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/primitives/core/src/crypto.rs

@@ -1006,6 +1006,8 @@ pub mod key_types {
 	pub const AUTHORITY_DISCOVERY: KeyTypeId = KeyTypeId(*b"audi");
 	/// Key type for staking, built-in. Identified as `stak`.
 	pub const STAKING: KeyTypeId = KeyTypeId(*b"stak");
+	/// Key type for equivocation reporting, built-in. Identified as `fish`.
+	pub const REPORTING: KeyTypeId = KeyTypeId(*b"fish");
 	/// A key type ID useful for tests.
 	pub const DUMMY: KeyTypeId = KeyTypeId(*b"dumy");
 }

substrate/primitives/finality-grandpa/src/lib.rs

@@ -352,21 +352,11 @@ where
 	H: Encode,
 	N: Encode,
 {
+	use sp_application_crypto::RuntimeAppPublic;
+
 	localized_payload_with_buffer(round, set_id, message, buf);
 
-	#[cfg(not(feature = "std"))]
-	let verify = || {
-		use sp_application_crypto::RuntimeAppPublic;
-		id.verify(&buf, signature)
-	};
-
-	#[cfg(feature = "std")]
-	let verify = || {
-		use sp_application_crypto::Pair;
-		AuthorityPair::verify(signature, &buf, &id)
-	};
-
-	if verify() {
+	if id.verify(&buf, signature) {
 		Ok(())
 	} else {
 		#[cfg(feature = "std")]
@@ -501,7 +491,10 @@ sp_api::decl_runtime_apis! {
 		/// provide the equivocation proof and a key ownership proof (should be
 		/// obtained using `generate_key_ownership_proof`). This method will
 		/// sign the extrinsic with any reporting keys available in the keystore
-		/// and will push the transaction to the pool.
+		/// and will push the transaction to the pool. This method returns `None`
+		/// when creation of the extrinsic fails, either due to unavailability
+		/// of keys to sign, or because equivocation reporting is disabled for
+		/// the given runtime (i.e. this method is hardcoded to return `None`).
 		/// Only useful in an offchain context.
 		fn submit_report_equivocation_extrinsic(
 			equivocation_proof: EquivocationProof<Block::Hash, NumberFor<Block>>,

substrate/primitives/session/src/lib.rs

@@ -63,18 +63,6 @@ pub struct MembershipProof {
 	pub validator_count: ValidatorCount,
 }
 
-impl MembershipProof {
-	/// Returns a session this proof was generated for.
-	pub fn session(&self) -> SessionIndex {
-		self.session
-	}
-
-	/// Returns the validator count of the session this proof was generated for.
-	pub fn validator_count(&self) -> ValidatorCount {
-		self.validator_count
-	}
-}
-
 /// Generate the initial session keys with the given seeds, at the given block and store them in
 /// the client's keystore.
 #[cfg(feature = "std")]