Produce a better error in case of malformed JSON

$ curl localhost:8777 -X POST -i -d '{"action": "version", "version": 6},' && echo ␄
HTTP/1.1 200 OK
Content-Type: text/json
Access-Control-Allow-Origin: http://localhost
Access-Control-Allow-Headers: *
Content-Length: 67

{"result": null, "error": "Extra data: line 1 column 36 (char 35)"}␄

$ curl localhost:8777 -X POST -i -d '{"action": "version", "version": 6},' -H "Origin: foo" && echo ␄
HTTP/1.1 403 Forbidden
Access-Control-Allow-Origin: http://localhost
Access-Control-Allow-Headers: *

␄

plugin/__init__.py

@@ -43,6 +43,7 @@ from anki.notes import Note
 from anki.errors import NotFoundError
 from aqt.qt import Qt, QTimer, QMessageBox, QCheckBox
 
+from .web import format_exception_reply, format_success_reply
 from .edit import Edit
 from . import web, util
 
@@ -99,7 +100,6 @@ class AnkiConnect:
         version = request.get('version', 4)
         params = request.get('params', {})
         key = request.get('key')
-        reply = {'result': None, 'error': None}
 
         try:
             if key != util.setting('apiKey') and name != 'requestPermission':
@@ -126,14 +126,12 @@ class AnkiConnect:
 
             if method is None:
                 raise Exception('unsupported action')
-            else:
-                reply['result'] = methodInst(**params)
 
-            if version <= 4:
+            api_return_value = methodInst(**params)
-                reply = reply['result']
+            reply = format_success_reply(version, api_return_value)
 
         except Exception as e:
-            reply['error'] = str(e)
+            reply = format_exception_reply(version, e)
 
         self.logEvent('reply', reply)
         return reply

plugin/web.py

@@ -175,18 +175,21 @@ class WebServer:
 
             return self.buildResponse(headers, body)
     
-        paramsError = False
-
         try:
             params = json.loads(req.body.decode('utf-8'))
-        except ValueError:
+        except ValueError as e:
-            body = json.dumps(None).encode('utf-8')
+            if allowed:
-            paramsError = True
-
-        if allowed or not paramsError and params.get('action', '') == 'requestPermission':
                 if len(req.body) == 0:
-                body = 'AnkiConnect v.{}'.format(util.setting('apiVersion')).encode('utf-8')
+                    body = f"AnkiConnect v.{util.setting('apiVersion')}".encode()
                 else:
+                    reply = format_exception_reply(util.setting('apiVersion'), e)
+                    body = json.dumps(reply).encode('utf-8')
+                headers = self.buildHeaders(corsOrigin, body)
+                return self.buildResponse(headers, body)
+            else:
+                params = {}  # trigger the 403 response below
+
+        if allowed or params.get('action', '') == 'requestPermission':
             if params.get('action', '') == 'requestPermission':
                 params['params'] = params.get('params', {})
                 params['params']['allowed'] = allowed
@@ -195,7 +198,6 @@ class WebServer:
                     corsOrigin = params['params']['origin']
                         
             body = json.dumps(self.handler(params)).encode('utf-8')
-                    
             headers = self.buildHeaders(corsOrigin, body)
         else :
             headers = [
@@ -273,3 +275,14 @@ class WebServer:
             client.close()
 
         self.clients = []
+
+
+def format_success_reply(api_version, result):
+    if api_version <= 4:
+        return result
+    else:
+        return {"result": result, "error": None}
+
+
+def format_exception_reply(_api_version, exception):
+    return {"result": None, "error": str(exception)}

tests/test_server.py

@@ -46,11 +46,14 @@ class Client:
         return {"action": action, "params": params, "version": 6}
 
     def send_request(self, action, **params):
-        request_url = f"http://localhost:{self.port}"
         request_data = self.make_request(action, **params)
-        request_json = json.dumps(request_data).encode("utf-8")
+        json_bytes = json.dumps(request_data).encode("utf-8")
-        request = urllib.request.Request(request_url, request_json)
+        return json.loads(self.send_bytes(json_bytes))
-        response = json.load(urllib.request.urlopen(request))
+
+    def send_bytes(self, bytes, headers={}):  # noqa
+        request_url = f"http://localhost:{self.port}"
+        request = urllib.request.Request(request_url, bytes, headers)
+        response = urllib.request.urlopen(request).read()
         return response
 
     def wait_for_web_server_to_come_live(self, at_most_seconds=30):
@@ -137,6 +140,11 @@ def test_multi_request(external_anki):
     }
 
 
+def test_request_with_empty_body_returns_version_banner(external_anki):
+    response = external_anki.send_bytes(b"")
+    assert response == b"AnkiConnect v.6"
+
+
 def test_failing_request_due_to_bad_arguments(external_anki):
     response = external_anki.send_request("addNote", bad="request")
     assert response["result"] is None
@@ -147,3 +155,24 @@ def test_failing_request_due_to_anki_raising_exception(external_anki):
     response = external_anki.send_request("suspend", cards=[-123])
     assert response["result"] is None
     assert "Card was not found" in response["error"]
+
+
+def test_failing_request_due_to_bad_encoding(external_anki):
+    response = json.loads(external_anki.send_bytes(b"\xe7\x8c"))
+    assert response["result"] is None
+    assert "can't decode" in response["error"]
+
+
+def test_failing_request_due_to_bad_json(external_anki):
+    response = json.loads(external_anki.send_bytes(b'{1: 2}'))
+    assert response["result"] is None
+    assert "in double quotes" in response["error"]
+
+
+def test_403_in_case_of_disallowed_origin(external_anki):
+    with pytest.raises(urllib.error.HTTPError, match="403"):  # good request/json
+        json_bytes = json.dumps(Client.make_request("version")).encode("utf-8")
+        external_anki.send_bytes(json_bytes, headers={b"origin": b"foo"})
+
+    with pytest.raises(urllib.error.HTTPError, match="403"):  # bad json
+        external_anki.send_bytes(b'{1: 2}', headers={b"origin": b"foo"})