Gracefully handle a missing subprocess module

As reported in #773, it seems that certain sandboxed environments like
CloudFlare Workers don't provide a `subprocess` module, and that causes
Node to throw when we try to require it.

Because the only thing we need `subprocess` for is getting `uname`, here
we extend the already-existing infrastructure built to gracefully handle
errors to also be able to handle this new error case.

Fixes #773.

lib/utils.js

@@ -1,12 +1,23 @@
 'use strict';
 
 const EventEmitter = require('events').EventEmitter;
-const exec = require('child_process').exec;
 const qs = require('qs');
 const crypto = require('crypto');
 
 const hasOwn = (obj, prop) => Object.prototype.hasOwnProperty.call(obj, prop);
 
+// Certain sandboxed environments (our known example right now are CloudFlare
+// Workers) may make `child_process` unavailable. Because `exec` isn't critical
+// to the operation of stripe-node, we handle this unavailability gracefully.
+var exec = null;
+try {
+  exec = require('child_process').exec;
+} catch (e) {
+  if (e.code !== 'MODULE_NOT_FOUND') {
+    throw e;
+  }
+}
+
 const OPTIONS_KEYS = [
   'apiKey',
   'idempotencyKey',
@@ -340,6 +351,13 @@ const utils = (module.exports = {
    * This unifies that interface.
    */
   safeExec: (cmd, cb) => {
+    // Occurs if we couldn't load the `child_process` module, which might
+    // happen in certain sandboxed environments like a CloudFlare Worker.
+    if (utils._exec === null) {
+      cb(new Error('exec not available'), null);
+      return;
+    }
+
     try {
       utils._exec(cmd, cb);
     } catch (e) {

test/utils.spec.js

@@ -506,6 +506,20 @@ describe('utils', () => {
       utils.safeExec('hello', myCb);
       expect(calls).to.deep.equal([[myErr, null]]);
     });
+
+    it ('handles being unable to require `child_process`', () => {
+      utils._exec = null;
+
+      var actualErr = null;
+      var actualRes = null;
+      function myCb(err, res) {
+        actualErr = err;
+        actualRes = res;
+      }
+      utils.safeExec('hello', myCb);
+      expect(actualErr.toString()).to.equal(new Error('exec not available').toString());
+      expect(actualRes).to.equal(null);
+    });
   });
 
   describe('flattenAndStringify', () => {