[networking] Add request handler preference framework (#7603)

Preference functions that take a request and a request handler instance can be registered to prioritize different request handlers per request.

Authored by: coletdjnz
Co-authored-by: pukkandan <pukkandan.ytdlp@gmail.com>

test/test_networking.py

@@ -1035,17 +1035,17 @@ class TestRequestDirector:
         assert isinstance(director.send(Request('http://')), FakeResponse)
 
     def test_unsupported_handlers(self):
-        director = RequestDirector(logger=FakeLogger())
-        director.add_handler(FakeRH(logger=FakeLogger()))
-
         class SupportedRH(RequestHandler):
             _SUPPORTED_URL_SCHEMES = ['http']
 
             def _send(self, request: Request):
                 return Response(fp=io.BytesIO(b'supported'), headers={}, url=request.url)
 
-        # This handler should by default take preference over FakeRH
+        director = RequestDirector(logger=FakeLogger())
         director.add_handler(SupportedRH(logger=FakeLogger()))
+        director.add_handler(FakeRH(logger=FakeLogger()))
+
+        # First should take preference
         assert director.send(Request('http://')).read() == b'supported'
         assert director.send(Request('any://')).read() == b''
 
@@ -1072,6 +1072,27 @@ class TestRequestDirector:
         director.add_handler(UnexpectedRH(logger=FakeLogger))
         assert director.send(Request('any://'))
 
+    def test_preference(self):
+        director = RequestDirector(logger=FakeLogger())
+        director.add_handler(FakeRH(logger=FakeLogger()))
+
+        class SomeRH(RequestHandler):
+            _SUPPORTED_URL_SCHEMES = ['http']
+
+            def _send(self, request: Request):
+                return Response(fp=io.BytesIO(b'supported'), headers={}, url=request.url)
+
+        def some_preference(rh, request):
+            return (0 if not isinstance(rh, SomeRH)
+                    else 100 if 'prefer' in request.headers
+                    else -1)
+
+        director.add_handler(SomeRH(logger=FakeLogger()))
+        director.preferences.add(some_preference)
+
+        assert director.send(Request('http://')).read() == b''
+        assert director.send(Request('http://', headers={'prefer': '1'})).read() == b'supported'
+
 
 # XXX: do we want to move this to test_YoutubeDL.py?
 class TestYoutubeDLNetworking: