TravisLintBear: Check for internet connection

Add a check for internet connection in the `check_prerequisites`
method without which the bear will fail to run.

Fixes #1978

bears/yaml/TravisLintBear.py

@@ -1,3 +1,5 @@
+import requests
+
 from coalib.bearlib.abstractions.Linter import linter
 
 from dependency_management.requirements.GemRequirement import GemRequirement
@@ -25,6 +27,32 @@ class TravisLintBear:
     CAN_DETECT = {'Formatting', 'Syntax'}
     SEE_MORE = 'https://docs.travis-ci.com/user/travis-lint'
 
+    @classmethod
+    def check_prerequisites(cls):
+        base_check = super().check_prerequisites()
+        if base_check is not True:
+            return base_check
+
+        check_connection_url = 'https://travis-ci.org/'
+        url_status = cls.get_url_status(check_connection_url)
+
+        try:
+            if url_status is None:
+                return 'You are not connected to the internet.'
+            else:
+                url_status.raise_for_status()
+                return True
+        except requests.exceptions.HTTPError:
+            return 'Failed to establish a connection to {}.'.format(
+                check_connection_url)
+
+    @staticmethod
+    def get_url_status(url):
+        try:
+            return requests.head(url, allow_redirects=False)
+        except requests.exceptions.RequestException:
+            return None
+
     @staticmethod
     def create_arguments(filename, file, config_file):
         return 'lint', filename

tests/yaml/TravisLintBearTest.py

@@ -1,5 +1,8 @@
 import os
+import requests
+import requests_mock
 from queue import Queue
+from unittest.mock import patch
 
 from coalib.results.Result import Result
 from coalib.results.RESULT_SEVERITY import RESULT_SEVERITY
@@ -71,3 +74,31 @@ def test_empty_file(self):
                                 file=get_testfile_path(file_name),
                                 severity=RESULT_SEVERITY.NORMAL)],
             filename=get_testfile_path(file_name))
+
+    def test_check_prerequisites(self):
+        with requests_mock.Mocker() as m:
+            check_connection_url = 'https://travis-ci.org/'
+            m.head(check_connection_url,
+                   status_code=200)
+            self.assertEqual(TravisLintBear.check_prerequisites(), True)
+
+            m.head(check_connection_url,
+                   exc=requests.exceptions.RequestException)
+            self.assertEqual(TravisLintBear.check_prerequisites(),
+                             'You are not connected to the internet.')
+
+            m.head(check_connection_url,
+                   status_code=404)
+            self.assertEqual(TravisLintBear.check_prerequisites(),
+                             'Failed to establish a connection to '
+                             'https://travis-ci.org/.')
+
+        # The primary base class is not the `LinterBase` inside `@linter`,
+        # but the class the user writes because of this mixin-technique
+        # `@linter` uses.
+        with patch.object(TravisLintBear.__bases__[1],
+                          'check_prerequisites') as mock_method:
+            base_check_fail_message = 'travis is not installed.'
+            mock_method.return_value = base_check_fail_message
+            self.assertEqual(base_check_fail_message,
+                             TravisLintBear.check_prerequisites())