Cleanup mount points automatically.

Manually listing the mount points that need cleanup is not
maintainable and makes it hard to write elements that use bind
mounting to inject resources into the build environment.

Change-Id: I7c9ade444f4ebe42552f8e321f257a7ec0a674ef

README.md

@@ -192,6 +192,11 @@ Conform to the following conventions:
     If they both use set -e and cp -n then the conflict will be caught and cause
     the build to fail.
 
+* If your element mounts anything into the image build tree ($TMP\_BUILD\_DIR)
+  then it will be automatically unmounted when the build tree is unmounted -
+  and not remounted into the filesystem image - if the mount point is needed
+  again, your element will need to remount it at that point.
+
 Make as many of the following subdirectories as you need, depending on what
 part of the process you need to customise:
 

elements/base/finalise.d/02-remove-ccache

@@ -0,0 +1,4 @@
+#!/bin/bash
+set -e
+
+rmdir /tmp/ccache || true

lib/img-functions

@@ -16,16 +16,14 @@
 function unmount_image () {
     # unmount from the chroot
     # Don't use TMP_MOUNT_PATH here, it might not have been set.
-    sudo umount -f $TMP_BUILD_DIR/mnt/sys || true
-    sudo umount -f $TMP_BUILD_DIR/mnt/proc || true
-    sudo umount -f $TMP_BUILD_DIR/mnt/dev || true
-    sudo umount -f $TMP_BUILD_DIR/mnt/tmp/in_target.d || true
-    sudo umount -f $TMP_BUILD_DIR/mnt/tmp/ccache || true
+    local MOUNTS
+    MOUNTS=$(grep < /proc/mounts " $TMP_BUILD_DIR/mnt" | awk '{ print $2 }' | sort -r)
+    for M in $MOUNTS; do
+        sudo umount -f $M || true
+    done
     # give it a second (ok really 5) to umount XXX - why? should instead track
     # the mount data / lsof etc.
     sleep 5
-    # oh ya don't want to forget to unmount the image
-    sudo umount -f $TMP_BUILD_DIR/mnt || true
     # having disk corruption issues; one possibility is qemu-nbd not flushing
     # dirty pages on disconnect?
     sync
@@ -84,8 +82,6 @@ function run_d_in_target () {
       check_break after-$1 run_in_target bash
       sudo umount -f $TMP_MOUNT_PATH/tmp/in_target.d
       sudo rmdir $TMP_MOUNT_PATH/tmp/in_target.d
-      sudo umount -f $TMP_MOUNT_PATH/tmp/ccache || true
-      sudo rmdir $TMP_MOUNT_PATH/tmp/ccache
     fi
 }