[Core] Add a cli flag for setting a max size of message cache (#3474)

* Add an arg in cli to change message cache size

* Add an arg in cli to change message cache size

* Changelog

* Actually pass None in message_cache_size

* Update cli.py

* Add a cli arg to disable message cache.

* Add a cli arg to disable message cache.

* well go away you useless

* you actually are an int

* Check if message cache is higher than 0 when set it.

* Use sys.maxsize as max cache size.

* Update cli.py

* Add bot.max_messages property.

* typos

* 🤦

* style

changelog.d/3474.feature.rst

@@ -0,0 +1 @@
+Add a cli flag for setting a max size of message cache

redbot/core/bot.py

@@ -149,6 +149,12 @@ class RedBase(
         if "command_not_found" not in kwargs:
             kwargs["command_not_found"] = "Command {} not found.\n{}"
 
+        message_cache_size = cli_flags.message_cache_size
+        if cli_flags.no_message_cache:
+            message_cache_size = None
+        kwargs["max_messages"] = message_cache_size
+        self._max_messages = message_cache_size
+
         self._uptime = None
         self._checked_time_accuracy = None
         self._color = discord.Embed.Empty  # This is needed or color ends up 0x000000
@@ -271,6 +277,10 @@ class RedBase(
     def colour(self) -> NoReturn:
         raise AttributeError("Please fetch the embed colour with `get_embed_colour`")
 
+    @property
+    def max_messages(self) -> Optional[int]:
+        return self._max_messages
+
     async def allowed_by_whitelist_blacklist(
         self,
         who: Optional[Union[discord.Member, discord.User]] = None,

redbot/core/cli.py

@@ -74,6 +74,22 @@ async def interactive_config(red, token_set, prefix_set, *, print_header=True):
     return token
 
 
+def positive_int(arg: str) -> int:
+    try:
+        x = int(arg)
+    except ValueError:
+        raise argparse.ArgumentTypeError("Message cache size has to be a number.")
+    if x < 1000:
+        raise argparse.ArgumentTypeError(
+            "Message cache size has to be greater than or equal to 1000."
+        )
+    if x > sys.maxsize:
+        raise argparse.ArgumentTypeError(
+            f"Message cache size has to be lower than or equal to {sys.maxsize}."
+        )
+    return x
+
+
 def parse_cli_flags(args):
     parser = argparse.ArgumentParser(
         description="Red - Discord Bot", usage="redbot <instance_name> [arguments]"
@@ -212,6 +228,15 @@ def parse_cli_flags(args):
             "all of the data on the host machine."
         ),
     )
+    parser.add_argument(
+        "--message-cache-size",
+        type=positive_int,
+        default=1000,
+        help="Set the maximum number of messages to store in the internal message cache.",
+    )
+    parser.add_argument(
+        "--no-message-cache", action="store_true", help="Disable the internal message cache.",
+    )
 
     args = parser.parse_args(args)