From a9934d48c193bc963e3d232ed83c5cbfa3e5152d Mon Sep 17 00:00:00 2001
From: Abdullah Osama <abdullahosama15@gmail.com>
Date: Tue, 11 Oct 2022 14:42:11 +0200
Subject: [PATCH] Making parse_server_name more consistent (#14007)

Fixes #12122
---
 changelog.d/14007.misc      | 1 +
 synapse/util/stringutils.py | 4 ++--
 tests/http/test_endpoint.py | 3 +++
 3 files changed, 6 insertions(+), 2 deletions(-)
 create mode 100644 changelog.d/14007.misc

diff --git a/changelog.d/14007.misc b/changelog.d/14007.misc
new file mode 100644
index 000000000000..3f0f3afe1cce
--- /dev/null
+++ b/changelog.d/14007.misc
@@ -0,0 +1 @@
+Make `parse_server_name` consistent in handling invalid server names.
\ No newline at end of file
diff --git a/synapse/util/stringutils.py b/synapse/util/stringutils.py
index 27a363d7e516..4961fe9313ac 100644
--- a/synapse/util/stringutils.py
+++ b/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,))
 
diff --git a/tests/http/test_endpoint.py b/tests/http/test_endpoint.py
index c8cc21cadd2f..a801f002a004 100644
--- a/tests/http/test_endpoint.py
+++ b/tests/http/test_endpoint.py
@@ -25,6 +25,8 @@ def test_parse_server_name(self):
             "[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 @@ def test_validate_bad_server_names(self):
             "newline.com\n",
             ".empty-label.com",
             "1234:5678:80",  # too many colons
+            ":80",
         ]
         for i in test_data:
             try: