proxy: Fix parsing of UnknownTopic with payload (#10339)

## Problem

When the proxy receives a `Notification` with an unknown topic it's
supposed to use the `UnknownTopic` unit variant. Unfortunately, in
adjacently tagged enums serde will not simply ignore the configured
content if found and try to deserialize a map/object instead.

## Summary of changes

* Use a custom deserialize function to ignore variant content.
* Add a little unit test covering both cases.

proxy/src/redis/notifications.rs

@@ -74,7 +74,11 @@ pub(crate) enum Notification {
     #[serde(rename = "/cancel_session")]
     Cancel(CancelSession),
 
-    #[serde(other, skip_serializing)]
+    #[serde(
+        other,
+        deserialize_with = "deserialize_unknown_topic",
+        skip_serializing
+    )]
     UnknownTopic,
 }
 
@@ -123,6 +127,15 @@ where
     serde_json::from_str(&s).map_err(<D::Error as serde::de::Error>::custom)
 }
 
+// https://github.com/serde-rs/serde/issues/1714
+fn deserialize_unknown_topic<'de, D>(deserializer: D) -> Result<(), D::Error>
+where
+    D: serde::Deserializer<'de>,
+{
+    deserializer.deserialize_any(serde::de::IgnoredAny)?;
+    Ok(())
+}
+
 struct MessageHandler<C: ProjectInfoCache + Send + Sync + 'static> {
     cache: Arc<C>,
     cancellation_handler: Arc<CancellationHandler<()>>,
@@ -458,4 +471,30 @@ mod tests {
 
         Ok(())
     }
+
+    #[test]
+    fn parse_unknown_topic() -> anyhow::Result<()> {
+        let with_data = json!({
+            "type": "message",
+            "topic": "/doesnotexist",
+            "data": {
+                "payload": "ignored"
+            },
+            "extra_fields": "something"
+        })
+        .to_string();
+        let result: Notification = serde_json::from_str(&with_data)?;
+        assert_eq!(result, Notification::UnknownTopic);
+
+        let without_data = json!({
+            "type": "message",
+            "topic": "/doesnotexist",
+            "extra_fields": "something"
+        })
+        .to_string();
+        let result: Notification = serde_json::from_str(&without_data)?;
+        assert_eq!(result, Notification::UnknownTopic);
+
+        Ok(())
+    }
 }