Light GRANDPA import handler (#1669)

* GrandpaLightBlockImport

* extract authorities in AuraVerifier

* post-merge fix

* restore authorities cache

* license

* new finality proof draft

* generalized PendingJustifications

* finality proof messages

* fixed compilation

* pass verifier to import_finality_proof

* do not fetch remote proof from light import directly

* FinalityProofProvider

* fixed authorities cache test

* restored finality proof tests

* finality_proof docs

* use DB backend in test client

* justification_is_fetched_by_light_client_when_consensus_data_changes

* restore justification_is_fetched_by_light_client_when_consensus_data_changes

* some more tests

* added authorities-related TODO

* removed unneeded clear_finality_proof_requests field

* truncated some long lines

* more granular light import tests

* only provide finality proof if it is generated by the requested set

* post-merge fix

* finality_proof_is_none_if_first_justification_is_generated_by_unknown_set

* make light+grandpa test rely on finality proofs (instead of simple justifications)

* empty_finality_proof_is_returned_to_light_client_when_authority_set_is_different

* missing trait method impl

* fixed proof-of-finality docs

* one more doc fix

* fix docs

* initialize authorities cache (post-merge fix)

* fixed cache initialization (post-merge fix)

* post-fix merge: fix light + GRANDPA tests (bad way)

* proper fix of empty_finality_proof_is_returned_to_light_client_when_authority_set_is_different

* fixed easy grumbles

* import finality proofs in BlockImportWorker thread

* allow import of finality proofs for non-requested blocks

* limit number of fragments in finality proof

* GRANDPA post-merge fix

* BABE: pos-merge fix

substrate/core/network/src/message.rs

@@ -23,6 +23,7 @@ pub use self::generic::{
 	BlockAnnounce, RemoteCallRequest, RemoteReadRequest,
 	RemoteHeaderRequest, RemoteHeaderResponse,
 	RemoteChangesRequest, RemoteChangesResponse,
+	FinalityProofRequest, FinalityProofResponse,
 	FromBlock, RemoteReadChildRequest,
 };
 
@@ -200,6 +201,10 @@ pub mod generic {
 		RemoteChangesResponse(RemoteChangesResponse<Number, Hash>),
 		/// Remote child storage read request.
 		RemoteReadChildRequest(RemoteReadChildRequest<Hash>),
+		/// Finality proof request.
+		FinalityProofRequest(FinalityProofRequest<Hash>),
+		/// Finality proof reponse.
+		FinalityProofResponse(FinalityProofResponse<Hash>),
 		/// Chain-specific message
 		#[codec(index = "255")]
 		ChainSpecific(Vec<u8>),
@@ -359,4 +364,26 @@ pub mod generic {
 		/// Missing changes tries roots proof.
 		pub roots_proof: Vec<Vec<u8>>,
 	}
+
+	#[derive(Debug, PartialEq, Eq, Clone, Encode, Decode)]
+	/// Finality proof request.
+	pub struct FinalityProofRequest<H> {
+		/// Unique request id.
+		pub id: RequestId,
+		/// Hash of the block to request proof for.
+		pub block: H,
+		/// Additional data blob (that both requester and provider understood) required for proving finality.
+		pub request: Vec<u8>,
+	}
+
+	#[derive(Debug, PartialEq, Eq, Clone, Encode, Decode)]
+	/// Finality proof response.
+	pub struct FinalityProofResponse<H> {
+		/// Id of a request this response was made for.
+		pub id: RequestId,
+		/// Hash of the block (the same as in the FinalityProofRequest).
+		pub block: H,
+		/// Finality proof (if available).
+		pub proof: Option<Vec<u8>>,
+	}
 }