making it concurrent-safe for indexing approval

storage/pebble/approvals.go

@@ -3,6 +3,7 @@ package pebble
 import (
 	"errors"
 	"fmt"
+	"sync"
 
 	"github.com/cockroachdb/pebble"
 
@@ -15,8 +16,9 @@ import (
 
 // ResultApprovals implements persistent storage for result approvals.
 type ResultApprovals struct {
-	db    *pebble.DB
-	cache *Cache[flow.Identifier, *flow.ResultApproval]
+	indexing *sync.Mutex // preventing concurrent indexing of approvals
+	db       *pebble.DB
+	cache    *Cache[flow.Identifier, *flow.ResultApproval]
 }
 
 func NewResultApprovals(collector module.CacheMetrics, db *pebble.DB) *ResultApprovals {
@@ -34,7 +36,8 @@ func NewResultApprovals(collector module.CacheMetrics, db *pebble.DB) *ResultApp
 	}
 
 	res := &ResultApprovals{
-		db: db,
+		indexing: new(sync.Mutex),
+		db:       db,
 		cache: newCache[flow.Identifier, *flow.ResultApproval](collector, metrics.ResourceResultApprovals,
 			withLimit[flow.Identifier, *flow.ResultApproval](flow.DefaultTransactionExpiry+100),
 			withStore[flow.Identifier, *flow.ResultApproval](store),
@@ -71,29 +74,29 @@ func (r *ResultApprovals) byChunk(resultID flow.Identifier, chunkIndex uint64) f
 
 func (r *ResultApprovals) index(resultID flow.Identifier, chunkIndex uint64, approvalID flow.Identifier) func(storage.PebbleReaderBatchWriter) error {
 	return func(tx storage.PebbleReaderBatchWriter) error {
+		// acquring the lock to prevent dirty reads of check conflicted approvals
+		r.indexing.Lock()
+		defer r.indexing.Unlock()
+
 		r, w := tx.ReaderWriter()
 
-		err := operation.IndexResultApproval(resultID, chunkIndex, approvalID)(w)
-		if err == nil {
-			return nil
-		}
+		var storedApprovalID flow.Identifier
+		err := operation.LookupResultApproval(resultID, chunkIndex, &storedApprovalID)(r)
+		if err != nil {
+			if !errors.Is(err, storage.ErrNotFound) {
+				return fmt.Errorf("could not lookup result approval ID: %w", err)
+			}
 
-		if !errors.Is(err, storage.ErrAlreadyExists) {
-			return err
+			// no approval found, index the approval
+
+			return operation.IndexResultApproval(resultID, chunkIndex, approvalID)(w)
 		}
 
-		// When trying to index an approval for a result, and there is already
-		// an approval for the result, double check if the indexed approval is
-		// the same.
+		// an approval is already indexed, double check if it is the same
 		// We don't allow indexing multiple approvals per chunk because the
 		// store is only used within Verification nodes, and it is impossible
 		// for a Verification node to compute different approvals for the same
 		// chunk.
-		var storedApprovalID flow.Identifier
-		err = operation.LookupResultApproval(resultID, chunkIndex, &storedApprovalID)(r)
-		if err != nil {
-			return fmt.Errorf("there is an approval stored already, but cannot retrieve it: %w", err)
-		}
 
 		if storedApprovalID != approvalID {
 			return fmt.Errorf("attempting to store conflicting approval (result: %v, chunk index: %d): storing: %v, stored: %v. %w",
@@ -104,14 +107,16 @@ func (r *ResultApprovals) index(resultID flow.Identifier, chunkIndex uint64, app
 	}
 }
 
-// Store stores a ResultApproval
+// Store stores a ResultApproval and indexes a ResultApproval by chunk (ResultID + chunk index).
+// this method is concurrent-safe
 func (r *ResultApprovals) Store(approval *flow.ResultApproval) error {
 	return operation.WithReaderBatchWriter(r.db, r.store(approval))
 }
 
 // Index indexes a ResultApproval by chunk (ResultID + chunk index).
 // operation is idempotent (repeated calls with the same value are equivalent to
 // just calling the method once; still the method succeeds on each call).
+// this method is concurrent-safe
 func (r *ResultApprovals) Index(resultID flow.Identifier, chunkIndex uint64, approvalID flow.Identifier) error {
 	err := operation.WithReaderBatchWriter(r.db, r.index(resultID, chunkIndex, approvalID))
 	if err != nil {