Reset LastUpdate on neuron replace

Author: ales-otf

pallets/subtensor/src/subnets/uids.rs

@@ -94,6 +94,13 @@ impl<T: Config> Pallet<T> {
 
         // 5. Reset new neuron's values.
         Self::clear_neuron(netuid, uid_to_replace);
+
+        // 6. Replacement creates a new logical neuron at the reused UID, so the weights timing
+        //    state should start from this registration block.
+        for mecid in 0..MechanismCountCurrent::<T>::get(netuid).into() {
+            let netuid_index = Self::get_mechanism_storage_index(netuid, mecid.into());
+            Self::set_last_update_for_uid(netuid_index, uid_to_replace, block_number);
+        }
     }
 
     /// Appends the uid to the network.

pallets/subtensor/src/tests/uids.rs

@@ -190,6 +190,41 @@ fn test_bonds_cleared_on_replace() {
     });
 }
 
+#[test]
+fn test_replace_neuron_resets_last_update() {
+    new_test_ext(1).execute_with(|| {
+        let registration_block: u64 = 0;
+        let replacement_block: u64 = 123;
+        let netuid = NetUid::from(1);
+        let tempo: u16 = 13;
+        let hotkey_account_id = U256::from(1);
+        let coldkey_account_id = U256::from(1234);
+        let new_hotkey_account_id = U256::from(2);
+
+        System::set_block_number(registration_block);
+        add_network(netuid, tempo, 0);
+        register_ok_neuron(netuid, hotkey_account_id, coldkey_account_id, 0);
+
+        let neuron_uid =
+            SubtensorModule::get_uid_for_net_and_hotkey(netuid, &hotkey_account_id).unwrap();
+        let netuid_index = NetUidStorageIndex::from(netuid);
+
+        LastUpdate::<Test>::insert(netuid_index, vec![7]);
+
+        SubtensorModule::replace_neuron(
+            netuid,
+            neuron_uid,
+            &new_hotkey_account_id,
+            replacement_block,
+        );
+
+        assert_eq!(
+            LastUpdate::<Test>::get(netuid_index)[neuron_uid as usize],
+            replacement_block
+        );
+    });
+}
+
 #[test]
 fn test_replace_neuron_multiple_subnets() {
     new_test_ext(1).execute_with(|| {