Making parse_server_name more consistent (#14007)

Fixes #12122

changelog.d/14007.misc

@@ -0,0 +1 @@
+Make `parse_server_name` consistent in handling invalid server names.

synapse/util/stringutils.py

@@ -86,7 +86,7 @@ def parse_server_name(server_name: str) -> Tuple[str, Optional[int]]:
         ValueError if the server name could not be parsed.
     """
     try:
-        if server_name[-1] == "]":
+        if server_name and server_name[-1] == "]":
             # ipv6 literal, hopefully
             return server_name, None
 
@@ -123,7 +123,7 @@ def parse_and_validate_server_name(server_name: str) -> Tuple[str, Optional[int]
     # that nobody is sneaking IP literals in that look like hostnames, etc.
 
     # look for ipv6 literals
-    if host[0] == "[":
+    if host and host[0] == "[":
         if host[-1] != "]":
             raise ValueError("Mismatched [...] in server name '%s'" % (server_name,))
 

tests/http/test_endpoint.py

@@ -25,6 +25,8 @@ class ServerNameTestCase(unittest.TestCase):
             "[0abc:1def::1234]": ("[0abc:1def::1234]", None),
             "1.2.3.4:1": ("1.2.3.4", 1),
             "[0abc:1def::1234]:8080": ("[0abc:1def::1234]", 8080),
+            ":80": ("", 80),
+            "": ("", None),
         }
 
         for i, o in test_data.items():
@@ -42,6 +44,7 @@ class ServerNameTestCase(unittest.TestCase):
             "newline.com\n",
             ".empty-label.com",
             "1234:5678:80",  # too many colons
+            ":80",
         ]
         for i in test_data:
             try: