lb_chroot_layered: if the unmount of the chroot mountpoint fails, dump mount information for debugging.

debian/changelog

@@ -2,6 +2,8 @@ livecd-rootfs (23.10.27) UNRELEASED; urgency=medium
 
   * Add ubuntu to the list of targets we autopkgtest for on amd64, to ensure
     coverage of the lb_chroot_layered behavior.
+  * lb_chroot_layered: if the unmount of the chroot mountpoint fails, dump
+    mount information for debugging.
 
  -- Steve Langasek <steve.langasek@ubuntu.com>  Tue, 29 Aug 2023 10:32:35 -0700
 

live-build/lb_chroot_layered

@@ -268,7 +268,13 @@ create_chroot_pass () {
 	lb chroot_devpts remove ${*}
 
 	if [ -n "$lowerdirs" ]; then
-		umount chroot
+		res=0
+		umount chroot || res=$?
+		if [ $res -ne 0 ]; then
+			Echo_message "Unmount failed, current mounts: "
+			cat /proc/$$/mounts
+			exit $res
+		fi
 		rmdir chroot
 	else
 		rm chroot