stages/disks: add logic to resize an existing partition

Fixes coreos#924
This fix will allow users to resize an existing partition without wiping
out an entry from the partition table.

internal/exec/stages/disks/partitions.go

@@ -339,6 +339,7 @@ func (s stage) partitionDisk(dev types.Disk, devAlias string) error {
 		}
 		matches := exists && matchErr == nil
 		wipeEntry := part.WipePartitionEntry != nil && *part.WipePartitionEntry
+		resize := part.Resize != nil && *part.Resize
 
 		// This is a translation of the matrix in the operator notes.
 		switch {
@@ -353,7 +354,23 @@ func (s stage) partitionDisk(dev types.Disk, devAlias string) error {
 		case exists && shouldExist && matches:
 			s.Logger.Info("partition %d found with correct specifications", part.Number)
 		case exists && shouldExist && !wipeEntry && !matches:
-			return fmt.Errorf("Partition %d didn't match: %v", part.Number, matchErr)
+			number := part.Number == info.Number
+			label := part.Label == nil || *part.Label == info.Label
+			start := part.StartSector == nil || *part.StartSector == info.StartSector
+			guid := part.GUID == nil || *part.GUID == "" || strings.ToLower(*part.GUID) == strings.ToLower(info.GUID)
+			typeGUID := part.TypeGUID == nil || *part.TypeGUID == "" || strings.ToLower(*part.TypeGUID) == strings.ToLower(info.TypeGUID)
+			if resize && number && label && start && guid && typeGUID {
+				s.Logger.Info("partition %d found with resize option, resizing partition", part.Number)
+				op.DeletePartition(part.Number)
+				part.Number = info.Number
+				part.GUID = &info.GUID
+				part.TypeGUID = &info.TypeGUID
+				part.Label = &info.Label
+				part.StartSector = &info.StartSector
+				op.CreatePartition(part)
+			} else {
+				return fmt.Errorf("Partition %d didn't match: %v", part.Number, matchErr)
+			}
 		case exists && shouldExist && wipeEntry && !matches:
 			s.Logger.Info("partition %d did not meet specifications, wiping partition entry and recreating", part.Number)
 			op.DeletePartition(part.Number)