Utils: add coerce_url_to_protocol()

Sometimes we have a url that may or may not be using the protocol we want.  In
those cases, it's nice to have a handy way to ensure the protocol.

r2/r2/lib/utils/utils.py

@@ -677,6 +677,13 @@ class UrlParser(object):
                            u.path, u.params, u.query, fragment))
 
 
+def coerce_url_to_protocol(url, protocol='http'):
+    '''Given an absolute (but potentially protocol-relative) url, coerce it to
+    a protocol.'''
+    parsed_url = UrlParser(url)
+    parsed_url.scheme = protocol
+    return parsed_url.unparse()
+
 def url_is_embeddable_image(url):
     """The url is on an oembed-friendly domain and looks like an image."""
     parsed_url = UrlParser(url)

r2/r2/tests/unit/lib/utils_test.py

@@ -70,6 +70,31 @@ class UtilsTest(unittest.TestCase):
             utils.extract_subdomain('internet-frontpage.com', 'reddit.com'),
             '')
 
+    def test_coerce_url_to_protocol(self):
+        self.assertEquals(
+            utils.coerce_url_to_protocol('http://example.com/foo'),
+            'http://example.com/foo')
+
+        self.assertEquals(
+            utils.coerce_url_to_protocol('https://example.com/foo'),
+            'http://example.com/foo')
+
+        self.assertEquals(
+            utils.coerce_url_to_protocol('//example.com/foo'),
+            'http://example.com/foo')
+
+        self.assertEquals(
+            utils.coerce_url_to_protocol('http://example.com/foo', 'https'),
+            'https://example.com/foo')
+
+        self.assertEquals(
+            utils.coerce_url_to_protocol('https://example.com/foo', 'https'),
+            'https://example.com/foo')
+
+        self.assertEquals(
+            utils.coerce_url_to_protocol('//example.com/foo', 'https'),
+            'https://example.com/foo')
+
 
 class TestCanonicalizeEmail(unittest.TestCase):
     def test_empty_string(self):