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

@@ -71,25 +71,57 @@ func (s stage) createPartitions(config types.Config) error {
 // also have non-zero start and size and non-nil start and size and label.
 // n.b. existing.{Size,Start}MiB must be converted to sectors first (yes the variable name becomes misleading)
 func partitionMatches(existing util.PartitionInfo, spec sgdisk.Partition) error {
+	result := partitionMatchesCommon(existing, spec)
+	switch result {
+	case "Number":
+		return fmt.Errorf("partition numbers did not match (specified %d, got %d). This should not happen, please file a bug", spec.Number, existing.Number)
+	case "StartSector":
+		return fmt.Errorf("starting sector did not match (specified %d, got %d)", *spec.StartSector, existing.StartSector)
+	case "SizeInSectors":
+		return fmt.Errorf("size did not match (specified %d, got %d)", *spec.SizeInSectors, existing.SizeInSectors)
+	case "GUID":
+		return fmt.Errorf("GUID did not match (specified %q, got %q)", *spec.GUID, existing.GUID)
+	case "TypeGUID":
+		return fmt.Errorf("type GUID did not match (specified %q, got %q)", *spec.TypeGUID, existing.TypeGUID)
+	case "Label":
+		return fmt.Errorf("label did not match (specified %q, got %q)", *spec.Label, existing.Label)
+	}
+	return nil
+}
+
+// partitionMatchesResize returns if the existing partition should be resized by evaluating if the `resize`
+// field is set to true and all of the specified attributes matches the existing partition's attributes except
+// the partition size.
+func partitionMatchesResize(existing util.PartitionInfo, spec sgdisk.Partition, resize bool) bool {
+	result := partitionMatchesCommon(existing, spec)
+	if resize && strings.Contains(result, "SizeInSectors") {
+		return true
+	}
+	return false
+}
+
+// partitionMatchesCommon handles the common tests to match the existing partition attributes with the
+// specified partition ones.
+func partitionMatchesCommon(existing util.PartitionInfo, spec sgdisk.Partition) string {
 	if spec.Number != existing.Number {
-		return fmt.Errorf("partition numbers did not match (specified %d, got %d). This should not happen, please file a bug.", spec.Number, existing.Number)
+		return "Number"
 	}
 	if spec.StartSector != nil && *spec.StartSector != existing.StartSector {
-		return fmt.Errorf("starting sector did not match (specified %d, got %d)", *spec.StartSector, existing.StartSector)
+		return "StatSector"
 	}
 	if spec.SizeInSectors != nil && *spec.SizeInSectors != existing.SizeInSectors {
-		return fmt.Errorf("size did not match (specified %d, got %d)", *spec.SizeInSectors, existing.SizeInSectors)
+		return "SizeInSectors"
 	}
 	if spec.GUID != nil && *spec.GUID != "" && strings.ToLower(*spec.GUID) != strings.ToLower(existing.GUID) {
-		return fmt.Errorf("GUID did not match (specified %q, got %q)", *spec.GUID, existing.GUID)
+		return "GUID"
 	}
 	if spec.TypeGUID != nil && *spec.TypeGUID != "" && strings.ToLower(*spec.TypeGUID) != strings.ToLower(existing.TypeGUID) {
-		return fmt.Errorf("type GUID did not match (specified %q, got %q)", *spec.TypeGUID, existing.TypeGUID)
+		return "TypeGUID"
 	}
 	if spec.Label != nil && *spec.Label != existing.Label {
-		return fmt.Errorf("label did not match (specified %q, got %q)", *spec.Label, existing.Label)
+		return "Label"
 	}
-	return nil
+	return ""
 }
 
 // partitionShouldBeInspected returns if the partition has zeroes that need to be resolved to sectors.
@@ -339,6 +371,7 @@ 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 +386,19 @@ 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)
+			resizePartition := partitionMatchesResize(info, part, resize)
+			if resizePartition {
+				s.Logger.Info("resizing partition %d", 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)