allow revoke with live_until_ledger 0 (OpenZeppelin#174)

Author: ozgunozerk

packages/tokens/non-fungible/src/non_fungible.rs

@@ -174,7 +174,8 @@ pub trait NonFungibleToken {
     ///
     /// Only a single account can be approved at a time for a `token_id`.
     /// To remove an approval, the approver can approve their own address,
-    /// effectively removing the previous approved address.
+    /// effectively removing the previous approved address. Alternatively,
+    /// setting the `live_until_ledger` to `0` will also revoke the approval.
     ///
     /// # Arguments
     ///
@@ -184,7 +185,7 @@ pub trait NonFungibleToken {
     /// * `approved` - The address receiving the approval.
     /// * `token_id` - Token id as a number.
     /// * `live_until_ledger` - The ledger number at which the allowance
-    ///   expires.
+    ///   expires. If `live_until_ledger` is `0`, the approval is revoked.
     ///
     /// # Errors
     ///

packages/tokens/non-fungible/src/storage.rs

@@ -322,7 +322,8 @@ impl Base {
     ///   `operator`).
     /// * `approved` - The address receiving the approval.
     /// * `token_id` - The identifier of the token to be approved.
-    /// * `live_until_ledger` - The ledger number at which the approval expires.
+    /// * `live_until_ledger` - The ledger number at which the allowance
+    ///   expires. If `live_until_ledger` is `0`, the approval is revoked.
     ///
     /// # Errors
     ///
@@ -497,12 +498,19 @@ impl Base {
             panic_with_error!(e, NonFungibleTokenError::InvalidApprover);
         }
 
+        let key = StorageKey::Approval(token_id);
+
+        if live_until_ledger == 0 {
+            e.storage().temporary().remove(&key);
+
+            emit_approve(e, approver, approved, token_id, live_until_ledger);
+            return;
+        }
+
         if live_until_ledger < e.ledger().sequence() {
             panic_with_error!(e, NonFungibleTokenError::InvalidLiveUntilLedger);
         }
 
-        let key = StorageKey::Approval(token_id);
-
         let approval_data = ApprovalData { approved: approved.clone(), live_until_ledger };
 
         e.storage().temporary().set(&key, &approval_data);