Implemented a fallback for DecodeSignerIDs when IdentitiesByEpoch fails with sentinel

Author: durkmurder

consensus/hotstuff/signature/block_signer_decoder.go

@@ -36,12 +36,19 @@ func (b *BlockSignerDecoder) DecodeSignerIDs(header *flow.Header) (flow.Identifi
 		return []flow.Identifier{}, nil
 	}
 
+	// we will use IdentitiesByEpoch since it's a faster call and avoids DB lookup
 	members, err := b.IdentitiesByEpoch(header.ParentView)
 	if err != nil {
 		if errors.Is(err, model.ErrViewForUnknownEpoch) {
-			return nil, fmt.Errorf("could not retrieve consensus participants for view %d: %w", header.ParentView, err)
+			// possibly, we request epoch which is far behind in the past, in this case we won't have it in cache.
+			// try asking by parent ID
+			members, err = b.IdentitiesByBlock(header.ParentID)
+			if err != nil {
+				return nil, fmt.Errorf("could not retrieve identities for block %x with QC view %d for parent %x: %w", header.ID(), header.ParentView, header.ParentID, err)
+			}
+		} else {
+			return nil, fmt.Errorf("unexpected error retrieving identities for block %v: %w", header.ID(), err)
 		}
-		return nil, fmt.Errorf("unexpected error retrieving identities for block %v: %w", header.ID(), err)
 	}
 	signerIDs, err := signature.DecodeSignerIndicesToIdentifiers(members.NodeIDs(), header.ParentVoterIndices)
 	if err != nil {

consensus/hotstuff/signature/block_signer_decoder_test.go

@@ -13,6 +13,7 @@ import (
 	"github.com/onflow/flow-go/model/flow"
 	"github.com/onflow/flow-go/model/flow/order"
 	"github.com/onflow/flow-go/module/signature"
+	"github.com/onflow/flow-go/state"
 	"github.com/onflow/flow-go/utils/unittest"
 )