Kinto Attachment's attachments can be replaced on read-only records

@@ -26,7 +26,11 @@
 
 class AttachmentRouteFactory(RouteFactory):
     def __init__(self, request):
-        """Attachment is not a Kinto resource.
+        """
+        This class is the `context` object being passed to the
+        :class:`kinto.core.authorization.AuthorizationPolicy`.
+
+        Attachment is not a Kinto resource.
 
         The required permission is:
         * ``write`` if the related record exists;
@@ -43,12 +47,19 @@ def __init__(self, request):
             existing = resource.get()
         except httpexceptions.HTTPNotFound:
             existing = None
+
         if existing:
+            # Request write permission on the existing record.
             self.permission_object_id = record_uri(request)
             self.required_permission = "write"
         else:
+            # Request create record permission on the parent collection.
             self.permission_object_id = collection_uri(request)
             self.required_permission = "create"
+        # Set the current object in context, since it is used in the
+        # authorization policy to distinguish operations on plural endpoints
+        # from individual objects. See Kinto/kinto#918
+        self.current_object = existing
 
 
 def sha256(content):

@@ -283,21 +283,57 @@ def test_upload_refused_if_not_authenticated(self):
         self.headers.pop("Authorization")
         self.upload(status=401)
 
+    def test_upload_replace_refused_if_not_authenticated(self):
+        self.upload(status=201)
+
+        self.headers.pop("Authorization")
+        self.upload(status=401)
+
     def test_upload_refused_if_not_allowed(self):
         self.headers.update(get_user_headers("jean-louis"))
         self.upload(status=403)
 
     def test_upload_replace_refused_if_only_create_allowed(self):
-        # Allow any authenticated to write in this bucket.
+        # Allow any authenticated to write in this collection.
         perm = {"permissions": {"record:create": ["system.Authenticated"]}}
         self.app.patch_json("/buckets/fennec/collections/fonts", perm, headers=self.headers)
         self.upload(status=201)
 
         self.headers.update(get_user_headers("jean-louis"))
         self.upload(status=403)
 
+    def test_upload_replace_refused_if_only_bucket_read_is_allowed(self):
+        # Create a record with attachment.
+        self.upload(status=201)
+
+        # Now allow anyone to read this bucket.
+        perm = {"permissions": {"read": ["system.Everyone"]}}
+        self.app.patch_json("/buckets/fennec", perm, headers=self.headers)
+
+        # And try to replace anonymously.
+        self.headers.pop("Authorization")
+        self.upload(status=401)
+
+    def test_upload_replace_refused_if_only_read_is_allowed(self):
+        # Create a record with attachment.
+        self.upload(status=201)
+
+        # Now allow anyone to read this collection.
+        perm_change = [
+            {"op": "add", "path": "/permissions", "value": {"read": ["system.Everyone"]}}
+        ]
+        self.app.patch_json(
+            "/buckets/fennec/collections/fonts",
+            perm_change,
+            headers={**self.headers, "Content-Type": "application/json-patch+json"},
+        )
+
+        # And try to replace anonymously.
+        self.headers.pop("Authorization")
+        self.upload(status=401)
+
     def test_upload_create_accepted_if_create_allowed(self):
-        # Allow any authenticated to write in this bucket.
+        # Allow any authenticated to write in this collection.
         perm = {"permissions": {"record:create": ["system.Authenticated"]}}
         self.app.patch_json("/buckets/fennec/collections/fonts", perm, headers=self.headers)