Add rd.kiwi.install.retain_past deployment option

As a less strict method to retain_last, now also the retain_past
deployment option exists. In the retain_past mode the partition
to preserve must not be part of the initial image. It can retain
the content of any last partition if its start address is past
the OS image. However, a gap in the partition table between the
OS end and the start of the retained partition can occur for
which this commit does not provide a resize method yet.

Author: schaefi

dracut/modules.d/55kiwi-dump/kiwi-dump-image.sh

@@ -346,8 +346,15 @@ function dump_image {
         dump=dump_local_image
     fi
 
+    # preserve pre-dump partition table
+    if getargbool 0 rd.kiwi.install.retain_past; then
+        fetch_local_partition_table "${image_target}" /tmp/parttable_pre
+    fi
+
     # setup blocks and blocksize to retain last
-    if getargbool 0 rd.kiwi.install.retain_last; then
+    if getargbool 0 rd.kiwi.install.retain_last || \
+       getargbool 0 rd.kiwi.install.retain_past
+    then
         if [ -n "${image_from_remote}" ];then
             image_size=$((blocks * blocksize))
             parttable=$(
@@ -409,15 +416,49 @@ function dump_image {
             report_and_quit "Failed to install image"
         fi
     fi
+
+    # recreate last partition from pre-dump table
+    if getargbool 0 rd.kiwi.install.retain_past; then
+        recreate_last_partition "${image_target}"
+    fi
 }
 
 function fetch_local_partition_table {
     local image_source=$1
-    local parttable=/tmp/parttable
+    local parttable=$2
+    if [ -z "${parttable}" ];then
+        parttable=/tmp/parttable
+    fi
     sfdisk -d "${image_source}" > "${parttable}" 2>/dev/null
     echo "${parttable}"
 }
 
+function recreate_last_partition {
+    # recreate last partition as it existed prior dump
+    local image_target=$1
+    local table_type
+    table_type=$(get_partition_table_type "${image_target}")
+    if [ "${table_type}" = "gpt" ];then
+        relocate_gpt_at_end_of_disk "${image_target}"
+    fi
+    # parttable after image dump, the OS
+    sfdisk -d "${image_target}" > /tmp/parttable_1 2>/dev/null
+    # last partition of table as it existed prior dump
+    tail -n 1 /tmp/parttable_pre > /tmp/parttable_2
+    # combine table snippets. Please note there can now be a gap
+    # between the end of the OS table and the start of the last
+    # partition of the table prior dump. Please also note, if the
+    # table prior dump had more partitions than only the last
+    # one to retain, this table concat will not retain those.
+    # and the OS dump might have overwritten them. A more
+    # sophisticated concat procedure would be needed to support
+    # this case here in this code and also in the
+    # compatible_to_retain() method
+    cat /tmp/parttable_1 /tmp/parttable_2 > /tmp/parttable
+    set_device_lock "${image_target}" \
+        sfdisk -f "${image_target}" < /tmp/parttable
+}
+
 function fetch_remote_partition_table {
     local image_source=$1
     local image_size=$2
@@ -452,8 +493,14 @@ function compatible_to_retain {
         touch /tmp/retain_not_applicable
         return 1
     fi
-    if [ ! "${source_start}" = "${target_start}" ];then
-        report_and_quit "Cannot retain partition, start address mismatch"
+    if getargbool 0 rd.kiwi.install.retain_last; then
+        if [ ! "${source_start}" = "${target_start}" ];then
+            report_and_quit "Cannot retain partition, start address mismatch"
+        fi
+    elif getargbool 0 rd.kiwi.install.retain_past; then
+        if [ "${source_start}" -gt "${target_start}" ];then
+            report_and_quit "Cannot retain partition, image overlaps"
+        fi
     fi
     return 0
 }
@@ -550,7 +597,9 @@ function check_image_integrity {
         # no verification wanted
         return
     fi
-    if getargbool 0 rd.kiwi.install.retain_last; then
+    if getargbool 0 rd.kiwi.install.retain_last || \
+       getargbool 0 rd.kiwi.install.retain_past
+    then
         if [ ! -e /tmp/retain_not_applicable ];then
             # no verification possible as only a portion of
             # the image got deployed intentionally