Universal python detection

package.json

@@ -13,7 +13,7 @@
   "browser": "./shim/vertical.js",
   "scripts": {
     "deploy": "rimraf gh-pages/closure-library/scripts/ci/CloseAdobeDialog.exe && gh-pages -t -d gh-pages -m \"Build for $(git log --pretty=format:%H -n1) [skip ci]\"",
-    "prepublish": "python3 build.py && webpack",
+    "prepublish": "node universal-python.js build.py && webpack",
     "test:unit": "node tests/jsunit/test_runner.js",
     "test:lint": "eslint .",
     "test:messages": "npm run translate && node i18n/test_scratch_msgs.js",
@@ -46,6 +46,7 @@
     "transifex": "1.6.6",
     "uglifyjs-webpack-plugin": "1.3.0",
     "webpack": "4.47.0",
-    "webpack-cli": "3.3.12"
+    "webpack-cli": "3.3.12",
+    "which": "^4.0.0"
   }
 }

universal-python.js

@@ -0,0 +1,82 @@
+/* eslint-env node */
+
+// There are a lot of different ways that people can install python, and there is no
+// universal name that they use for the actual executable. We can already assume
+// that there is a working Node.js environment, so this script can figure it out.
+
+var child_process = require('child_process');
+var _which = require('which');
+
+function which(command) {
+  return _which.sync(command, {
+    nothrow: true
+  });
+}
+
+function run(command, args) {
+  console.log('universal-python: Running ' + command + ' with arguments ' + args.join(', '));
+  var subprocess = child_process.spawn(command, args, {
+    windowsHide: true,
+    shell: false,
+    stdio: 'inherit'
+  });
+  subprocess.on('exit', function(code) {
+    // Microsoft Store install shim exits with code 9009
+    if (process.platform === 'win32' && code === 9009) {
+      // eslint-disable-next-line max-len
+      console.log('universal-python: Attempted to start python, but got the Microsoft Store installation shim. Install python from the Microsoft Store or python.org. (code 9009)');
+    } else if (code === 0) {
+      console.log('universal-python: Success');
+    } else {
+      console.log('universal-python: Failed with code ' + code);
+    }
+    process.exit(code);
+  });
+  subprocess.on('error', function(error) {
+    console.log('universal-python: Error starting python; please ensure you have python installed: ' + error);
+    process.exit(1);
+  });
+  process.on('SIGINT', function() {
+    subprocess.kill('SIGINT');
+  });
+}
+
+var argv = process.argv.slice(2);
+
+if (process.platform === 'win32') {
+  // Python in Windows is weird. Most commonly it will be installed using the python.org installers or
+  // from the Microsoft Store. Interestingly the Microsoft Store shims always exist on PATH and are
+  // also weird fake files so Node.js' fs does not work very well with them.
+  // Python.org installers will usually install the py launcher, so we'll try that first so that we
+  // don't open up the Microsoft Store for people who installed Python from there.
+  var py = which('py.exe');
+  if (py) {
+    run(py, ['-3'].concat(argv));
+  } else {
+    // This will either run the python installation from PATH or the Microsoft Store install shim.
+    // Don't know what version we'll get, but 2 and 3 both work so it doesn't matter.
+    run('python.exe', argv);
+  }
+} else {
+  // Python 3 is usually installed as python3
+  var python3 = which('python3');
+  if (python3) {
+    run(python3, argv);
+  } else {
+    // Sometimes it might only be installed as python
+    // Don't know what version we'll get, but 2 and 3 both work so it doesn't matter
+    var python = which('python');
+    if (python) {
+      run(python, argv);
+    } else {
+      // Some old systems might only have python 2
+      var python2 = which('python2');
+      if (python2) {
+        run(python2, argv);
+      } else {
+        console.log("universal-python: Could not find python on your PATH; please ensure you have python installed.");
+        process.exit(1);
+      }
+    }
+  }
+}