Move /tmp/ccache setup to base element

Commit c7d80dd (Cleanup mount points automatically) removed the unmount
of $TMP_MOUNT_PATH/tmp/ccache in run_d_in_target() and moved the
"rm /tmp/ccache" to elements/base/finalise.d/02-remove-ccache.  There
are two problems with this:

1) Not unmounting at the end of run_d_in_target() results in tmp/ccache
   being bind mounted muliple times on top of itself (three times, if you
   just run `disk-image-create base`).  It is eventually unmounted, but
   somehow the auto unmount code is confused, and tries to unmount it
   one more time than it was mounted, which results in an error like
   "umount: /tmp/image.THQkZxQa/mnt/tmp/ccache: not mounted".
   This doesn't actually break anything, but it's a little messy.

2) "rm /tmp/ccache" in elements/base/finalise.d/02-remove-ccache never
   succeeds in removing /tmp/ccache, because that hook is invoked by
   run_d_in_target(), *while* /tmp/ccache is mounted.

This present commit solves the above by moving the ccache setup glue out
of img-functions and into the base element's root.d.  This has the
following implications:

1) lib/img-functions is a little cleaner.

2) /tmp/ccache is available in the chroot during the root, extra-data,
   pre-install, install and post-install stages.  It is not available
   during block-device, finalise and cleanup stages as it will have been
   automatically unmounted by then.

3) /tmp/ccache won't be setup if you're building an image that doesn't
   include the base element.

Change-Id: Ief4c0a6f4ec622db6c6f652776215684178d8943

elements/base/README.md

@@ -10,3 +10,8 @@ Notes:
    This may be as simple as having language-pack-XX installed in the
    pre-install stage
 
+ * This element ensures /tmp/ccache will be available in the chroot
+   during the root, extra-data, pre-install, install and post-install
+   stages.  /tmp/ccache is unavailable during block-device, finalise
+   and cleanup stages as it will have been automatically unmounted
+   by then.

elements/base/cleanup.d/01-ccache

@@ -0,0 +1,7 @@
+#!/bin/bash
+
+set -e
+
+# As this is run in cleanup, it's already been automatically unmounted,
+# so all we need to do here is remove the directory
+sudo rmdir $TMP_MOUNT_PATH/tmp/ccache || true

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

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

elements/base/root.d/01-ccache

@@ -0,0 +1,15 @@
+#!/bin/bash
+
+set -e
+
+# Don't do anything if already mounted (if disk-image-create is invoked with
+# no elements specified, this hook actually fires twice, once during
+# `run_d root` for the base element, then again when `run_d root` is called
+# after automatically pulling in the Ubuntu element)
+grep " $TMP_MOUNT_PATH/tmp/ccache" /proc/mounts && exit
+
+DIB_CCACHE_DIR=${DIB_CCACHE_DIR:-$HOME/.cache/image-create/ccache}
+mkdir -p $DIB_CCACHE_DIR
+
+sudo mkdir -p $TMP_MOUNT_PATH/tmp/ccache
+sudo mount --bind $DIB_CCACHE_DIR $TMP_MOUNT_PATH/tmp/ccache

lib/common-functions

@@ -241,8 +241,10 @@ function create_base () {
     export TMP_MOUNT_PATH=$TMP_BUILD_DIR/mnt
     # Copy data in to the root.
     TARGET_ROOT=$TMP_MOUNT_PATH run_d root
-    if [ -z "$(ls $TMP_MOUNT_PATH | grep -v lost+found)" ] ; then
-        # Nothing copied in, use Ubuntu.
+    if [ -z "$(ls $TMP_MOUNT_PATH | grep -v '^lost+found\|tmp$')" ] ; then
+        # Nothing copied in, use Ubuntu.  Note the test above allows
+        # root.d elements to put things in /tmp, and still have the
+        # automatic Ubuntu behaviour.
         echo "Adding ubuntu element as / had no contents"
         IMAGE_ELEMENT=$($SCRIPT_HOME/element-info --expand-dependencies $IMAGE_ELEMENT ubuntu)
         generate_hooks

lib/img-defaults

@@ -20,4 +20,3 @@ FS_TYPE=${FS_TYPE:-ext4}
 # Used to set the file extension only at this stage.
 IMAGE_TYPE=${IMAGE_TYPE:-qcow2}
 IMAGE_NAME=${IMAGE_NAME:-image}
-DIB_CCACHE_DIR=${DIB_CCACHE_DIR:-~/.cache/image-create/ccache}

lib/img-functions

@@ -75,9 +75,6 @@ function run_d_in_target () {
       sudo mkdir $TMP_MOUNT_PATH/tmp/in_target.d
       sudo mount --bind ${TMP_HOOKS_PATH} $TMP_MOUNT_PATH/tmp/in_target.d
       sudo mount -o remount,ro,bind ${TMP_HOOKS_PATH} $TMP_MOUNT_PATH/tmp/in_target.d
-      sudo mkdir -p $TMP_MOUNT_PATH/tmp/ccache
-      mkdir -p $DIB_CCACHE_DIR
-      sudo mount --bind $DIB_CCACHE_DIR $TMP_MOUNT_PATH/tmp/ccache
       check_break before-$1 run_in_target bash
       trap "check_break after-error run_in_target bash" ERR
       run_in_target dib-run-parts /tmp/in_target.d/$1.d