Refactor slot by blockroot (#16040)

**Review after #16033 is merged**

**What type of PR is this?**
Other

**What does this PR do? Why is it needed?**
This PR refactors the code to find the corresponding slot of a given
block root using state summary or the block itself, into its own
function `SlotByBlockRoot(ctx, blockroot)`.

Note that there exists a function `slotByBlockRoot(ctx context.Context,
tx *bolt.Tx, blockRoot []byte)` immediately below the new function. Also
note that this function has two drawbacks, which led to creation of the
new function:
- the old function requires a boltdb tx, which is not necessarily
available to the caller.
- the old function does NOT make use of the state summary cache. 

edit: 
- the old function also uses the state bucket to retrieve the state and
it's slot. this is not something we want in the state diff feature,
since there is no state bucket.

beacon-chain/db/kv/state.go

@@ -8,7 +8,6 @@ import (
 	"github.com/OffchainLabs/prysm/v7/beacon-chain/state"
 	statenative "github.com/OffchainLabs/prysm/v7/beacon-chain/state/state-native"
 	"github.com/OffchainLabs/prysm/v7/config/features"
-	"github.com/OffchainLabs/prysm/v7/consensus-types/blocks"
 	"github.com/OffchainLabs/prysm/v7/consensus-types/primitives"
 	"github.com/OffchainLabs/prysm/v7/encoding/bytesutil"
 	"github.com/OffchainLabs/prysm/v7/genesis"
@@ -481,7 +480,7 @@ func (s *Store) DeleteState(ctx context.Context, blockRoot [32]byte) error {
 			return nil
 		}
 
-		slot, err := s.slotByBlockRoot(ctx, tx, blockRoot[:])
+		slot, err := s.SlotByBlockRoot(ctx, blockRoot)
 		if err != nil {
 			return err
 		}
@@ -823,50 +822,45 @@ func (s *Store) stateBytes(ctx context.Context, blockRoot [32]byte) ([]byte, err
 	return dst, err
 }
 
-// slotByBlockRoot retrieves the corresponding slot of the input block root.
-func (s *Store) slotByBlockRoot(ctx context.Context, tx *bolt.Tx, blockRoot []byte) (primitives.Slot, error) {
-	ctx, span := trace.StartSpan(ctx, "BeaconDB.slotByBlockRoot")
+// SlotByBlockRoot returns the slot of the input block root, based on state summary, block, or state.
+// Check for state is only done if state diff feature is not enabled.
+func (s *Store) SlotByBlockRoot(ctx context.Context, blockRoot [32]byte) (primitives.Slot, error) {
+	ctx, span := trace.StartSpan(ctx, "BeaconDB.SlotByBlockRoot")
 	defer span.End()
 
-	bkt := tx.Bucket(stateSummaryBucket)
-	enc := bkt.Get(blockRoot)
-
-	if enc == nil {
-		// Fall back to check the block.
-		bkt := tx.Bucket(blocksBucket)
-		enc := bkt.Get(blockRoot)
-
-		if enc == nil {
-			// Fallback and check the state.
-			bkt = tx.Bucket(stateBucket)
-			enc = bkt.Get(blockRoot)
-			if enc == nil {
-				return 0, errors.New("state enc can't be nil")
-			}
-			// no need to construct the validator entries as it is not used here.
-			s, err := s.unmarshalState(ctx, enc, nil)
-			if err != nil {
-				return 0, errors.Wrap(err, "could not unmarshal state")
-			}
-			if s == nil || s.IsNil() {
-				return 0, errors.New("state can't be nil")
-			}
-			return s.Slot(), nil
-		}
-		b, err := unmarshalBlock(ctx, enc)
-		if err != nil {
-			return 0, errors.Wrap(err, "could not unmarshal block")
-		}
-		if err := blocks.BeaconBlockIsNil(b); err != nil {
-			return 0, err
-		}
-		return b.Block().Slot(), nil
+	// check state summary first
+	stateSummary, err := s.StateSummary(ctx, blockRoot)
+	if err != nil {
+		return 0, err
 	}
-	stateSummary := &ethpb.StateSummary{}
-	if err := decode(ctx, enc, stateSummary); err != nil {
-		return 0, errors.Wrap(err, "could not unmarshal state summary")
+	if stateSummary != nil {
+		return stateSummary.Slot, nil
 	}
-	return stateSummary.Slot, nil
+
+	// fall back to block if state summary is not found
+	blk, err := s.Block(ctx, blockRoot)
+	if err != nil {
+		return 0, err
+	}
+	if blk != nil && !blk.IsNil() {
+		return blk.Block().Slot(), nil
+	}
+
+	// fall back to state, only if state diff feature is not enabled
+	if features.Get().EnableStateDiff {
+		return 0, errors.New("neither state summary nor block found")
+	}
+
+	st, err := s.State(ctx, blockRoot)
+	if err != nil {
+		return 0, err
+	}
+	if st != nil && !st.IsNil() {
+		return st.Slot(), nil
+	}
+
+	// neither state summary, block nor state found
+	return 0, errors.New("neither state summary, block nor state found")
 }
 
 // HighestSlotStatesBelow returns the states with the highest slot below the input slot
@@ -1044,24 +1038,10 @@ func (s *Store) isStateValidatorMigrationOver() (bool, error) {
 }
 
 func (s *Store) getStateUsingStateDiff(ctx context.Context, blockRoot [32]byte) (state.BeaconState, error) {
-	var slot primitives.Slot
-
-	stateSummary, err := s.StateSummary(ctx, blockRoot)
+	slot, err := s.SlotByBlockRoot(ctx, blockRoot)
 	if err != nil {
 		return nil, err
 	}
-	if stateSummary == nil {
-		blk, err := s.Block(ctx, blockRoot)
-		if err != nil {
-			return nil, err
-		}
-		if blk == nil || blk.IsNil() {
-			return nil, errors.New("neither state summary nor block found")
-		}
-		slot = blk.Block().Slot()
-	} else {
-		slot = stateSummary.Slot
-	}
 
 	st, err := s.stateByDiff(ctx, slot)
 	if err != nil {

changelog/bastin_refactor-slot-by-blkroot.md

@@ -0,0 +1,3 @@
+### Ignored
+
+- Refactor finding slot by block root using state summary and block to its own function.