Add mock tests to cover subcommand error handling (#1474)

tests/command.executableSubcommand.mock.test.js

@@ -0,0 +1,64 @@
+const commander = require('../');
+const childProcess = require('child_process');
+const EventEmitter = require('events');
+
+// Using mock to allow try/catch around what is otherwise out-of-stack error handling.
+// Injecting errors, these are not end-to-end tests.
+
+function makeSystemError(code) {
+  // We can not make an actual SystemError, but our usage is lightweight so easy to match.
+  const err = new Error();
+  err.code = code;
+  return err;
+}
+
+test('when subcommand executable missing (ENOENT) then throw custom message', () => {
+  // If the command is not found, we show a custom error with an explanation and offer
+  // some advice for possible fixes.
+  const mockProcess = new EventEmitter();
+  const spawnSpy = jest.spyOn(childProcess, 'spawn').mockImplementation(() => { return mockProcess; });
+  const program = new commander.Command();
+  program.exitOverride();
+  program.command('executable', 'executable description');
+  program.parse(['executable'], { from: 'user' });
+  expect(() => {
+    mockProcess.emit('error', makeSystemError('ENOENT'));
+  }).toThrow('use the executableFile option to supply a custom name'); // part of custom message
+  spawnSpy.mockRestore();
+});
+
+test('when subcommand executable not executable (EACCES) then throw custom message', () => {
+  // Side note: this error does not actually happen on Windows! But we can still simulate the behaviour on other platforms.
+  const mockProcess = new EventEmitter();
+  const spawnSpy = jest.spyOn(childProcess, 'spawn').mockImplementation(() => { return mockProcess; });
+  const program = new commander.Command();
+  program.exitOverride();
+  program.command('executable', 'executable description');
+  program.parse(['executable'], { from: 'user' });
+  expect(() => {
+    mockProcess.emit('error', makeSystemError('EACCES'));
+  }).toThrow('not executable'); // part of custom message
+  spawnSpy.mockRestore();
+});
+
+test('when subcommand executable fails with other error then return in custom wrapper', () => {
+  // The existing behaviour is to just silently fail for unexpected errors, as it is happening
+  // asynchronously in spawned process and client can not catch errors.
+  const mockProcess = new EventEmitter();
+  const spawnSpy = jest.spyOn(childProcess, 'spawn').mockImplementation(() => { return mockProcess; });
+  const program = new commander.Command();
+  program.exitOverride((err) => {
+    throw err;
+  });
+  program.command('executable', 'executable description');
+  program.parse(['executable'], { from: 'user' });
+  let caughtErr;
+  try {
+    mockProcess.emit('error', makeSystemError('OTHER'));
+  } catch (err) {
+    caughtErr = err;
+  }
+  expect(caughtErr.code).toEqual('commander.executeSubCommandAsync');
+  expect(caughtErr.nestedError.code).toEqual('OTHER');
+  spawnSpy.mockRestore();
+});