grandpa: missing equivocation reporting nits (#5953)

* primitives: move reporting key type to common key types

* session: remove useless methods on MembershipProof

* grandpa: remove std special-casing when checking signatures

* grandpa: add some more docs

* grandpa: use proper error types rather than strings

substrate/frame/grandpa/src/lib.rs

@@ -184,6 +184,10 @@ decl_error! {
 		ChangePending,
 		/// Cannot signal forced change so soon after last.
 		TooSoon,
+		/// A key ownership proof provided as part of an equivocation report is invalid.
+		InvalidKeyOwnershipProof,
+		/// A given equivocation report is valid but already previously reported.
+		DuplicateOffenceReport,
 	}
 }
 
@@ -228,8 +232,9 @@ decl_module! {
 		/// against the extracted offender. If both are valid, the offence
 		/// will be reported.
 		///
-		/// Since the weight is 0 in order to avoid DoS pre-validation is implemented in a
-		/// `SignedExtension`.
+		/// Since the weight of the extrinsic is 0, in order to avoid DoS by
+		/// submission of invalid equivocation reports, a mandatory pre-validation of
+		/// the extrinsic is implemented in a `SignedExtension`.
 		#[weight = 0]
 		fn report_equivocation(
 			origin,
@@ -249,7 +254,7 @@ decl_module! {
 				T::KeyOwnerProofSystem::check_proof(
 					(fg_primitives::KEY_TYPE, equivocation_proof.offender().clone()),
 					key_owner_proof,
-				).ok_or("Invalid key ownership proof.")?;
+				).ok_or(Error::<T>::InvalidKeyOwnershipProof)?;
 
 			// the set id and round when the offence happened
 			let set_id = equivocation_proof.set_id();
@@ -265,7 +270,7 @@ decl_module! {
 					set_id,
 					round,
 				),
-			).map_err(|_| "Duplicate offence report.")?;
+			).map_err(|_| Error::<T>::DuplicateOffenceReport)?;
 		}
 
 		fn on_finalize(block_number: T::BlockNumber) {
@@ -440,9 +445,9 @@ impl<T: Trait> Module<T> {
 			Self::set_grandpa_authorities(authorities);
 		}
 
-		// NOTE: initialize first session of first set. this is necessary
-		// because we only update this `on_new_session` which isn't called
-		// for the genesis session.
+		// NOTE: initialize first session of first set. this is necessary for
+		// the genesis set and session since we only update the set -> session
+		// mapping whenever a new session starts, i.e. through `on_new_session`.
 		SetIdSession::insert(0, 0);
 	}
 
@@ -454,10 +459,7 @@ impl<T: Trait> Module<T> {
 		equivocation_proof: EquivocationProof<T::Hash, T::BlockNumber>,
 		key_owner_proof: T::KeyOwnerProof,
 	) -> Option<()> {
-		T::HandleEquivocation::submit_equivocation_report(equivocation_proof, key_owner_proof)
-			.ok()?;
-
-		Some(())
+		T::HandleEquivocation::submit_equivocation_report(equivocation_proof, key_owner_proof).ok()
 	}
 }