refactor: Build query in find_first_unread_anchor().

This is a bit more than a pure refactor, because we duplicate a
chunk of code to calculate a query inside of
find_first_unread_anchor(), so we're doing a bit more work
than before.

We need this refactoring to start decoupling find_first_unread_anchor
from get_messages_backend for the case where include_history is
True.  This will happen in a subsequent commit.

The only test that changes here is a direct test on
find_first_unread_anchor().  All other tests pass without
modification, and we have decent coverage on get_messages_backend.

zerver/tests/test_narrow.py

@@ -1760,21 +1760,10 @@ class GetOldMessagesTest(ZulipTestCase):
 
         user_profile = hamlet
 
-        # TODO: Make it so that find_first_unread_anchor() does not require
-        #       the incoming query to join to zerver_usermessage.
-        query = select([column("message_id"), column("flags")],
-                       column("user_profile_id") == literal(user_profile.id),
-                       join(table("zerver_usermessage"), table("zerver_message"),
-                            literal_column("zerver_usermessage.message_id") ==
-                            literal_column("zerver_message.id")))
-        inner_msg_id_col = column("message_id")
-
         anchor = find_first_unread_anchor(
             sa_conn=sa_conn,
-            inner_msg_id_col=inner_msg_id_col,
             user_profile=user_profile,
             narrow=[],
-            query=query,
         )
         self.assertEqual(anchor, first_message_id)
 

zerver/views/messages.py

@@ -622,10 +622,32 @@ def add_narrow_conditions(user_profile: UserProfile,
     return (query, is_search)
 
 def find_first_unread_anchor(sa_conn: Any,
-                             inner_msg_id_col: ColumnElement,
                              user_profile: UserProfile,
-                             narrow: List[Dict[str, Any]],
-                             query: Query) -> int:
+                             narrow: List[Dict[str, Any]]) -> int:
+    # We always need UserMessage in our query, because it has the unread
+    # flag for the user.
+    need_user_message = True
+
+    # TODO:
+    # We err on the side of putting Message in our query, but there are
+    # probably situations that we don't need it.  We may eventually try
+    # to make add_narrow_conditions() and similar functions help us make
+    # possible optimizations.
+    need_message = True
+
+    query, inner_msg_id_col = get_base_query_for_search(
+        user_profile=user_profile,
+        need_message=need_message,
+        need_user_message=need_user_message,
+    )
+
+    query, is_search = add_narrow_conditions(
+        user_profile=user_profile,
+        inner_msg_id_col=inner_msg_id_col,
+        query=query,
+        narrow=narrow,
+    )
+
     condition = column("flags").op("&")(UserMessage.flags.read.mask) == 0
 
     # We exclude messages on muted topics when finding the first unread
@@ -716,10 +738,8 @@ def get_messages_backend(request: HttpRequest, user_profile: UserProfile,
     if use_first_unread_anchor:
         anchor = find_first_unread_anchor(
             sa_conn,
-            inner_msg_id_col,
             user_profile,
             narrow,
-            query
         )
 
     anchored_to_left = (anchor == 0)