New forbidden() plugin hook, closes #812

datasette/app.py

@@ -975,22 +975,24 @@ class DatasetteRouter:
                         await response.asgi_send(send)
                     return
                 except NotFound as exception:
-                    return await self.handle_404(scope, receive, send, exception)
+                    return await self.handle_404(request, send, exception)
                 except Exception as exception:
-                    return await self.handle_500(scope, receive, send, exception)
+                    return await self.handle_500(request, send, exception)
-        return await self.handle_404(scope, receive, send)
+        return await self.handle_404(request, send)
 
-    async def handle_404(self, scope, receive, send, exception=None):
+    async def handle_404(self, request, send, exception=None):
         # If URL has a trailing slash, redirect to URL without it
-        path = scope.get("raw_path", scope["path"].encode("utf8"))
+        path = request.scope.get("raw_path", request.scope["path"].encode("utf8"))
         if path.endswith(b"/"):
             path = path.rstrip(b"/")
-            if scope["query_string"]:
+            if request.scope["query_string"]:
-                path += b"?" + scope["query_string"]
+                path += b"?" + request.scope["query_string"]
             await asgi_send_redirect(send, path.decode("latin1"))
         else:
             # Is there a pages/* template matching this path?
-            template_path = os.path.join("pages", *scope["path"].split("/")) + ".html"
+            template_path = (
+                os.path.join("pages", *request.scope["path"].split("/")) + ".html"
+            )
             try:
                 template = self.ds.jinja_env.select_template([template_path])
             except TemplateNotFound:
@@ -1019,7 +1021,7 @@ class DatasetteRouter:
                         "custom_status": custom_status,
                         "custom_redirect": custom_redirect,
                     },
-                    request=Request(scope, receive),
+                    request=request,
                     view_name="page",
                 )
                 # Pull content-type out into separate parameter
@@ -1035,11 +1037,9 @@ class DatasetteRouter:
                     content_type=content_type,
                 )
             else:
-                await self.handle_500(
+                await self.handle_500(request, send, exception or NotFound("404"))
-                    scope, receive, send, exception or NotFound("404")
-                )
 
-    async def handle_500(self, scope, receive, send, exception):
+    async def handle_500(self, request, send, exception):
         title = None
         if isinstance(exception, NotFound):
             status = 404
@@ -1049,6 +1049,17 @@ class DatasetteRouter:
             status = 403
             info = {}
             message = exception.args[0]
+            # Try the forbidden() plugin hook
+            for custom_response in pm.hook.forbidden(
+                datasette=self.ds, request=request, message=message
+            ):
+                if callable(custom_response):
+                    custom_response = custom_response()
+                if asyncio.iscoroutine(custom_response):
+                    custom_response = await custom_response
+                if custom_response is not None:
+                    await custom_response.asgi_send(send)
+                    return
         elif isinstance(exception, DatasetteError):
             status = exception.status
             info = exception.error_dict
@@ -1070,7 +1081,7 @@ class DatasetteRouter:
         headers = {}
         if self.ds.cors:
             headers["Access-Control-Allow-Origin"] = "*"
-        if scope["path"].split("?")[0].endswith(".json"):
+        if request.path.split("?")[0].endswith(".json"):
             await asgi_send_json(send, info, status=status, headers=headers)
         else:
             template = self.ds.jinja_env.select_template(templates)

datasette/hookspecs.py

@@ -88,3 +88,8 @@ def canned_queries(datasette, database, actor):
 @hookspec
 def register_magic_parameters(datasette):
     "Return a list of (name, function) magic parameter functions"
+
+
+@hookspec
+def forbidden(datasette, request, message):
+    "Custom response for a 403 forbidden error"

datasette/views/database.py

@@ -10,7 +10,7 @@ from datasette.utils import (
     path_with_added_args,
     path_with_removed_args,
 )
-from datasette.utils.asgi import AsgiFileDownload, Response
+from datasette.utils.asgi import AsgiFileDownload, Response, Forbidden
 from datasette.plugins import pm
 
 from .base import DatasetteError, DataView
@@ -120,7 +120,7 @@ class DatabaseDownload(DataView):
         if db.is_memory:
             raise DatasetteError("Cannot download :memory: database", status=404)
         if not self.ds.config("allow_download") or db.is_mutable:
-            raise DatasetteError("Database download is forbidden", status=403)
+            raise Forbidden("Database download is forbidden")
         if not db.path:
             raise DatasetteError("Cannot download database", status=404)
         filepath = db.path

datasette/views/special.py

@@ -1,5 +1,5 @@
 import json
-from datasette.utils.asgi import Response
+from datasette.utils.asgi import Response, Forbidden
 from .base import BaseView
 import secrets
 
