[fix] Fix accounting stop handling for empty octets

Certain NAS devices, such as hostapd, may omit input/output octets
in accounting-stop requests. Consequently, FreeRADIUS represents
these fields as empty strings in API requests to this API. Django
REST Framework's validation rejects these requests as the fields
are defined as Integer fields and do not accept empty strings.

This patch addresses this issue by gracefully handling empty strings,
making the API more tolerant of varying system behaviors and
enhancing compatibility across heterogeneous systems.

openwisp_radius/api/serializers.py

@@ -151,13 +151,13 @@ class RadiusAccountingSerializer(serializers.ModelSerializer):
     framed_ipv6_address = serializers.IPAddressField(
         required=False, allow_blank=True, protocol='IPv6'
     )
-    session_time = serializers.IntegerField(required=False, default=0)
+    session_time = serializers.IntegerField(required=False)
     stop_time = serializers.DateTimeField(required=False)
     update_time = serializers.DateTimeField(required=False)
-    input_octets = serializers.IntegerField(required=False, default=0)
-    output_octets = serializers.IntegerField(required=False, default=0)
-    # this is needed otherwise serialize will ignore status_type from accounting packet
-    # as it's not a model field
+    input_octets = serializers.IntegerField(required=False)
+    output_octets = serializers.IntegerField(required=False)
+    # this is needed otherwise serializer will ignore status_type
+    # from the accounting request because it's not a model field
     status_type = serializers.ChoiceField(
         write_only=True, required=True, choices=STATUS_TYPE_CHOICES
     )
@@ -191,9 +191,24 @@ def is_valid(self, raise_exception=False):
             raise error
 
     def run_validation(self, data):
+        """
+        Custom validation to handle empty strings in:
+            - session_time
+            - input_octets
+            - output_octets
+        """
         for field in ['session_time', 'input_octets', 'output_octets']:
-            if data.get('status_type', None) == 'Start' and data[field] == '':
+            # missing data in accounting start,
+            # let's set zero as default value
+            if data.get('status_type', None) == 'Start' and data.get(field) == '':
                 data[field] = 0
+            # missing data in accounting stop,
+            # let's remove the empty string to
+            # prevent the API from failing
+            # the existing values stored in previous
+            # interim-updates won't be changed
+            if data.get('status_type', None) == 'Stop' and data.get(field) == '':
+                del data[field]
         return super().run_validation(data)
 
     def validate(self, data):

openwisp_radius/tests/test_api/test_freeradius_api.py

@@ -991,6 +991,41 @@ def test_accounting_stop_200(self):
         self.assertEqual(ra.start_time, start_time)
         self.assertAcctData(ra, data)
 
+    @freeze_time(START_DATE)
+    def test_accounting_stop_empty_octets(self):
+        self.assertEqual(RadiusAccounting.objects.count(), 0)
+        data = self.acct_post_data
+        data.update(
+            dict(
+                input_octets=9900909,
+                output_octets=1513075509,
+            )
+        )
+        ra = self._create_radius_accounting(**data)
+        ra.refresh_from_db()
+        start_time = ra.start_time
+        data = self.acct_post_data
+        data.update(
+            {
+                'status_type': 'Stop',
+                'terminate_cause': '',
+                'input_octets': '',
+                'output_octets': '',
+            }
+        )
+        data = self._get_accounting_params(**data)
+        response = self.post_json(data)
+        self.assertEqual(response.status_code, 200)
+        self.assertIsNone(response.data)
+        self.assertEqual(RadiusAccounting.objects.count(), 1)
+        ra.refresh_from_db()
+        self.assertEqual(ra.update_time.timetuple(), now().timetuple())
+        self.assertEqual(ra.stop_time.timetuple(), now().timetuple())
+        self.assertEqual(ra.start_time, start_time)
+        self.assertEqual(ra.input_octets, 9900909)
+        self.assertEqual(ra.output_octets, 1513075509)
+        self.assertEqual(ra.session_time, 261)
+
     @freeze_time(START_DATE)
     @capture_any_output()
     def test_accounting_stop_201(self):