feat(parquet): add uses_key_retriever method to FileDecryptionProperties

parquet/src/encryption/decrypt.rs

@@ -432,6 +432,14 @@ impl FileDecryptionProperties {
         }
         (column_names, column_keys)
     }
+
+    /// Whether these decryption properties use a key retriever.
+    /// When false, explicit keys were provided up front and can
+    /// be retrieved without providing key metadata, rather than
+    /// resolved on demand.
+    pub fn uses_key_retriever(&self) -> bool {
+        matches!(self.keys, DecryptionKeys::ViaRetriever(_))
+    }
 }
 
 impl std::fmt::Debug for FileDecryptionProperties {

parquet/tests/encryption/encryption.rs

@@ -1474,3 +1474,36 @@ fn test_decrypt_page_index(
 
     Ok(())
 }
+
+#[test]
+fn test_decryption_properties_uses_key_retriever() {
+    let key_retriever = TestKeyRetriever::new()
+        .with_key(
+            AES_128_FOOTER_KEY_NAME.to_owned(),
+            AES_128_FOOTER_KEY.to_vec(),
+        )
+        .with_key(
+            AES_128_KEY_NAMES[0].to_owned(),
+            AES_128_COLUMN_KEYS[0].to_vec(),
+        );
+
+    let properties_with_retriever =
+        FileDecryptionProperties::with_key_retriever(Arc::new(key_retriever))
+            .build()
+            .unwrap();
+
+    assert!(properties_with_retriever.uses_key_retriever());
+
+    let properties_with_keys = FileDecryptionProperties::builder(AES_128_FOOTER_KEY.to_vec())
+        .with_column_key(AES_128_COLUMN_NAMES[0], AES_128_COLUMN_KEYS[0].to_vec())
+        .build()
+        .unwrap();
+
+    assert!(!properties_with_keys.uses_key_retriever());
+
+    let uniform_properties = FileDecryptionProperties::builder(AES_128_FOOTER_KEY.to_vec())
+        .build()
+        .unwrap();
+
+    assert!(!uniform_properties.uses_key_retriever());
+}