fix handling of null groupConfiguration

Fixes #1129 .

I added the "nullish" serde function and test emulating similar treatment for nullish boolean and map fields. i think the new deserialize_nullish function could supersede deserialize_nullish_boolean, deserialize_lambda_map, and possibly deserialize_lambda_dynamodb_item. but i left that out for a potential follow-up PR, to keep the blast radius small.

Author: jct-tympanon

lambda-events/src/custom_serde/mod.rs

@@ -58,6 +58,16 @@ where
     serializer.serialize_str(&base64::engine::general_purpose::STANDARD.encode(value))
 }
 
+/// Deserializes any `Default` type, mapping JSON `null` to `T::default()`.
+pub(crate) fn deserialize_nullish<'de, D, T>(deserializer: D) -> Result<T, D::Error>
+where
+    D: Deserializer<'de>,
+    T: Default + Deserialize<'de>,
+{
+    let opt = Option::deserialize(deserializer)?;
+    Ok(opt.unwrap_or_default())
+}
+
 /// Deserializes `HashMap<_>`, mapping JSON `null` to an empty map.
 pub(crate) fn deserialize_lambda_map<'de, D, K, V>(deserializer: D) -> Result<HashMap<K, V>, D::Error>
 where
@@ -178,6 +188,28 @@ mod test {
         assert_eq!(serde_dynamo::Item::from(HashMap::new()), decoded.v);
     }
 
+    #[test]
+    fn test_deserialize_nullish() {
+        #[derive(Debug, Default, Deserialize, PartialEq)]
+        struct Inner {
+            x: u32,
+        }
+        #[derive(Deserialize)]
+        struct Test {
+            #[serde(default, deserialize_with = "deserialize_nullish")]
+            v: Inner,
+        }
+
+        let decoded: Test = serde_json::from_str(r#"{"v": null}"#).unwrap();
+        assert_eq!(decoded.v, Inner::default());
+
+        let decoded: Test = serde_json::from_str(r#"{}"#).unwrap();
+        assert_eq!(decoded.v, Inner::default());
+
+        let decoded: Test = serde_json::from_str(r#"{"v": {"x": 42}}"#).unwrap();
+        assert_eq!(decoded.v, Inner { x: 42 });
+    }
+
     #[test]
     fn test_deserialize_nullish_boolean() {
         #[derive(Deserialize)]

lambda-events/src/event/cognito/mod.rs

@@ -4,7 +4,7 @@ use serde::{de::DeserializeOwned, Deserialize, Serialize};
 use serde_json::Value;
 use std::collections::HashMap;
 
-use crate::custom_serde::{deserialize_lambda_map, deserialize_nullish_boolean};
+use crate::custom_serde::{deserialize_lambda_map, deserialize_nullish, deserialize_nullish_boolean};
 
 /// `CognitoEvent` contains data from an event sent from AWS Cognito Sync
 #[non_exhaustive]
@@ -433,6 +433,7 @@ pub struct CognitoEventUserPoolsPreTokenGenRequest {
     #[serde(deserialize_with = "deserialize_lambda_map")]
     #[serde(default)]
     pub user_attributes: HashMap<String, String>,
+    #[serde(default, deserialize_with = "deserialize_nullish")]
     pub group_configuration: GroupConfiguration,
     #[serde(deserialize_with = "deserialize_lambda_map")]
     #[serde(default)]
@@ -495,6 +496,7 @@ pub struct CognitoEventUserPoolsPreTokenGenRequestV2 {
     #[serde(deserialize_with = "deserialize_lambda_map")]
     #[serde(default)]
     pub user_attributes: HashMap<String, String>,
+    #[serde(default, deserialize_with = "deserialize_nullish")]
     pub group_configuration: GroupConfiguration,
     #[serde(deserialize_with = "deserialize_lambda_map")]
     #[serde(default)]