Add try_mutate_exists to StorageDoubleMap. (#6694)

substrate/frame/support/src/storage/generator/double_map.rs

@@ -246,6 +246,25 @@ impl<K1, K2, V, G> storage::StorageDoubleMap<K1, K2, V> for G where
 		ret
 	}
 
+	fn try_mutate_exists<KArg1, KArg2, R, E, F>(k1: KArg1, k2: KArg2, f: F) -> Result<R, E>
+	where
+		KArg1: EncodeLike<K1>,
+		KArg2: EncodeLike<K2>,
+		F: FnOnce(&mut Option<V>) -> Result<R, E>,
+	{
+		let final_key = Self::storage_double_map_final_key(k1, k2);
+		let mut val = unhashed::get(final_key.as_ref());
+
+		let ret = f(&mut val);
+		if ret.is_ok() {
+			match val {
+				Some(ref val) => unhashed::put(final_key.as_ref(), val),
+				None => unhashed::kill(final_key.as_ref()),
+			}
+		}
+		ret
+	}
+
 	fn append<Item, EncodeLikeItem, KArg1, KArg2>(
 		k1: KArg1,
 		k2: KArg2,

substrate/frame/support/src/storage/mod.rs

@@ -366,6 +366,13 @@ pub trait StorageDoubleMap<K1: FullEncode, K2: FullEncode, V: FullCodec> {
 		KArg2: EncodeLike<K2>,
 		F: FnOnce(&mut Self::Query) -> Result<R, E>;
 
+	/// Mutate the item, only if an `Ok` value is returned. Deletes the item if mutated to a `None`.
+	fn try_mutate_exists<KArg1, KArg2, R, E, F>(k1: KArg1, k2: KArg2, f: F) -> Result<R, E>
+	where
+		KArg1: EncodeLike<K1>,
+		KArg2: EncodeLike<K2>,
+		F: FnOnce(&mut Option<V>) -> Result<R, E>;
+
 	/// Append the given item to the value in the storage.
 	///
 	/// `V` is required to implement [`StorageAppend`].