@@ -60,7 +60,7 @@ class AuthTokenView(BaseView):
     async def get(self, request):
         token = request.args.get("token") or ""
         if not self.ds._root_token:
-            return Response("Root token has already been used", status=403)
+            raise Forbidden("Root token has already been used")
         if secrets.compare_digest(token, self.ds._root_token):
             self.ds._root_token = None
             response = Response.redirect("/")
@@ -69,7 +69,7 @@ class AuthTokenView(BaseView):
             )
             return response
         else:
-            return Response("Invalid token", status=403)
+            raise Forbidden("Invalid token")
 
 
 class LogoutView(BaseView):
@@ -99,7 +99,7 @@ class PermissionsDebugView(BaseView):
     async def get(self, request):
         await self.check_permission(request, "view-instance")
         if not await self.ds.permission_allowed(request.actor, "permissions-debug"):
-            return Response("Permission denied", status=403)
+            raise Forbidden("Permission denied")
         return await self.render(
             ["permissions_debug.html"],
             request,

docs/plugin_hooks.rst

@@ -946,3 +946,46 @@ This example registers two new magic parameters: ``:_request_http_version`` retu
             ("request", request),
             ("uuid", uuid),
         ]
+
+.. _plugin_hook_forbidden:
+
+forbidden(datasette, request, message)
+--------------------------------------
+
+``datasette`` - :ref:`internals_datasette`
+    You can use this to access plugin configuration options via ``datasette.plugin_config(your_plugin_name)``, or to execute SQL queries.
+
+``request`` - object
+    The current HTTP :ref:`internals_request`.
+
+``message`` - string
+    A message hinting at why the request was forbidden.
+
+Plugins can use this to customize how Datasette responds when a 403 Forbidden error occurs - usually because a page failed a permission check, see :authentication_permissions:.
+
+If a plugin hook wishes to react to the error, it should return a :ref:`Response object <internals_response>`.
+
+This example returns a redirect to a ``/-/login`` page:
+
+.. code-block:: python
+
+    from datasette import hookimpl
+    from urllib.parse import urlencode
+
+    @hookimpl
+    def forbidden(request, message):
+        return Response.redirect("/-/login?=" + urlencode({"message": message}))
+
+The function can alternatively return an awaitable function if it needs to make any asynchronous method calls. This example renders a template:
+
+.. code-block:: python
+
+    from datasette import hookimpl
+    from datasette.utils.asgi import Response
+
+    @hookimpl
+    def forbidden(datasette):
+        async def inner():
+            return Response.html(await datasette.render_template("forbidden.html"))
+
+        return inner

tests/fixtures.py

@@ -45,6 +45,7 @@ EXPECTED_PLUGINS = [
             "extra_css_urls",
             "extra_js_urls",
             "extra_template_vars",
+            "forbidden",
             "permission_allowed",
             "prepare_connection",
             "prepare_jinja2_environment",

tests/plugins/my_plugin.py

@@ -245,3 +245,10 @@ def register_magic_parameters():
         ("request", request),
         ("uuid", uuid),
     ]
+
+
+@hookimpl
+def forbidden(datasette, request, message):
+    datasette._last_forbidden_message = message
+    if request.path == "/data2":
+        return Response.redirect("/login?message=" + message)

tests/test_plugins.py

@@ -684,3 +684,16 @@ def test_register_magic_parameters(restore_working_directory):
         assert 200 == response_get.status
         new_uuid = response_get.json[0][":_uuid_new"]
         assert 4 == new_uuid.count("-")
+
+
+def test_forbidden(restore_working_directory):
+    with make_app_client(
+        extra_databases={"data2.db": "create table logs (line text)"},
+        metadata={"allow": {}},
+    ) as client:
+        response = client.get("/")
+        assert 403 == response.status
+        response2 = client.get("/data2", allow_redirects=False)
+        assert 302 == response2.status
+        assert "/login?message=view-database" == response2.headers["Location"]
+        assert "view-database" == client.ds._last_forbidden_message