Extract finish_body() method on HTTP parser

Refactor the body-drain logic out of Parser.__next__() into a public
finish_body() method. No behavior change — __next__() calls it in the
same place. This makes the drain available for future use (e.g. explicit
keepalive body discard once thread pool exhaustion protection is added).
Ported from gunicorn b43dc6d.

Author: davegaeddert

plain/plain/server/http/parser.py

@@ -42,16 +42,24 @@ def __init__(
     def __iter__(self) -> Iterator[Request]:
         return self
 
+    def finish_body(self) -> None:
+        """Discard any unread body of the current message.
+
+        Call before returning a keepalive connection to the poller so the
+        socket doesn't appear readable due to leftover body bytes.
+        """
+        if self.mesg and self.mesg.body:
+            data = self.mesg.body.read(8192)
+            while data:
+                data = self.mesg.body.read(8192)
+
     def __next__(self) -> Request:
         # Stop if HTTP dictates a stop.
         if self.mesg and self.mesg.should_close():
             raise StopIteration()
 
         # Discard any unread body of the previous message
-        if self.mesg and self.mesg.body:
-            data = self.mesg.body.read(8192)
-            while data:
-                data = self.mesg.body.read(8192)
+        self.finish_body()
 
         # Parse the next request
         self.req_count += 1