Eliminate the parse_usermessage_flags interface.

This commit puts the guts of parse_usermessage_flags into
UserMessage.flags_list_for_flags, since it was slightly faster
than the old implementation and produced the same results.
(Both algorithms were super fast, actually.)

And then all callers use the model method now.

zerver/models.py

@@ -1318,18 +1318,19 @@ class AbstractUserMessage(models.Model):
         return self.flags_list_for_flags(flags)
 
     @staticmethod
-    def flags_list_for_flags(flags):
+    def flags_list_for_flags(val):
         # type: (int) -> List[str]
         '''
         This function is highly optimized, because it actually slows down
         sending messages in a naive implementation.
         '''
-        names = AbstractUserMessage.ALL_FLAGS
-        return [
-            names[i]
-            for i in range(len(names))
-            if flags & (2 ** i)
-        ]
+        flags = []
+        mask = 1
+        for flag in UserMessage.ALL_FLAGS:
+            if val & mask:
+                flags.append(flag)
+            mask <<= 1
+        return flags
 
     def __str__(self):
         # type: () -> Text
@@ -1347,17 +1348,6 @@ class UserMessage(AbstractUserMessage):
     message = models.ForeignKey(Message, on_delete=CASCADE)  # type: Message
 
 
-def parse_usermessage_flags(val):
-    # type: (int) -> List[str]
-    flags = []
-    mask = 1
-    for flag in UserMessage.ALL_FLAGS:
-        if val & mask:
-            flags.append(flag)
-        mask <<= 1
-    return flags
-
-
 class AbstractAttachment(models.Model):
     file_name = models.TextField(db_index=True)  # type: Text
     # path_id is a storage location agnostic representation of the path of the file.

zerver/views/messages.py

@@ -36,7 +36,7 @@ from zerver.lib.validator import \
     check_list, check_int, check_dict, check_string, check_bool
 from zerver.models import Message, UserProfile, Stream, Subscription, \
     Realm, RealmDomain, Recipient, UserMessage, bulk_get_recipients, get_personal_recipient, \
-    get_stream, parse_usermessage_flags, email_to_domain, get_realm, get_active_streams, \
+    get_stream, email_to_domain, get_realm, get_active_streams, \
     get_user_including_cross_realm, get_stream_recipient
 
 from sqlalchemy import func
@@ -739,7 +739,7 @@ def get_messages_backend(request, user_profile,
         for row in query_result:
             message_id = row[0]
             flags = row[1]
-            user_message_flags[message_id] = parse_usermessage_flags(flags)
+            user_message_flags[message_id] = UserMessage.flags_list_for_flags(flags)
             message_ids.append(message_id)
 
     search_fields = dict()  # type: Dict[int, Dict[str, Text]]