fail with nice error when trying amend the very first commit

icechunk/src/session.rs

@@ -1140,6 +1140,13 @@ impl Session {
         // Icechunk 1 doesn't support amend
         self.asset_manager.fail_unless_spec_at_least(SpecVersionBin::V2dot0)?;
 
+        // Cannot amend the initial commit (which has no parent and no transaction log)
+        let (repo_info, _) = self.asset_manager.fetch_repo_info().await?;
+        let current_snapshot_info = repo_info.find_snapshot(self.snapshot_id())?;
+        if current_snapshot_info.is_initial() {
+            return Err(SessionErrorKind::NoAmendForInitialCommit.into());
+        }
+
         self._commit(message, properties, false, CommitMethod::Amend).await
     }
 
@@ -2606,7 +2613,7 @@ async fn do_commit_v2(
             (CommitMethod::NewCommit, _) => parent_snapshot_id.clone(),
             (CommitMethod::Amend, Some(parent_id)) => parent_id,
             (CommitMethod::Amend, None) => {
-                return Err(RepositoryErrorKind::NoAmendForInitialCommit.into());
+                unreachable!("amend() checks for initial commit before calling _commit")
             }
         };
 
@@ -4184,6 +4191,15 @@ mod tests {
     #[tokio_test]
     async fn test_amend() -> Result<(), Box<dyn Error>> {
         let repo = create_memory_store_repository().await;
+
+        // Test that amending the initial commit fails
+        let mut session = repo.writable_session("main").await?;
+        session.add_group(Path::root(), Bytes::copy_from_slice(b"")).await?;
+        let amend_result = session.amend("cannot amend initial commit", None).await;
+        assert!(amend_result.is_err());
+        assert!(amend_result.unwrap_err().to_string().contains("first commit"));
+
+        // Now make a proper first commit
         let mut session = repo.writable_session("main").await?;
         session.add_group(Path::root(), Bytes::copy_from_slice(b"")).await?;
         session.commit("make root", None).await?;