Add translate API for storage values

srml/support/src/storage/generator/value.rs

@@ -16,7 +16,7 @@
 
 #[cfg(not(feature = "std"))]
 use rstd::prelude::*;
-use codec::{FullCodec, Encode, EncodeAppend, EncodeLike};
+use codec::{FullCodec, Encode, EncodeAppend, EncodeLike, Decode};
 use crate::{storage::{self, unhashed}, hash::{Twox128, StorageHasher}, traits::Len};
 
 /// Generator for `StorageValue` used by `decl_storage`.
@@ -60,6 +60,23 @@ impl<T: FullCodec, G: StorageValue<T>> storage::StorageValue<T> for G {
 		G::from_optional_value_to_query(value)
 	}
 
+	fn translate<O: Decode, F: FnOnce(Option<O>) -> Option<T>>(f: F) -> Result<Option<T>, ()> {
+		let key = Self::storage_value_final_key();
+
+		// attempt to get the length directly.
+		let maybe_old = match unhashed::get_raw(&key) {
+			Some(old_data) => Some(O::decode(&mut &old_data[..]).map_err(|_| ())?),
+			None => None,
+		};
+		let maybe_new = f(maybe_old);
+		if let Some(new) = maybe_new.as_ref() {
+			new.using_encoded(|d| unhashed::put_raw(&key, d));
+		} else {
+			unhashed::kill(&key);
+		}
+		Ok(maybe_new)
+	}
+
 	fn put<Arg: EncodeLike<T>>(val: Arg) {
 		unhashed::put(&Self::storage_value_final_key(), &val)
 	}

srml/support/src/storage/mod.rs

@@ -17,7 +17,7 @@
 //! Stuff to do with the runtime's storage.
 
 use rstd::prelude::*;
-use codec::{FullCodec, FullEncode, Encode, EncodeAppend, EncodeLike};
+use codec::{FullCodec, FullEncode, Encode, EncodeAppend, EncodeLike, Decode};
 use crate::traits::Len;
 
 #[macro_use]
@@ -41,6 +41,28 @@ pub trait StorageValue<T: FullCodec> {
 	/// Load the value from the provided storage instance.
 	fn get() -> Self::Query;
 
+	/// Translate a value from some previous type (`O`) to the current type.
+	///
+	/// `f: F` is the translation function.
+	///
+	/// Returns `Err` if the storage item could not be interpreted as the old type, and Ok, along
+	/// with the new value if it could.
+	///
+	/// NOTE: This operates from and to `Option<>` types; no effort is made to respect the default
+	/// value of the original type.
+	///
+	/// # Warning
+	///
+	/// This function must be used with care as every other call to the storage still contains the
+	/// old type.
+	///
+	/// # Usage
+	///
+	/// This would typically be called inside the module implementation of on_initialize, while
+	/// ensuring no usage of this storage are made before the call to on_initialize.
+	/// (More precisely prior initialized modules doesn't make use of this storage).
+	fn translate<O: Decode, F: FnOnce(Option<O>) -> Option<T>>(f: F) -> Result<Option<T>, ()>;
+
 	/// Store a value under this key into the provided storage instance.
 	fn put<Arg: EncodeLike<T>>(val: Arg);