port ratelimiting logic to its own class

src/pinnwand/handler/api_v1.py

@@ -1,6 +1,6 @@
 import json
 from datetime import datetime, timedelta
-from typing import Any
+from typing import Any, Awaitable, List, Optional
 from urllib.parse import urljoin
 
 import tornado.web
@@ -10,25 +10,43 @@
 log = logger.get_logger(__name__)
 
 
+class RateLimiterMixin:
+    """
+    A mixin that provides behavior for ratelimiting.
+
+    The subclass must define a list of areas where the ratelimit should be applied.
+
+    Due to MRO, the mixin needs to be the first to be inherited in the child classes.
+
+    Raises `RatelimitError` when a ratelimit is hit for any of the areas defined in the subclass.
+    """
+
+    areas: List[str] = []
+
+    def prepare(self) -> Optional[Awaitable[None]]:
+        for area in self.areas:
+            if defensive.ratelimit(self.request, area=area):
+                raise error.RatelimitError()
+        return None
+
+
 class Base(tornado.web.RequestHandler):
     def write_error(self, status_code: int, **kwargs: Any) -> None:
         _, exc, _ = kwargs["exc_info"]
         self.write({"state": "error", "code": status_code, "message": str(exc)})
 
 
-class Lexer(Base):
-    async def get(self) -> None:
-        if defensive.ratelimit(self.request, area="read"):
-            raise error.RatelimitError()
+class Lexer(RateLimiterMixin, Base):
+    areas = ["read"]
 
+    async def get(self) -> None:
         self.write(utility.list_languages())
 
 
-class Expiry(Base):
-    async def get(self) -> None:
-        if defensive.ratelimit(self.request, area="read"):
-            raise error.RatelimitError()
+class Expiry(RateLimiterMixin, Base):
+    areas = ["read"]
 
+    async def get(self) -> None:
         self.write(
             {
                 name: str(timedelta(seconds=delta))
@@ -37,17 +55,16 @@ async def get(self) -> None:
         )
 
 
-class Paste(Base):
+class Paste(RateLimiterMixin, Base):
+    areas = ["create"]
+
     def check_xsrf_cookie(self) -> None:
         return
 
     async def get(self) -> None:
         raise tornado.web.HTTPError(405)
 
     async def post(self) -> None:
-        if defensive.ratelimit(self.request, area="create"):
-            raise error.RatelimitError()
-
         try:
             data = tornado.escape.json_decode(self.request.body)
         except json.decoder.JSONDecodeError:
@@ -123,11 +140,10 @@ async def post(self) -> None:
             self.write({"link": url_paste, "removal": url_removal})
 
 
-class PasteDetail(Base):
-    async def get(self, slug: str) -> None:
-        if defensive.ratelimit(self.request, area="read"):
-            raise error.RatelimitError()
+class PasteDetail(RateLimiterMixin, Base):
+    areas = ["read"]
 
+    async def get(self, slug: str) -> None:
         with database.session() as session:
             paste = (
                 session.query(database.Paste)