__ENV should not be shared between VUs

[mstoykov]

Environment

• k6 version: v0.26.0, but probably all versions

Expected Behavior

changing __ENV in one VU should not change it in another

Actual Behavior

it changes :)

Steps to Reproduce the Problem

1. script.js:

console.log(__ENV.something)
__ENV.something = Math.random()
export default function() {}

2.k6 run script.js

$ k6 run -u 5 script.js

          /\      |‾‾|  /‾‾/  /‾/
     /\  /  \     |  |_/  /  / /
    /  \/    \    |      |  /  ‾‾\
   /          \   |  |‾\  \ | (_) |
  / __________ \  |__|  \__\ \___/ .io

INFO[0001] undefined
WARN[0001] All iterations (1 in this test run) are shared between all VUs, so some of the 5 VUs will not execute even a single iteration!
INFO[0001] 0.44183025883093746
INFO[0001] 0.1016693686145117
INFO[0001] 0.3351747792404137
INFO[0001] 0.9595724221546064
INFO[0001] 0.6634652530245149
  execution: local
     output: -
     script: env.js

    duration: -,  iterations: 1
         vus: 5, max: 5

INFO[0001] 0.9781430971546957
INFO[0001] 0.5929989320809331
    done [==========================================================] 1 / 1

    data_received........: 0 B 0 B/s
    data_sent............: 0 B 0 B/s
    iteration_duration...: avg=3.78µs min=3.78µs med=3.78µs max=3.78µs p(90)=3.78µs p(95)=3.78µs
    iterations...........: 1   0/s
    vus..................: 5   min=5 max=5
    vus_max..............: 5   min=5 max=5

