Cache prefixes (#3150)

* Cache prefixes

 - This works towards #3148
 - Ends up centralizing some logic
   - Including that prefixes should be a reverse sorted list

* handle global prefix attempts at none

* fix prefix set for server

* cache using guild id

redbot/core/settings_caches.py

@@ -0,0 +1,53 @@
+from __future__ import annotations
+
+from typing import Dict, List, Optional
+from argparse import Namespace
+
+import discord
+
+from .config import Config
+
+
+class PrefixManager:
+    def __init__(self, config: Config, cli_flags: Namespace):
+        self._config: Config = config
+        self._global_prefix_overide: Optional[List[str]] = sorted(
+            cli_flags.prefix, reverse=True
+        ) or None
+        self._cached: Dict[Optional[int], List[str]] = {}
+
+    async def get_prefixes(self, guild: Optional[discord.Guild] = None) -> List[str]:
+        ret: List[str]
+
+        gid: Optional[int] = guild.id if guild else None
+
+        if gid in self._cached:
+            ret = self._cached[gid].copy()
+        else:
+            if gid is not None:
+                ret = await self._config.guild_from_id(gid).prefix()
+                if not ret:
+                    ret = await self.get_prefixes(None)
+            else:
+                ret = self._global_prefix_overide or (await self._config.prefix())
+
+            self._cached[gid] = ret.copy()
+
+        return ret
+
+    async def set_prefixes(
+        self, guild: Optional[discord.Guild] = None, prefixes: Optional[List[str]] = None
+    ):
+        gid: Optional[int] = guild.id if guild else None
+        prefixes = prefixes or []
+        if not isinstance(prefixes, list) and not all(isinstance(pfx, str) for pfx in prefixes):
+            raise TypeError("Prefixes must be a list of strings")
+        prefixes = sorted(prefixes, reverse=True)
+        if gid is None:
+            if not prefixes:
+                raise ValueError("You must have at least one prefix.")
+            self._cached.clear()
+            await self._config.prefix.set(prefixes)
+        else:
+            del self._cached[gid]
+            await self._config.guild_from_id(gid).prefix.set(prefixes)