Do not panic in ‘os’ module if argv.Length() is 0

[samhocevar]

No description provided.

[flimzy]

Hi, @samhocevar , thanks for the contribution. To help understand the implications, can you explain when you ran into argv having a length of 0?

[samhocevar]

Sure; I built the following Go program:

func main() {
    fmt.Printf("%v", len(os.Args))
}

I am using GopherJS 1.17.2+go1.17.9 on the Windows platform, using: gopherjs build ./src.

I tweaked the GopherJS-generated src.js so that it declares a my_code() function, which lets me first initialise BrowserFS (the version from npm), and then call that main function:

<script src="node_modules/browserfs/dist/browserfs.js"></script>
<script src="src.js"></script>
<script type="text/javascript">
  BrowserFS.install(window);
  BrowserFS.configure({ fs: "XmlHttpRequest", options: { index: "index.json" }, }, function(e) { if (e) { throw e; }
    window.fs = BrowserFS.BFSRequire('fs');
    window.fs.constants = { O_WRONLY: 1, O_RDWR: 2, O_CREAT: 64, O_TRUNC: 512, O_APPEND: 1024, O_EXCL: 128 };
    my_code();
  });
</script>

The stacktrace looks like this:

src.js:1561 Uncaught Error: runtime error: makeslice: len out of range
    at $callDeferred (src.js:1561:17)
    at $panic (src.js:1616:3)
    at throw$1 (gopherjs__runtime.go:432:3)
    at $makeSlice (src.js:1399:5)
    at init$1 (gopherjs__os.go:22:5)
    at Object.$init (src.js:36392:3)
    at Object.$init (src.js:40487:11)
    at Object.$init (src.js:46644:12)
    at $init (src.js:270206:12)
    at $goroutine (src.js:1636:19)
    at $runScheduled (src.js:1682:7)
    at $schedule (src.js:1706:5)
    at $go (src.js:1668:3)
    at src.js:270285:1
    at src.js:270288:4

[nevkontakte]

@samhocevar this change looks good to me and I don't mind merging it, but I would like to better understand why it is needed in the first place. In a browser, I wouldn't expect it make it past the argv != js.Undefined. Usually such counter-intuitive behavior is a sign of another latent bug and I'd like to find it.

Do you know where argv gets set? Or could you attach an archive or a git repo which reproduces the crash so that I could poke at it?

[samhocevar]

Hi! I have investigated this issue more thoroughly, and I believe this was all caused by me calling the main GopherJS-generated function without arguments: my_code() instead of my_code(this)… I’m sorry for wasting your time with this!

[nevkontakte]

No worries, glad you figured it out!

[JounQin]

@nevkontakte Hi, when would this be released? See mvdan/sh#864

[nevkontakte]

@JounQin our plan was to release it with GopherJS 1.18. It was merged after #1111, which is a big enough of a change that we didn't feel appropriate to release as a patch-level version. So far I haven't had much time to really dig into 1.18, but I have a vacation coming up, so that should change :)

If you are comfortable doing so, you should be able to install the unreleased GopherJS version from master: go get github.com/gopherjs/gopherjs@master.