Don't fail on empty bodies when sending out read receipts (#10531)

Fixes a bug introduced in rc1 that would cause Synapse to 400 on read receipts requests with empty bodies.

Broken in #10413

changelog.d/10531.bugfix

+Fix a bug introduced in Synapse v1.40.0rc1 that would cause Synapse to respond with an error when clients would update their read receipts.

synapse/rest/client/v2_alpha/receipts.py

@@ -43,7 +43,7 @@ class ReceiptRestServlet(RestServlet):
         if receipt_type != "m.read":
             raise SynapseError(400, "Receipt type must be 'm.read'")
 
-        body = parse_json_object_from_request(request)
+        body = parse_json_object_from_request(request, allow_empty_body=True)
         hidden = body.get(ReadReceiptEventFields.MSC2285_HIDDEN, False)
 
         if not isinstance(hidden, bool):

tests/rest/client/v2_alpha/test_sync.py

@@ -418,6 +418,18 @@ class ReadReceiptsTestCase(unittest.HomeserverTestCase):
         # Test that the first user can't see the other user's hidden read receipt
         self.assertEqual(self._get_read_receipt(), None)
 
+    def test_read_receipt_with_empty_body(self):
+        # Send a message as the first user
+        res = self.helper.send(self.room_id, body="hello", tok=self.tok)
+
+        # Send a read receipt for this message with an empty body
+        channel = self.make_request(
+            "POST",
+            "/rooms/%s/receipt/m.read/%s" % (self.room_id, res["event_id"]),
+            access_token=self.tok2,
+        )
+        self.assertEqual(channel.code, 200)
+
     def _get_read_receipt(self):
         """Syncs and returns the read receipt."""