use serde_ignored to warn about ignored unknown fields

test_runner/regress/test_pageserver_config.py

@@ -0,0 +1,35 @@
+import pytest
+from fixtures.neon_fixtures import NeonEnv
+
+
+@pytest.mark.parametrize("what", ["default", "top_level", "nested"])
+def test_unknown_config_items_handling(neon_simple_env: NeonEnv, what: str):
+    env = neon_simple_env
+
+    def edit_fn(config) -> str | None:
+        if what == "default":
+            return None
+        elif what == "top_level":
+            config["unknown_top_level_config_item"] = 23
+            return r".*unknown_top_level_config_item.*"
+        elif what == "nested":
+            config["remote_storage"]["unknown_config_item"] = 23
+            return r".*remote_storage\.unknown_config_item.*"
+        else:
+            raise ValueError(f"Unknown what: {what}")
+
+    expect_warn_re = env.pageserver.edit_config_toml(edit_fn)
+
+    if expect_warn_re is not None:
+        env.pageserver.allowed_errors.append(expect_warn_re)
+
+    if expect_warn_re is not None:
+        assert not env.pageserver.log_contains(expect_warn_re)
+
+    # in any way, unknown config items should not fail pageserver to start
+    # TODO: extend this test with the config validator mode once we introduce it
+    # https://github.com/neondatabase/cloud/issues/24349
+    env.pageserver.restart()
+
+    if expect_warn_re is not None:
+        assert env.pageserver.log_contains(expect_warn_re)