fix: add a check for overlay mounts in installer pre-flight checks

Overlay mount in `mountinfo` don't show up as mounts for any particular
block device, so the existing check doesn't catch them.

This was discovered as our current master can't upgrade because of
overlay mount for `/opt` and `apid` image in `/opt/apid` (which will be
fixed in a separate PR).

Without the check, installer fails on resetting partition table for the
disk effectively wiping the node (`device or resource busy` error).

Signed-off-by: Andrey Smirnov <smirnov.andrey@gmail.com>

cmd/installer/pkg/install/manifest.go

@@ -31,6 +31,7 @@ type Manifest struct {
 	PartitionOptions *runtime.PartitionOptions
 	Devices          map[string]Device
 	Targets          map[string][]*Target
+	ImageCreation    bool
 }
 
 // Device represents device options.
@@ -52,6 +53,8 @@ func NewManifest(label string, sequence runtime.Sequence, bootPartitionFound boo
 	manifest = &Manifest{
 		Devices: map[string]Device{},
 		Targets: map[string][]*Target{},
+
+		ImageCreation: sequence == runtime.SequenceNoop,
 	}
 
 	if opts.Board != constants.BoardNone {
@@ -170,6 +173,8 @@ func (m *Manifest) Execute() (err error) {
 }
 
 // checkMounts verifies that no active mounts in any mount namespace exist for the device.
+//
+//nolint:gocyclo
 func (m *Manifest) checkMounts(device Device) error {
 	matches, err := filepath.Glob("/proc/*/mountinfo")
 	if err != nil {
@@ -198,6 +203,22 @@ func (m *Manifest) checkMounts(device Device) error {
 					continue
 				}
 
+				if !m.ImageCreation && fields[len(fields)-2] == "overlay" {
+					// parsing options (last column) in the overlay mount line which looks like:
+					// 163 70 0:52 /apid / ro,relatime - overlay overlay rw,lowerdir=/opt,upperdir=/var/system/overlays/opt-diff,workdir=/var/system/overlays/opt-workdir
+
+					options := strings.Split(fields[len(fields)-1], ",")
+
+					for _, option := range options {
+						parts := strings.SplitN(option, "=", 2)
+						if len(parts) == 2 {
+							if strings.HasPrefix(parts[1], "/var/") {
+								return fmt.Errorf("found overlay mount in %q: %s", path, scanner.Text())
+							}
+						}
+					}
+				}
+
 				if fields[len(fields)-2] == device.Device {
 					return fmt.Errorf("found active mount in %q for %q: %s", path, device.Device, scanner.Text())
 				}