Introduce entropy function into frame System

frame/system/src/lib.rs

@@ -66,6 +66,7 @@
 
 #[cfg(feature = "std")]
 use serde::Serialize;
+use sp_io::hashing::blake2_256;
 #[cfg(feature = "runtime-benchmarks")]
 use sp_runtime::traits::TrailingZeroInput;
 use sp_runtime::{
@@ -1316,6 +1317,8 @@ impl<T: Config> Pallet<T> {
 		// populate environment
 		ExecutionPhase::<T>::put(Phase::Initialization);
 		storage::unhashed::put(well_known_keys::EXTRINSIC_INDEX, &0u32);
+		let entropy = (b"frame_system::initialize", parent_hash).using_encoded(blake2_256);
+		storage::unhashed::put(well_known_keys::INTRABLOCK_ENTROPY, &entropy[..]);
 		<Number<T>>::put(number);
 		<Digest<T>>::put(digest);
 		<ParentHash<T>>::put(parent_hash);
@@ -1365,6 +1368,7 @@ impl<T: Config> Pallet<T> {
 		);
 		ExecutionPhase::<T>::kill();
 		AllExtrinsicsLen::<T>::kill();
+		storage::unhashed::kill(well_known_keys::INTRABLOCK_ENTROPY);
 
 		// The following fields
 		//
@@ -1633,6 +1637,16 @@ impl<T: Config> Pallet<T> {
 	}
 }
 
+/// Returns a 32 byte datum which is guaranteed to be universally unique. `entropy` is provided
+/// as a facility to reduce the potential for precalculating results.
+pub fn unique(entropy: impl Encode) -> [u8; 32] {
+	let mut last = [0u8; 32];
+	sp_io::storage::read(well_known_keys::INTRABLOCK_ENTROPY, &mut last[..], 0);
+	let next = (b"frame_system::unique", entropy, last).using_encoded(blake2_256);
+	sp_io::storage::set(well_known_keys::INTRABLOCK_ENTROPY, &next.encode());
+	next
+}
+
 /// Event handler which registers a provider when created.
 pub struct Provider<T>(PhantomData<T>);
 impl<T: Config> HandleLifetime<T::AccountId> for Provider<T> {

frame/system/src/tests.rs

@@ -34,6 +34,27 @@ fn origin_works() {
 	assert_eq!(x.unwrap(), RawOrigin::<u64>::Signed(1u64));
 }
 
+#[test]
+fn unique_datum_works() {
+	new_test_ext().execute_with(|| {
+		System::initialize(&1, &[0u8; 32].into(), &Default::default());
+		assert!(sp_io::storage::exists(well_known_keys::INTRABLOCK_ENTROPY));
+
+		let h1 = unique(b"");
+		let h2 = unique(b"");
+		assert_ne!(h1, h2);
+
+		let h3 = unique(b"Hello");
+		assert_ne!(h2, h3);
+
+		let h4 = unique(b"Hello");
+		assert_ne!(h3, h4);
+
+		System::finalize();
+		assert!(!sp_io::storage::exists(well_known_keys::INTRABLOCK_ENTROPY));
+	});
+}
+
 #[test]
 fn stored_map_works() {
 	new_test_ext().execute_with(|| {

primitives/storage/src/lib.rs

@@ -204,6 +204,9 @@ pub mod well_known_keys {
 	/// Encodes to `0x3a65787472696e7369635f696e646578`.
 	pub const EXTRINSIC_INDEX: &[u8] = b":extrinsic_index";
 
+	/// Current intra-block entropy (a universally unique `[u8; 32]` value) is stored here.
+	pub const INTRABLOCK_ENTROPY: &[u8] = b":intrablock_entropy";
+
 	/// Prefix of child storage keys.
 	pub const CHILD_STORAGE_KEY_PREFIX: &[u8] = b":child_storage:";