child_process execFile / spawn throw non-descript exception on windows if exe requires elevation

[TanninOne]

• Version: v6.5.0
• Platform: Windows 10 (x64)
• Subsystem: child_process

let cp = require('child_process');
let util = require('util');
try {
  cp.execFileSync('SomeTool.exe')
} catch (err) {
  console.log('err', util.inspect(err));
}

outputs

err { Error: spawnSync SomeTool.exe UNKNOWN
    at exports._errnoException (util.js:1026:11)
    at spawnSync (child_process.js:461:20)
    at Object.execFileSync (child_process.js:498:13)
    at repl:2:4
    at sigintHandlersWrap (vm.js:22:35)
    at sigintHandlersWrap (vm.js:96:12)
    at ContextifyScript.Script.runInThisContext (vm.js:21:12)
    at REPLServer.defaultEval (repl.js:313:29)
    at bound (domain.js:280:14)
    at REPLServer.runBound [as eval] (domain.js:293:12)
  code: 'UNKNOWN',
  errno: 'UNKNOWN',
  ...
}

spawn generates the same error.

Obviously it would be better if this error was returned through the callback but I would also love to have a proper error code to react to the situation.
Not sure how execFile and spawn are implemented but it should be possible to report a proper errorcode as CreateProcess generates errorcode 740 if elevation is required.

[silverwind]

Does SomeTool.exe actually exit with this error level? Try in PowerShell:

SomeTool.exe
echo %ERRORLEVEL%

[TanninOne]

It doesn't work like this. As I said: SomeTool.exe requires elevation, which means if you start it in cmd.exe or powershell it displays a UAC dialog and if the user accepts it the application is run in a separate administrator shell. The orignal shell doesn't receive an exit code.

However, running execFile/spawn from node does not display an UAC dialog to begin with so SomeTool.exe isn't started at all. It can't be - unless node exploits a security bug in Windows to circumvent security measures. Whatever api node/libuv is using to spawn a process (I assume CreateProcess because ShellExecute would work and display a UAC dialog) fails immediately with an error code that says the user doesn't have the permission to run that exe and node doesn't report that error code back to the application.

I just updated to node 6.9.1 to repeat the test. I also realized that using execFileSync might hide an aspect of the problem:

let cp = require('child_process');
let util = require('util');
try {
  cp.execFile('SomeTool.exe', (err, out) => {
    if (err) {
      console.log('err', err);
    }
  });
} catch (ex) {
  console.log('exception', ex);
}

reports:

exception { Error: spawn UNKNOWN
    at exports._errnoException (util.js:1026:11)
    at ChildProcess.spawn (internal/child_process.js:313:11)
    at exports.spawn (child_process.js:380:9)
    ...

As you see, the error is reported synchronously as an exception, not as an error through the callback.

[silverwind]

FYI, It's using CreateProcess.

[richardlau]

UNKNOWN is because ERROR_ELEVATION_REQUIRED (740) is not translated in uv_translate_sys_error(). I'll submit a fix upstream.

[richardlau]

libuv/libuv#1154