sealing: round parts in fastPledgeCommitment

storage/sealing/garbage.go

@@ -5,6 +5,7 @@ import (
 	"context"
 	"io"
 	"math"
+	"math/bits"
 	"math/rand"
 	"runtime"
 	"sync"
@@ -18,6 +19,8 @@ import (
 )
 
 func (m *Sealing) fastPledgeCommitment(size uint64, parts uint64) (commP [sectorbuilder.CommLen]byte, err error) {
+	parts = 1 << bits.Len64(parts) // round down to nearest power of 2
+
 	piece := sectorbuilder.UserBytesForSectorSize((size/127 + size) / parts)
 	out := make([]sectorbuilder.PublicPieceInfo, parts)
 	var lk sync.Mutex