help: if groff is not found, try mandoc

This will allow 'help' to work on BSDs, Illumos and Linux
distributions using the mandoc formatter.

awscli/help.py

@@ -106,9 +106,12 @@ class PosixHelpRenderer(PagingHelpRenderer):
 
     def _convert_doc_content(self, contents):
         man_contents = publish_string(contents, writer=manpage.Writer())
-        if not self._exists_on_path('groff'):
-            raise ExecutableNotFoundError('groff')
-        cmdline = ['groff', '-m', 'man', '-T', 'ascii']
+        if self._exists_on_path('groff'):
+            cmdline = ['groff', '-m', 'man', '-T', 'ascii']
+        elif self._exists_on_path('mandoc'):
+            cmdline = ['mandoc', '-T', 'ascii']
+        else:
+            raise ExecutableNotFoundError('groff or mandoc')
         LOG.debug("Running command: %s", cmdline)
         p3 = self._popen(cmdline, stdin=PIPE, stdout=PIPE, stderr=PIPE)
         groff_output = p3.communicate(input=man_contents)[0]

tests/unit/test_help.py

@@ -99,8 +99,8 @@ def test_pager_with_args(self):
     def test_no_groff_exists(self):
         renderer = FakePosixHelpRenderer()
         renderer.exists_on_path['groff'] = False
-        expected_error = 'Could not find executable named "groff"'
-        with self.assertRaisesRegex(ExecutableNotFoundError, expected_error):
+        expected_error = 'Could not find executable named "groff or mandoc"'
+        with self.assertRaisesRegexp(ExecutableNotFoundError, expected_error):
             renderer.render('foo')
 
     @skip_if_windows('Requires POSIX system.')