tenant create / update-config: reject unknown fields (#4267)

This PR enforces that the tenant create / update-config APIs reject
requests with unknown fields.

This is a desirable property because some tenant config settings control
the lifetime of user data (e.g., GC horizon or PITR interval).

Suppose we inadvertently rename the `pitr_interval` field in the Rust
code.
Then, right now, a client that still uses the old name will send a
tenant config request to configure a new PITR interval.
Before this PR, we would accept such a request, ignore the old name
field, and use the pageserver.toml default value for what the new PITR
interval is.
With this PR, we will instead reject such a request.

One might argue that the client could simply check whether the config it
sent has been applied, using the `/v1/tenant/.../config` endpoint.
That is correct for tenant create and update-config.

But, attach will soon [^1] grow the ability to have attach-time config
as well.
If we ignore unknown fields and fall back to global defaults in that
case, we risk data loss.
Example:
1. Default PITR in pageservers is 7 days.
2. Create a tenant and set its PITR to 30 days.
3. For 30 days, fill the tenant continuously with data.
4. Detach the tenant.
5. Attach tenant.

Attach must use the 30-day PITR setting in this scenario.
If it were to fall back to the 7-day default value, we would lose 23
days of PITR capability for the tenant.

So, the PR that adds attach-time tenant config will build on the
(clunky) infrastructure added in this PR

[^1]: https://github.com/neondatabase/neon/pull/4255

Implementation Notes
====================

This could have been a simple `#[serde(deny_unknown_fields)]` but sadly,
that is documented- but silent-at-compile-time-incompatible with
`#[serde(flatten)]`. But we are still using this by adding on outer struct and use unit tests to ensure it is correct.

`neon_local tenant config` now uses the `.remove()` pattern + bail if
there are leftover config args. That's in line with what
`neon_local tenant create` does. We should dedupe that logic in a future
PR.

---------

Signed-off-by: Alex Chi <iskyzh@gmail.com>
Co-authored-by: Alex Chi <iskyzh@gmail.com>

libs/pageserver_api/src/models.rs

@@ -131,13 +131,14 @@ pub struct TimelineCreateRequest {
 }
 
 #[serde_as]
-#[derive(Serialize, Deserialize, Default)]
+#[derive(Serialize, Deserialize, Debug, Default)]
+#[serde(deny_unknown_fields)]
 pub struct TenantCreateRequest {
     #[serde(default)]
     #[serde_as(as = "Option<DisplayFromStr>")]
     pub new_tenant_id: Option<TenantId>,
     #[serde(flatten)]
-    pub config: TenantConfig,
+    pub config: TenantConfig, // as we have a flattened field, we should reject all unknown fields in it
 }
 
 impl std::ops::Deref for TenantCreateRequest {
@@ -148,7 +149,7 @@ impl std::ops::Deref for TenantCreateRequest {
     }
 }
 
-#[derive(Serialize, Deserialize, Default)]
+#[derive(Serialize, Deserialize, Debug, Default)]
 pub struct TenantConfig {
     pub checkpoint_distance: Option<u64>,
     pub checkpoint_timeout: Option<String>,
@@ -192,12 +193,13 @@ impl TenantCreateRequest {
 }
 
 #[serde_as]
-#[derive(Serialize, Deserialize)]
+#[derive(Serialize, Deserialize, Debug)]
+#[serde(deny_unknown_fields)]
 pub struct TenantConfigRequest {
     #[serde_as(as = "DisplayFromStr")]
     pub tenant_id: TenantId,
     #[serde(flatten)]
-    pub config: TenantConfig,
+    pub config: TenantConfig, // as we have a flattened field, we should reject all unknown fields in it
 }
 
 impl std::ops::Deref for TenantConfigRequest {
@@ -768,4 +770,31 @@ mod tests {
         assert!(format!("{:?}", &original_broken.state).contains("reason"));
         assert!(format!("{:?}", &original_broken.state).contains("backtrace info"));
     }
+
+    #[test]
+    fn test_reject_unknown_field() {
+        let id = TenantId::generate();
+        let create_request = json!({
+            "new_tenant_id": id.to_string(),
+            "unknown_field": "unknown_value".to_string(),
+        });
+        let err = serde_json::from_value::<TenantCreateRequest>(create_request).unwrap_err();
+        assert!(
+            err.to_string().contains("unknown field `unknown_field`"),
+            "expect unknown field `unknown_field` error, got: {}",
+            err
+        );
+
+        let id = TenantId::generate();
+        let config_request = json!({
+            "tenant_id": id.to_string(),
+            "unknown_field": "unknown_value".to_string(),
+        });
+        let err = serde_json::from_value::<TenantConfigRequest>(config_request).unwrap_err();
+        assert!(
+            err.to_string().contains("unknown field `unknown_field`"),
+            "expect unknown field `unknown_field` error, got: {}",
+            err
+        );
+    }
 }