Improve NFT locking (#14510)

* Update docs

* Prevent locking of the same NFT twice

* Validate item is not locked on burn

* Cover with tests

* chore

substrate/frame/nft-fractionalization/src/lib.rs

@@ -338,12 +338,12 @@ pub mod pallet {
 			T::PalletId::get().into_account_truncating()
 		}
 
-		/// Transfer the NFT from the account holding that NFT to the pallet's account.
+		/// Prevent further transferring of NFT.
 		fn do_lock_nft(nft_collection_id: T::NftCollectionId, nft_id: T::NftId) -> DispatchResult {
 			T::Nfts::disable_transfer(&nft_collection_id, &nft_id)
 		}
 
-		/// Transfer the NFT to the account returning the tokens.
+		/// Remove the transfer lock and transfer the NFT to the account returning the tokens.
 		fn do_unlock_nft(
 			nft_collection_id: T::NftCollectionId,
 			nft_id: T::NftId,

substrate/frame/nft-fractionalization/src/tests.rs

@@ -122,6 +122,33 @@ fn fractionalize_should_work() {
 			beneficiary: account(2),
 		}));
 
+		// owner can't burn an already fractionalized NFT
+		assert_noop!(
+			Nfts::burn(RuntimeOrigin::signed(account(1)), nft_collection_id, nft_id),
+			DispatchError::Module(ModuleError {
+				index: 4,
+				error: [12, 0, 0, 0],
+				message: Some("ItemLocked")
+			})
+		);
+
+		// can't fractionalize twice
+		assert_noop!(
+			NftFractionalization::fractionalize(
+				RuntimeOrigin::signed(account(1)),
+				nft_collection_id,
+				nft_id,
+				asset_id + 1,
+				account(2),
+				fractions,
+			),
+			DispatchError::Module(ModuleError {
+				index: 4,
+				error: [12, 0, 0, 0],
+				message: Some("ItemLocked")
+			})
+		);
+
 		let nft_id = nft_id + 1;
 		assert_noop!(
 			NftFractionalization::fractionalize(