").Call("createElement", "div")

func AddEventListener(event string, target js.Value){
	cb:= js.FuncOf(func(this js.Value, args []js.Value)any{
        // RETRIEVING EVENT INFO
        evt := args[0]
		typ := evt.Get("type").String()
        log.Print("== HANDLING " + typ + " EVENT ==\n")
        
        
        // EVENT HANDLING
        log.Print("LOCKING\n")
		b:= mutex.TryLock() // locking here as we need to access Mutable
        if !b{
            log.Print("ERROR: UNLOCKING phase seems to have been skipped in previous handler. Wrapped Functions are prempted, interleaved?")
            mutex.Lock() // just to cause the deadlock
        }
        log.Print("LOCKED\n")
		Mutable++
		evtcounter.Set("textContent", "# of event triggers: " + strconv.Itoa(Mutable))
        
		evtcounter.Call("focus")

        log.Print("UNLOCKING\n")
		mutex.Unlock() // Unlocking needs to happen before another callback starts.
        log.Print("UNLOCKED ... ready for locking\n")
		return nil
	})
	target.Call("addEventListener", event, cb)
}

func main(){
	
	body:= js.Global().Get("document").Get("body")
	button:= js.Global().Get("document").Call("createElement", "btn")
	button.Set("textContent","click me")
    
    evtcounter.Call("setAttribute", "tabindex","-1") // makes it focusable programmatically


	body.Call("appendChild",evtcounter)
	body.Call("appendChild",button)

	AddEventListener("click",button)
	AddEventListener("focusin",body)


	c := make(chan struct{}, 0)
	<-c
}