Add the ability to break into a shell during builds.

Change-Id: I87af952d892f8622e4c916085fc896c735a35438

README.md

@@ -125,6 +125,20 @@ Ramdisk elements support the following files in their element directories:
 * init : a POSIX shell script fragment that will be appended to the default
   script executed as the ramdisk is booted (/init)
 
+Debugging elements
+------------------
+
+Export 'break' to drop to a shell during the image build. Break points can be
+set either before or after any of the hook points by exporting
+"break=[before|after]-hook-name". Multiple break points can be specified as a
+comma-delimited string. Some examples:
+
+* break=before-block-device-size will break before the block device size hooks
+  are called.
+
+* break=after-first-boot,before-pre-install will break after the first-boot
+  hooks and before the pre-install hooks.
+
 Third party elements
 --------------------
 

lib/common-functions

@@ -44,6 +44,20 @@ function generate_hooks () {
   done
 }
 
+# Call the supplied break-in routine if the named point is listed in the break
+# list.
+# $1 the break point.
+# $2.. what to call if a break is needed
+function check_break () {
+  if echo "$break" | egrep -e "(,|^)$1(,|$)" -q; then
+    echo "Starting debug shell. Exit to resume building." >&2
+    echo At stage $1 >&2
+    shift
+    "$@"
+    echo "Resuming" >&2
+  fi
+}
+
 # Check that a real element has been chosen (prevents foot-guns)
 function check_element () {
   [ -d $TMP_HOOKS_PATH ] || generate_hooks

lib/img-functions

@@ -122,7 +122,9 @@ 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
+      check_break before-$1 run_in_target bash
       run_in_target run-parts -v /tmp/in_target.d/$1.d
+      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
     fi
@@ -131,12 +133,15 @@ function run_d_in_target() {
 # Run a directory of hooks outside the target.
 function run_d() {
     check_element
+    check_break before-$1 bash
     if [ -d ${TMP_HOOKS_PATH}/$1.d ] ; then
       run-parts ${TMP_HOOKS_PATH}/$1.d
     fi
+    check_break after-$1 bash
 }
 
 function prepare_first_boot () {
+    check_break before-first-boot run_in_target bash
     if [ -d ${TMP_HOOKS_PATH}/first-boot.d ] ; then
       sudo cp -t $TMP_MOUNT_PATH/etc/ -a $TMP_HOOKS_PATH/first-boot.d
       run_in_target mv /etc/rc.local /etc/rc.local.REAL
@@ -152,6 +157,7 @@ exit 0
 EOF
       run_in_target chmod 755 /etc/rc.local
     fi
+    check_break after-first-boot run_in_target bash
 }
 
 function finalise_base () {