topic_link_util: Add support for empty topics in the backend.

Fallback links with empty topics now have correct
url and text for the "general chat" topic.

zerver/lib/topic_link_util.py

@@ -3,6 +3,8 @@
 import re
 import urllib.parse
 
+from zerver.models.messages import Message
+
 invalid_stream_topic_regex = re.compile(r"[`>*&\[\]]|(\$\$)")
 
 
@@ -42,13 +44,6 @@ def encode_hash_component(s: str) -> str:
     return "".join(hash_replacements.get(c, c) for c in encoded)
 
 
-def channel_topic_url(stream_id: int, stream_name: str, topic_name: str | None = None) -> str:
-    link = f"#narrow/channel/{stream_id}-{encode_hash_component(stream_name.replace(' ', '-'))}"
-    if topic_name:
-        link += f"/topic/{encode_hash_component(topic_name)}"
-    return link
-
-
 def get_fallback_markdown_link(
     stream_id: int, stream_name: str, topic_name: str | None = None
 ) -> str:
@@ -56,15 +51,19 @@ def get_fallback_markdown_link(
     Generates the markdown link syntax for a stream or topic link.
     """
     escape = escape_invalid_stream_topic_characters
-    url = channel_topic_url(stream_id, stream_name, topic_name)
+    link = f"#narrow/channel/{stream_id}-{encode_hash_component(stream_name.replace(' ', '-'))}"
-    if topic_name:
+    text = f"#{escape(stream_name)}"
-        return f"[#{escape(stream_name)} > {escape(topic_name)}]({url})"
+    if topic_name is not None:
+        link += f"/topic/{encode_hash_component(topic_name)}"
+        if topic_name == "":
+            topic_name = Message.EMPTY_TOPIC_FALLBACK_NAME
+        text += f" > {escape(topic_name)}"
 
-    return f"[#{escape(stream_name)}]({url})"
+    return f"[{text}]({link})"
 
 
 def get_stream_topic_link_syntax(stream_id: int, stream_name: str, topic_name: str) -> str:
-    # If the topic name is such that it will generate an invalid #**stream>topic** syntax,
+    # If the stream/topic name is such that it will generate an invalid #**stream>topic** syntax,
     # we revert to generating the normal markdown syntax for a link.
     if will_produce_broken_stream_topic_link(topic_name) or will_produce_broken_stream_topic_link(
         stream_name
@@ -74,7 +73,7 @@ def get_stream_topic_link_syntax(stream_id: int, stream_name: str, topic_name: s
 
 
 def get_stream_link_syntax(stream_id: int, stream_name: str) -> str:
-    # If the topic name is such that it will generate an invalid #**stream>topic** syntax,
+    # If the stream name is such that it will generate an invalid #**stream** syntax,
     # we revert to generating the normal markdown syntax for a link.
     if will_produce_broken_stream_topic_link(stream_name):
         return get_fallback_markdown_link(stream_id, stream_name)

zerver/tests/test_topic_link_util.py

@@ -68,3 +68,11 @@ class TestTopicLinkUtil(ZulipTestCase):
             get_stream_topic_link_syntax(sweden_id, "Sweden", "&a[b"),
             f"[#Sweden > &a[b](#narrow/channel/{sweden_id}-Sweden/topic/.26a.5Bb)",
         )
+        self.assertEqual(
+            get_stream_topic_link_syntax(sweden_id, "Sweden", ""),
+            "#**Sweden>**",
+        )
+        self.assertEqual(
+            get_stream_topic_link_syntax(sweden_id, "Sw*den", ""),
+            f"[#Sw*den > general chat](#narrow/channel/{sweden_id}-Sw*den/topic/)",
+        )