Zerofree the image if possible

If the image has an ext filesystem and the zerofree utility is present
on the build system then run zerofree. This should make images as
compressable as possible which is a nice feature when building
compressed qcow2 images.

Change-Id: Ia6062c291f7a3f58b85a4f408ecb3d0574c65d53

bin/disk-image-create

@@ -335,7 +335,9 @@ if [ -n "$MAX_ONLINE_RESIZE" ]; then
 fi
 
 LOOPDEV=$(sudo losetup --show -f $TMP_IMAGE_PATH)
-export EXTRA_UNMOUNT="detach_loopback $LOOPDEV"
+# Definied here for backward compat against set -u
+export EXTRA_UNMOUNT=""
+export EXTRA_CLEANUP="detach_loopback $LOOPDEV"
 export IMAGE_BLOCK_DEVICE=$LOOPDEV
 eval_run_d block-device "IMAGE_BLOCK_DEVICE="
 sudo mkfs $MKFS_OPTS -t $FS_TYPE -L ${DIB_ROOT_LABEL} ${IMAGE_BLOCK_DEVICE}
@@ -382,6 +384,13 @@ done
 unmount_image
 cleanup_build_dir
 
+# Write zeros to free blocks to aid compression of images.
+if echo "$FS_TYPE" | grep -q "^ext" && which zerofree ; then
+    sudo zerofree $IMAGE_BLOCK_DEVICE
+fi
+# We are done with the loopback device make sure it is detached.
+$EXTRA_CLEANUP
+
 has_raw_type=
 if [ "$IS_RAMDISK" == "0" ]; then
   for IMAGE_TYPE in ${IMAGE_TYPES[@]} ; do

doc/source/user_guide/installation.rst

@@ -27,6 +27,11 @@ It is recommended you check the documentation for each element you are using
 to determine if there are any additional dependencies. Of particular note is
 the need for the `dev-python/pyyaml` package on Gentoo hosts.
 
+If the zerofree utility is present on the build system and you are building
+an image with an ext filesystem zerofree will be used to zero out free
+blocks in the image. This makes the resulting images much smaller if
+compressed.
+
 
 Source Installation
 -------------------

lib/img-functions

@@ -24,6 +24,8 @@ function unmount_image () {
     # unmount from the chroot
     # Don't use TMP_MOUNT_PATH here, it might not have been set.
     unmount_dir "$TMP_BUILD_DIR/mnt"
+
+    # Left here for backward compatibility
     if [ -n "$EXTRA_UNMOUNT" ]; then
         $EXTRA_UNMOUNT
     fi
@@ -39,6 +41,9 @@ function cleanup () {
     unmount_image
     cleanup_build_dir
     cleanup_image_dir
+    if [ -n "$EXTRA_CLEANUP" ]; then
+        $EXTRA_CLEANUP
+    fi
 }
 
 function ensure_nbd () {

tests/install_test_deps.sh

@@ -8,8 +8,10 @@ sudo apt-get install -y \
         debootstrap \
         inetutils-ping \
         kpartx \
+        zerofree \
         qemu-utils || \
     sudo yum -y install \
         debootstrap \
         kpartx \
+        zerofree \
         qemu-img