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# Refs - refs https://github.com/neondatabase/neon/issues/8915 - discussion thread: https://neondb.slack.com/archives/C033RQ5SPDH/p1742406381132599 - stacked atop https://github.com/neondatabase/neon/pull/11298 - corresponding internal docs update that illustrates how this PR removes friction: https://github.com/neondatabase/docs/pull/404 # Problem Rejecting `pageserver.toml`s with unknown fields adds friction, especially when using `pageserver.toml` fields as feature flags that need to be decommissioned. See the added paragraphs on `pageserver_api::models::ConfigToml` for details on what kind of friction it causes. Also read the corresponding internal docs update linked above to see a more imperative guide for using `pageserver.toml` flags as feature flags. # Solution ## Ignoring unknown fields Ignoring is the serde default behavior. So, just remove `serde(deny_unknown_fields)` from all structs in `pageserver_api::config::ConfigToml` `pageserver_api::config::TenantConfigToml`. I went through all the child fields and verified they don't use `deny_unknown_fields` either, including those shared with `pageserver_api::models`. ## Warning about unknown fields We still want to warn about unknown fields to - be informed about typos in the config template - be reminded about feature-flag style configs that have been cleaned up in code but not yet in config templates We tried `serde_ignore` (cf draft #11319) but it doesn't work with `serde(flatten)`. The solution we arrived at is to compare the on-disk TOML with the TOML that we produce if we serialize the `ConfigToml` again. Any key specified in the on-disk TOML but not present in the serialized TOML is flagged as an ignored key. The mechanism to do it is a tiny recursive decent visitor on the `toml_edit::DocumentMut`. # Future Work Invalid config _values_ in known fields will continue to fail pageserver startup. See - https://github.com/neondatabase/cloud/issues/24349 for current worst case impact to deployments & ideas to improve.