::jsstack frame shows only one arg when two args were passed

[trentm]

Repro (sorry this is from running in an imgapi zone I have handy -- a smartos zone running node v4.6.1 with the verror module already installed):

[root@d934df7f-313d-470c-809b-8528fcefb51d (nightly-1:imgapi0) /opt/smartdc/imgapi]# cat boom.js
var WError = require('verror').WError;
var err = new WError({
        restCode: 'ResourceNotFound',
        statusCode: 404,
        constructorOpt: 'blah'
    }, '/docs/%2C%27%29..%28%28%28%2C%27');
[root@d934df7f-313d-470c-809b-8528fcefb51d (nightly-1:imgapi0) /opt/smartdc/imgapi]# node --abort-on-uncaught-exception  boom.js
XXX WError: arguments: {"0":{"restCode":"ResourceNotFound","statusCode":404,"constructorOpt":"blah"},"1":"/docs/%2C%27%29..%28%28%28%2C%27"}
Uncaught Error: too few args to sprintf

FROM
jsSprintf (/opt/smartdc/imgapi/node_modules/extsprintf/lib/extsprintf.js:71:4)
new WError (/opt/smartdc/imgapi/node_modules/verror/lib/verror.js:208:41)
Object.<anonymous> (/opt/smartdc/imgapi/boom.js:2:11)
Module._compile (module.js:409:26)
Object.Module._extensions..js (module.js:416:10)
Module.load (module.js:343:32)
Function.Module._load (module.js:300:12)
Function.Module.runMain (module.js:441:10)
startup (node.js:139:18)
node.js:974:3
Illegal Instruction (core dumped)

And from the core for that:

js:     WError
          file: /opt/smartdc/imgapi/node_modules/verror/lib/verror.js
          posn: line 194
          this: 8f06aa85 (JSObject: WError)
          arg1: 8f06aa65 (JSObject: Object)

            189 /*
            190  * Like JavaScript's built-in Error class, but supports a "cause" argument which
            191  * is wrapped, not "folded in" as with VError.  Accepts a printf-style message.
            192  * The cause argument can be null.
            193  */
            194 function WError(options)
            195 {
            196         Error.call(this);
            197 console.log('XXX WError: arguments: %j', arguments);
            198
            199         var args, cause, ctor;
            200         if (typeof (options) === 'object') {
            201                 args = Array.prototype.slice.call(arguments, 1);
            202         } else {
            203                 args = Array.prototype.slice.call(arguments, 0);
            204                 options = undefined;
            205         }

[trentm]

mdb_v8 version:

> ::load v8
mdb_v8 version: 1.1.4 (release, from 3a6fad0)
V8 version: 4.5.103.37
Autoconfigured V8 support from target
C++ symbol demangling enabled

[davepacheco]

One problem is that the way we tell how many arguments were passed is by looking at how many arguments the function takes, which as I understand it is determined by the way the function is defined. In this case WError only nominally takes one argument here. In terms of doing better: the first question would be to figure out if the other arguments are actually there on the stack, or if they're stored elsewhere when there's a varargs situation like this. Relatedly, we need to determine if there is a way to tell how many arguments were actually passed.