For the record running

export default function() {
        __ENV.something = Math.random()
}

reliably panics 2 VUs and 2000iterations with

fatal error: concurrent map writes----------------------------------] starting

goroutine 32 [running]:
runtime.throw(0xeaad9b, 0x15)
        /home/mstoykov/.gvm/gos/go1.13.5/src/runtime/panic.go:774 +0x72 fp=0xc002229760 sp=0xc002229730 pc=0x430a92
runtime.mapassign(0xd53940, 0xc00039d350, 0xc002ca5440, 0x40b79b)
        /home/mstoykov/.gvm/gos/go1.13.5/src/runtime/map.go:588 +0x5c3 fp=0xc0022297e8 sp=0xc002229760 pc=0x410143
reflect.mapassign(0xd53940, 0xc00039d350, 0xc002ca5440, 0xc002ca5450)
        /home/mstoykov/.gvm/gos/go1.13.5/src/runtime/map.go:1329 +0x3f fp=0xc002229818 sp=0xc0022297e8 pc=0x411b2f
reflect.Value.SetMapIndex(0xd53940, 0xc00039d350, 0x15, 0xd1c520, 0xc002ca5440, 0x98, 0xd1c520, 0xc002ca5450, 0x98)
        /home/mstoykov/.gvm/gos/go1.13.5/src/reflect/value.go:1679 +0x268 fp=0xc002229898 sp=0xc002229818 pc=0x4c72d8
github.com/loadimpact/k6/vendor/github.com/dop251/goja.(*objectGoMapReflect).putStr(0xc001ee3130, 0xc000042972, 0x9, 0x10cf580, 0xc002b08b10, 0x1)
        /home/mstoykov/.gvm/pkgsets/go1.13.5/global/src/github.com/loadimpact/k6/vendor/github.com/dop251/goja/object_gomap_reflect.go:107 +0x100 fp=0xc002229908 sp=0xc002229898 pc=0x8d6300
github.com/loadimpact/k6/vendor/github.com/dop251/goja.setPropStrict.exec(0xc000042972, 0x9, 0xc00042b930)
        /home/mstoykov/.gvm/pkgsets/go1.13.5/global/src/github.com/loadimpact/k6/vendor/github.com/dop251/goja/vm.go:1069 +0xde fp=0xc002229960 sp=0xc002229908 pc=0x8fcd7e
github.com/loadimpact/k6/vendor/github.com/dop251/goja.(*setPropStrict).exec(0xc002308c60, 0xc00042b930)
        <autogenerated>:1 +0x4f fp=0xc002229988 sp=0xc002229960 pc=0x921c9f
github.com/loadimpact/k6/vendor/github.com/dop251/goja.(*vm).run(0xc00042b930)
        /home/mstoykov/.gvm/pkgsets/go1.13.5/global/src/github.com/loadimpact/k6/vendor/github.com/dop251/goja/vm.go:289 +0x99 fp=0xc0022299c8 sp=0xc002229988 pc=0x8f8409
github.com/loadimpact/k6/vendor/github.com/dop251/goja.(*funcObject).Call(0xc001f4f500, 0x10cf760, 0x1990980, 0xc002ca5430, 0x1, 0x1, 0x10cf760, 0x1990980)
        /home/mstoykov/.gvm/pkgsets/go1.13.5/global/src/github.com/loadimpact/k6/vendor/github.com/dop251/goja/func.go:130 +0x2cc fp=0xc002229a40 sp=0xc0022299c8 pc=0x8ce70c
github.com/loadimpact/k6/vendor/github.com/dop251/goja.(*funcObject).Call-fm(0x10cf760, 0x1990980, 0xc002ca5430, 0x1, 0x1, 0x100000000203000, 0xc002229b60)
        /home/mstoykov/.gvm/pkgsets/go1.13.5/global/src/github.com/loadimpact/k6/vendor/github.com/dop251/goja/func.go:102 +0x48 fp=0xc002229a90 sp=0xc002229a40 pc=0x916d28
github.com/loadimpact/k6/vendor/github.com/dop251/goja.AssertFunction.func1.2()
        /home/mstoykov/.gvm/pkgsets/go1.13.5/global/src/github.com/loadimpact/k6/vendor/github.com/dop251/goja/runtime.go:1461 +0x96 fp=0xc002229b08 sp=0xc002229a90 pc=0x90bf66
github.com/loadimpact/k6/vendor/github.com/dop251/goja.(*vm).try(0xc00042b930, 0xc002229c50, 0x0)
        /home/mstoykov/.gvm/pkgsets/go1.13.5/global/src/github.com/loadimpact/k6/vendor/github.com/dop251/goja/vm.go:379 +0x147 fp=0xc002229bf0 sp=0xc002229b08 pc=0x8f8a27
github.com/loadimpact/k6/vendor/github.com/dop251/goja.AssertFunction.func1(0x10cf760, 0x1990980, 0xc002ca5430, 0x1, 0x1, 0x0, 0x0, 0x0, 0x0)
        /home/mstoykov/.gvm/pkgsets/go1.13.5/global/src/github.com/loadimpact/k6/vendor/github.com/dop251/goja/runtime.go:1460 +0x13a fp=0xc002229ca0 sp=0xc002229bf0 pc=0x90c0ea
github.com/loadimpact/k6/js.(*VU).runFn(0xc0026c03c0, 0x10c25a0, 0xc0003cae40, 0xc000af1380, 0x1, 0xc000556b80, 0xc002ca5430, 0x1, 0x1, 0x432800, ...)
        /home/mstoykov/.gvm/pkgsets/go1.13.5/global/src/github.com/loadimpact/k6/js/runner.go:477 +0x398 fp=0xc002229da0 sp=0xc002229ca0 pc=0xc4e998
github.com/loadimpact/k6/js.(*VU).RunOnce(0xc0026c03c0, 0x10c25a0, 0xc0003cae40, 0x0, 0x0)
        /home/mstoykov/.gvm/pkgsets/go1.13.5/global/src/github.com/loadimpact/k6/js/runner.go:427 +0x277 fp=0xc002229ec8 sp=0xc002229da0 pc=0xc4e317
github.com/loadimpact/k6/core/local.(*vuHandle).run(0xc0017dbb00, 0xc0000f6070, 0xc00045c240, 0xc0000d2420)
        /home/mstoykov/.gvm/pkgsets/go1.13.5/global/src/github.com/loadimpact/k6/core/local/local.go:70 +0x152 fp=0xc002229f90 sp=0xc002229ec8 pc=0xc643b2
github.com/loadimpact/k6/core/local.(*Executor).scale.func1(0xc0017dbb00, 0xc000a50b40, 0xc00045c240, 0xc0000d2420)
        /home/mstoykov/.gvm/pkgsets/go1.13.5/global/src/github.com/loadimpact/k6/core/local/local.go:349 +0x4d fp=0xc002229fc0 sp=0xc002229f90 pc=0xc6792d
runtime.goexit()
        /home/mstoykov/.gvm/gos/go1.13.5/src/runtime/asm_amd64.s:1357 +0x1 fp=0xc002229fc8 sp=0xc002229fc0 pc=0x45de91
created by github.com/loadimpact/k6/core/local.(*Executor).scale
        /home/mstoykov/.gvm/pkgsets/go1.13.5/global/src/github.com/loadimpact/k6/core/local/local.go:348 +0x328