Better

cmd/conformance/mysql/main.go

@@ -165,8 +165,8 @@ func configureTilesReadAPI(mux *http.ServeMux, storage *mysql.Storage) {
 			}
 			return
 		}
-		impliedSize := (index*256 + width) << (level * 8)
-		tile, err := storage.ReadTile(r.Context(), level, index, impliedSize)
+		inferredMinTreeSize := (index*256 + width) << (level * 8)
+		tile, err := storage.ReadTile(r.Context(), level, index, inferredMinTreeSize)
 		if err != nil {
 			if os.IsNotExist(err) {
 				w.WriteHeader(http.StatusNotFound)

storage/mysql/mysql.go

@@ -244,7 +244,7 @@ func (s *Storage) writeTreeState(ctx context.Context, tx *sql.Tx, size uint64, r
 // 3. Partial tile request with full/larger partial tile output: Return trimmed partial tile with correct tile width.
 // 4. Partial tile request with partial tile (same width) output: Return partial tile.
 // 5. Partial tile request with smaller partial tile output: Return error.
-func (s *Storage) ReadTile(ctx context.Context, level, index, treeSize uint64) ([]byte, error) {
+func (s *Storage) ReadTile(ctx context.Context, level, index, minTreeSize uint64) ([]byte, error) {
 	row := s.db.QueryRowContext(ctx, selectSubtreeByLevelAndIndexSQL, level, index)
 	if err := row.Err(); err != nil {
 		return nil, err
@@ -258,20 +258,17 @@ func (s *Storage) ReadTile(ctx context.Context, level, index, treeSize uint64) (
 
 		return nil, fmt.Errorf("scan tile: %v", err)
 	}
-	if false {
-		numEntries := uint64(len(tile) / 32)
 
-		// A tile at this height implies this many
-		//	modulus := 1 << (level * 8)
-		width := treeSize % 256
-		if width == 0 {
-			width = 256
-		}
+	// Do some validation to make sure we don't return data at a tree size we haven't reached yet
+	// Doing so could break caching at a higher level
+	entriesPerLevel := 1 << level * 8
+	numEntries := uint64(len(tile) / 32)
 
-		if width > numEntries {
-			// If the user has requested more entries than we have, then return not found.
-			return nil, os.ErrNotExist
-		}
+	minPermanentEntries := numEntries * uint64(entriesPerLevel)
+
+	if minTreeSize > minPermanentEntries {
+		// If the user has requested a size larger than we have, they can't have it
+		return nil, os.ErrNotExist
 	}
 
 	return tile, nil