Can retrieve cached initial sync block and db block (#10568)

* Save cached initial sync blocks before getting head block

* Add better abstraction to get block

* Move unlock read to a better location

* Feedbacks

* Add head changed logging

* Harder hasBlock requirement

* Update beacon-chain/blockchain/service.go

Co-authored-by: Preston Van Loon <preston@prysmaticlabs.com>

* Update receive_attestation.go

* Don't process head if the block is unknown

* Use a helper method

* Fix test

Co-authored-by: Nishant Das <nishdas93@gmail.com>
Co-authored-by: Preston Van Loon <preston@prysmaticlabs.com>
Co-authored-by: Raul Jordan <raul@prysmaticlabs.com>

beacon-chain/blockchain/init_sync_process_block.go

@@ -1,9 +1,15 @@
 package blockchain
 
 import (
+	"context"
+
+	"github.com/pkg/errors"
+	"github.com/prysmaticlabs/prysm/beacon-chain/core/helpers"
 	"github.com/prysmaticlabs/prysm/proto/prysm/v1alpha1/block"
 )
 
+var errBlockNotFoundInCacheOrDB = errors.New("block not found in cache or db")
+
 // This saves a beacon block to the initial sync blocks cache.
 func (s *Service) saveInitSyncBlock(r [32]byte, b block.SignedBeaconBlock) {
 	s.initSyncBlocksLock.Lock()
@@ -20,13 +26,33 @@ func (s *Service) hasInitSyncBlock(r [32]byte) bool {
 	return ok
 }
 
-// This retrieves a beacon block from the initial sync blocks cache using the root of
-// the block.
-func (s *Service) getInitSyncBlock(r [32]byte) block.SignedBeaconBlock {
+// Returns true if a block for root `r` exists in the initial sync blocks cache or the DB.
+func (s *Service) hasBlockInInitSyncOrDB(ctx context.Context, r [32]byte) bool {
+	if s.hasInitSyncBlock(r) {
+		return true
+	}
+	return s.cfg.BeaconDB.HasBlock(ctx, r)
+}
+
+// Returns block for a given root `r` from either the initial sync blocks cache or the DB.
+// Error is returned if the block is not found in either cache or DB.
+func (s *Service) getBlock(ctx context.Context, r [32]byte) (block.SignedBeaconBlock, error) {
 	s.initSyncBlocksLock.RLock()
-	defer s.initSyncBlocksLock.RUnlock()
-	b := s.initSyncBlocks[r]
-	return b
+
+	// Check cache first because it's faster.
+	b, ok := s.initSyncBlocks[r]
+	s.initSyncBlocksLock.RUnlock()
+	var err error
+	if !ok {
+		b, err = s.cfg.BeaconDB.Block(ctx, r)
+		if err != nil {
+			return nil, errors.Wrap(err, "could not retrieve block from db")
+		}
+	}
+	if err := helpers.BeaconBlockIsNil(b); err != nil {
+		return nil, errBlockNotFoundInCacheOrDB
+	}
+	return b, nil
 }
 
 // This retrieves all the beacon blocks from the initial sync blocks cache, the returned