Work around kpartx's nonzero exit due to "Device or resource busy" errors

which still permit kpartx to finally remove loop device (LP: #1684090).
Also sync before calling kpartx to let writing to loop devices finish.

debian/changelog

@@ -5,6 +5,11 @@ livecd-rootfs (2.442) UNRELEASED; urgency=medium
     comparison prior to aa-series (LP: #1681548)
   * live-build/ubuntu-cpc/hooks/999-extras.binary: Exit on first failure
 
+  [Balint Reczey]
+  * sync before calling kpartx to let writing to loop devices finish
+  * Work around kpartx's nonzero exit due to "Device or resource busy" errors
+    which still permit kpartx to finally remove loop device (LP: #1684090)
+
  -- Steve Langasek <steve.langasek@ubuntu.com>  Thu, 13 Apr 2017 10:38:04 -0700
 
 livecd-rootfs (2.441) zesty; urgency=medium

live-build/functions

@@ -11,9 +11,22 @@ backing_img=
 apt-get -qqy install dosfstools gdisk
 
 clean_loops() {
+    local kpartx_ret
+    local kpartx_stdout
 
     if [ -n "${backing_img}" ]; then
-        kpartx -v -d "${backing_img}"
+        # sync before removing loop to avoid "Device or resource busy" errors
+        sync
+        kpartx_ret=""
+        kpartx_stdout=$(kpartx -v -d "${backing_img}") || kpartx_ret=$?
+        echo "$kpartx_stdout"
+        if [ -n "$kpartx_ret" ]; then
+            if echo "$kpartx_stdout" | grep -q "loop deleted: "; then
+                echo "Suppressing kpartx returning error (#860894)"
+            else
+                exit $kpartx_ret
+            fi
+        fi
         unset backing_img
     fi