Deprecate OBSERVE_REQUEST_CALLBACK

With the addition of the UPDATE_ON_FETCH setting, the
OBSERVE_REQUEST_CALLBACK setting is redundant.  Thus add a check
debug_toolbar.W008 to warn if it is present in DEBUG_TOOLBAR_CONFIG,
but do not remove it yet.

See jazzband#1886

debug_toolbar/apps.py

@@ -206,3 +206,18 @@ def js_mimetype_check(app_configs, **kwargs):
             )
         ]
     return []
+
+
+@register()
+def check_settings(app_configs, **kwargs):
+    errors = []
+    USER_CONFIG = getattr(settings, "DEBUG_TOOLBAR_CONFIG", {})
+    if "OBSERVE_REQUEST_CALLBACK" in USER_CONFIG:
+        errors.append(
+            Warning(
+                "Specifying OBSERVE_REQUEST_CALLBACK in DEBUG_TOOLBAR_CONFIG is deprecated.",
+                hint="Use the UPDATE_ON_FETCH and SHOW_TOOLBAR_CALLBACK settings instead.",
+                id="debug_toolbar.W008",
+            )
+        )
+    return errors

docs/changes.rst

@@ -17,6 +17,9 @@ Pending
   :class:`StaticFilesPanel <debug_toolbar.panels.staticfiles.StaticFilesPanel>`
   since that check is made redundant by a similar check in Django 4.0 and
   later.
+* Deprecated the ``OBSERVE_REQUEST_CALLBACK`` setting and added check
+  ``debug_toolbar.W008`` to warn when it is specified in
+  ``DEBUG_TOOLBAR_SETTINGS``.
 
 4.3.0 (2024-02-01)
 ------------------

tests/test_checks.py

@@ -235,3 +235,19 @@ def test_check_w007_invalid(self, mocked_guess_type):
                 )
             ],
         )
+
+    @override_settings(
+        DEBUG_TOOLBAR_CONFIG={"OBSERVE_REQUEST_CALLBACK": lambda request: False}
+    )
+    def test_observe_request_callback_specified(self):
+        errors = run_checks()
+        self.assertEqual(
+            errors,
+            [
+                Warning(
+                    "Specifying OBSERVE_REQUEST_CALLBACK in DEBUG_TOOLBAR_CONFIG is deprecated.",
+                    hint="Use the UPDATE_ON_FETCH and SHOW_TOOLBAR_CALLBACK settings instead.",
+                    id="debug_toolbar.W008",
+                )
+            ],
+        )