grandpa: allow authority set hard forks to be forced (#10444)

* grandpa: allow authority set hard forks to be forced

* grandpa: fix authority set hard forks in warp proof provider

* grandpa: make AuthoritySetHardFork public

* grandpa: extend comment

client/finality-grandpa/src/lib.rs

@@ -519,6 +519,24 @@ where
 	)
 }
 
+/// A descriptor for an authority set hard fork. These are authority set changes
+/// that are not signalled by the runtime and instead are defined off-chain
+/// (hence the hard fork).
+pub struct AuthoritySetHardFork<Block: BlockT> {
+	/// The new authority set id.
+	pub set_id: SetId,
+	/// The block hash and number at which the hard fork should be applied.
+	pub block: (Block::Hash, NumberFor<Block>),
+	/// The authorities in the new set.
+	pub authorities: AuthorityList,
+	/// The latest block number that was finalized before this authority set
+	/// hard fork. When defined, the authority set change will be forced, i.e.
+	/// the node won't wait for the block above to be finalized before enacting
+	/// the change, and the given finalized number will be used as a base for
+	/// voting.
+	pub last_finalized: Option<NumberFor<Block>>,
+}
+
 /// Make block importer and link half necessary to tie the background voter to
 /// it. A vector of authority set hard forks can be passed, any authority set
 /// change signaled at the given block (either already signalled or in a further
@@ -528,14 +546,8 @@ pub fn block_import_with_authority_set_hard_forks<BE, Block: BlockT, Client, SC>
 	client: Arc<Client>,
 	genesis_authorities_provider: &dyn GenesisAuthoritySetProvider<Block>,
 	select_chain: SC,
-	authority_set_hard_forks: Vec<(SetId, (Block::Hash, NumberFor<Block>), AuthorityList)>,
-) -> Result<
-	(
-		GrandpaBlockImport<BE, Block, Client, SC>,
-		LinkHalf<Block, Client, SC>,
-	),
-	ClientError,
->
+	authority_set_hard_forks: Vec<AuthoritySetHardFork<Block>>,
+) -> Result<(GrandpaBlockImport<BE, Block, Client, SC>, LinkHalf<Block, Client, SC>), ClientError>
 where
 	SC: SelectChain<Block>,
 	BE: Backend<Block> + 'static,
@@ -562,19 +574,24 @@ where
 	let (justification_sender, justification_stream) =
 		GrandpaJustificationStream::channel();
 
-	// create pending change objects with 0 delay and enacted on finality
-	// (i.e. standard changes) for each authority set hard fork.
+	// create pending change objects with 0 delay for each authority set hard fork.
 	let authority_set_hard_forks = authority_set_hard_forks
 		.into_iter()
-		.map(|(set_id, (hash, number), authorities)| {
+		.map(|fork| {
+			let delay_kind = if let Some(last_finalized) = fork.last_finalized {
+				authorities::DelayKind::Best { median_last_finalized: last_finalized }
+			} else {
+				authorities::DelayKind::Finalized
+			};
+
 			(
-				set_id,
+				fork.set_id,
 				authorities::PendingChange {
-					next_authorities: authorities,
+					next_authorities: fork.authorities,
 					delay: Zero::zero(),
-					canon_hash: hash,
-					canon_height: number,
-					delay_kind: authorities::DelayKind::Finalized,
+					canon_hash: fork.block.0,
+					canon_height: fork.block.1,
+					delay_kind,
 				},
 			)
 		})