On-chain storage version on new chain seems to be incorrect

Question

I modified the template pallet from the substrate node template 681a9d8e3e8becf99aabf1cc76e1c08016db2e1a to contain a storage version:

const STORAGE_VERSION: StorageVersion = StorageVersion::new(123);

// -- snip --

#[pallet::pallet]
#[pallet::storage_version(STORAGE_VERSION)]
#[pallet::generate_store(pub(super) trait Store)]
pub struct Pallet<T>(_);

Then I wrote the following test:

#[test]
fn storage_version_is_correct() {
    new_test_ext().execute_with(|| {
        assert_eq!(<Pallet<Test> as GetStorageVersion>::current_storage_version(), 123);
        assert_eq!(<Pallet<Test> as GetStorageVersion>::on_chain_storage_version(), 123);
        assert_eq!(StorageVersion::get::<Pallet<Test>>(), 123);
    });
}

The first assert_eq! succeeds, the other two fail. Is this expected behavior? I would have expected the storage version of the pallet to be written to storage at genesis. If this is expected behavior, why is this the case?

I expanded lib.rs and found the following snippet:

impl<T: Config> frame_support::traits::OnGenesis for Pallet<T> {
    fn on_genesis() {
        let storage_version = STORAGE_VERSION;
        storage_version.put::<Self>();
    }
}

So it appears that the storage version is supposed to be written to the on-chain storage on genesis, right? Why didn't it happen in this case?

Answer 1

The following patch will make it work:

diff --git a/pallets/template/src/tests.rs b/pallets/template/src/tests.rs
index 550ffad..6ca03f1 100644
--- a/pallets/template/src/tests.rs
+++ b/pallets/template/src/tests.rs
@@ -3,7 +3,7 @@ use frame_support::{
        assert_noop, assert_ok,
        pallet_prelude::*,
        storage::storage_prefix,
-       traits::{GetStorageVersion, StorageVersion},
+       traits::{GetStorageVersion, StorageVersion, OnGenesis},
 };
 
 #[test]
@@ -27,6 +27,7 @@ fn correct_error_for_none_value() {
 #[test]
 fn storage_version_is_correct() {
        new_test_ext().execute_with(|| {
+        Pallet::<Test>::on_genesis();
                println!(
                        "{:?}",
                        storage_prefix(<Pallet<Test>>::name().as_bytes(), b":__STORAGE_VERSION__:")

The problem here being that on_genesis is not being called magically and you don't call it in your code anywhere. StorageVersion defaults to 0 when it is not present in the storage.

Normally on_genesis being called as part of the runtime GenesisConfig. Here you see the code that is being generated that will then call this method.

When you want to test migrations, I would just put the expected StorageVersion into the storage, before calling the migration code.

Answer 2

1. on_chain_storage_version is the version number saved in the DB.
2. And the current_storage_version's real meaning is the number which is annotated in the pallet's pallet::storage_version macro.