impl Randomness trait for Babe and remove unused RandomBeacon trait (#4886)

* impl Randomness trait for Babe and remove unused RandomBeacon trait

* bump runtime version

substrate/frame/babe/src/lib.rs

@@ -23,10 +23,10 @@
 pub use pallet_timestamp;
 
 use sp_std::{result, prelude::*};
-use frame_support::{decl_storage, decl_module, traits::FindAuthor, traits::Get};
+use frame_support::{decl_storage, decl_module, traits::{FindAuthor, Get, Randomness as RandomnessT}};
 use sp_timestamp::OnTimestampSet;
 use sp_runtime::{generic::DigestItem, ConsensusEngineId, Perbill};
-use sp_runtime::traits::{IsMember, SaturatedConversion, Saturating, RandomnessBeacon};
+use sp_runtime::traits::{IsMember, SaturatedConversion, Saturating, Hash};
 use sp_staking::{
 	SessionIndex,
 	offence::{Offence, Kind},
@@ -191,9 +191,13 @@ decl_module! {
 	}
 }
 
-impl<T: Trait> RandomnessBeacon for Module<T> {
-	fn random() -> [u8; VRF_OUTPUT_LENGTH] {
-		Self::randomness()
+impl<T: Trait> RandomnessT<<T as frame_system::Trait>::Hash> for Module<T> {
+	fn random(subject: &[u8]) -> T::Hash {
+		let mut subject = subject.to_vec();
+		subject.reserve(VRF_OUTPUT_LENGTH);
+		subject.extend_from_slice(&Self::randomness()[..]);
+
+		<T as frame_system::Trait>::Hashing::hash(&subject[..])
 	}
 }
 

substrate/frame/support/src/traits.rs

@@ -767,7 +767,10 @@ pub trait Randomness<Output> {
 	/// Get a "random" value
 	///
 	/// Being a deterministic blockchain, real randomness is difficult to come by. This gives you
-	/// something that approximates it. `subject` is a context identifier and allows you to get a
+	/// something that approximates it. At best, this will be randomness which was
+	/// hard to predict a long time ago, but that has become easy to predict recently.
+	///
+	/// `subject` is a context identifier and allows you to get a
 	/// different result to other callers of this function; use it like
 	/// `random(&b"my context"[..])`.
 	fn random(subject: &[u8]) -> Output;