disk: add new method minSize() to VolumeContainer

Add a new method to the VolumeContainer interface that calculates the
minimum size in bytes that the container needs to hold all its children
and metadata.

pkg/disk/btrfs.go

@@ -96,6 +96,20 @@ func (b *Btrfs) MetadataSize() uint64 {
 	return 0
 }
 
+func (b *Btrfs) minSize(size uint64) uint64 {
+	var subvolsum uint64
+	for _, sv := range b.Subvolumes {
+		subvolsum += sv.Size
+	}
+	minSize := subvolsum + b.MetadataSize()
+
+	if minSize > size {
+		size = minSize
+	}
+
+	return b.AlignUp(size)
+}
+
 type BtrfsSubvolume struct {
 	Name       string
 	Size       uint64

pkg/disk/disk.go

@@ -138,6 +138,16 @@ type VolumeContainer interface {
 	// bytes), i.e. the storage space that needs to be reserved for
 	// the container itself, in contrast to the data it contains.
 	MetadataSize() uint64
+
+	// minSize returns the size for the VolumeContainer that is either the
+	// provided desired size value or the sum of all children if that is
+	// larger. It will also add any space required for metadata. The returned
+	// value should, at minimum, be large enough to fit all the children, their
+	// metadata, and the VolumeContainer's metadata. In other words, the
+	// VolumeContainer's size, or its parent size, will be able to hold the
+	// VolumeContainer if it is created with the exact size returned by the
+	// function.
+	minSize(size uint64) uint64
 }
 
 // FSSpec for a filesystem (UUID and Label); the first field of fstab(5)

pkg/disk/luks.go

@@ -109,3 +109,16 @@ func (lc *LUKSContainer) MetadataSize() uint64 {
 	// 16 MiB is the default size for the LUKS2 header
 	return 16 * common.MiB
 }
+
+func (lc *LUKSContainer) minSize(size uint64) uint64 {
+	// since a LUKS container can contain pretty much any payload, but we only
+	// care about the ones that have a size, or contain children with sizes
+	minSize := lc.MetadataSize()
+	switch payload := lc.Payload.(type) {
+	case VolumeContainer:
+		minSize += payload.minSize(size)
+	case Sizeable:
+		minSize += payload.GetSize()
+	}
+	return minSize
+}

pkg/disk/lvm.go

@@ -150,6 +150,20 @@ func (vg *LVMVolumeGroup) MetadataSize() uint64 {
 	return 1 * common.MiB
 }
 
+func (vg *LVMVolumeGroup) minSize(size uint64) uint64 {
+	var lvsum uint64
+	for _, lv := range vg.LogicalVolumes {
+		lvsum += lv.Size
+	}
+	minSize := lvsum + vg.MetadataSize()
+
+	if minSize > size {
+		size = minSize
+	}
+
+	return vg.AlignUp(size)
+}
+
 type LVMLogicalVolume struct {
 	Name    string
 	Size    uint64