fix(d/postinst): retain handle_preseed functions for MAAS provision

Commit cf13fd1 removed support for MAAS preseed functions from
debian/cloud-init.postinst which is the only operation which
allows MAAS to send preseed config to cloud-init.

Fixes canonicalGH-5685

debian/cloud-init.postinst

@@ -51,6 +51,80 @@ with open(fname, "w") as fp:
     fp.write(yaml.dump(cfg))' "$@"
 }
 
+handle_preseed_maas() {
+   local cfg_file="/etc/cloud/cloud.cfg.d/90_dpkg_maas.cfg"
+   local md_url="" creds_all="" c_key="" t_key="" t_sec="" c_sec="";
+
+   db_get "cloud-init/maas-metadata-url" && md_url="$RET" || :
+   db_get "cloud-init/maas-metadata-credentials" && creds_all="$RET" || :
+
+   # nothing to do
+   [ -n "$md_url" -o -n "$creds_all" ] || return 0
+
+   # change a url query string format into : delimited
+   if [ -n "$creds_all" -a "${creds_all#*&}" != "${creds_all}" ]; then
+      # the command here ends up looking like:
+      # python3 -c '...' 'oauth_consumer_key=v1&oauth_token_key=v2...' \
+      #   oauth_consumer_key oauth_token_key oauth_token_secret
+      creds_all=$(python3 -c 'from six.moves.urllib.parse import parse_qs;
+import sys;
+keys = parse_qs(sys.argv[1])
+for k in sys.argv[2:]:
+   sys.stdout.write("%s:" % keys.get(k,[""])[0])' "$creds_all" \
+   oauth_consumer_key oauth_token_key oauth_token_secret
+)
+   fi
+
+   # now, if non-empty creds_all is: consumer_key:token_key:token_secret
+   if [ -n "$creds_all" ]; then
+      OIFS="$IFS"; IFS=:; set -- $creds_all; IFS="$OIFS"
+      c_key=$1; t_key=$2; t_sec=$3
+   fi
+
+   if [ "$md_url" = "_" -a "${c_key}:${t_key}:${t_sec}" = "_:_:_" ]; then
+      # if all these values were '_', the delete value, just delete the file.
+      rm -f "$cfg_file"
+   else
+      local header="# written by cloud-init debian package per preseed entries
+# cloud-init/{maas-metadata-url,/maas-metadata-credentials}"
+
+      local pair="" k="" v="" pload="" orig_umask=""
+      for pair in "metadata_url:$md_url" "consumer_key:${c_key}" \
+         "token_key:${t_key}" "token_secret:$t_sec"; do
+         k=${pair%%:*}
+         v=${pair#${k}:}
+         [ -n "$v" ] && pload="${pload} $k: \"$v\","
+      done
+
+      # '_' would indicate "delete", otherwise, existing entries are left
+      orig_umask=$(umask)
+      umask 066
+      : >> "$cfg_file" && chmod 600 "$cfg_file"
+      update_cfg "$cfg_file" "$header" "datasource: { MAAS: { ${pload%,} } }" _
+      umask ${orig_umask}
+   fi
+
+   # now clear the database of the values, as they've been consumed
+   db_unregister "cloud-init/maas-metadata-url" || :
+   db_unregister "cloud-init/maas-metadata-credentials" || :
+}
+
+handle_preseed_local_cloud_config() {
+   local ccfg="" debconf_name="cloud-init/local-cloud-config"
+   local cfg_file="/etc/cloud/cloud.cfg.d/90_dpkg_local_cloud_config.cfg"
+   local header="# written by cloud-init debian package per preseed entry
+# $debconf_name"
+
+   db_get "${debconf_name}" && ccfg="$RET" || :
+
+   if [ "$ccfg" = "_" ]; then
+      rm -f "$cfg_file"
+   elif [ -n "$ccfg" ]; then
+      { echo "$header"; echo "$ccfg"; } > "$cfg_file"
+   fi
+   db_unregister "${debconf_name}" || :
+}
+
 fix_1336855() {
   ### Begin fix for LP: 1336855
   # fix issue where cloud-init misidentifies the location of grub and
@@ -191,6 +265,12 @@ datasource_list: [ $values ]
 EOF
    fi
 
+   # if there are maas settings pre-seeded apply them
+   handle_preseed_maas
+
+   # if there is generic cloud-config preseed, apply them
+   handle_preseed_local_cloud_config
+
    # fix issue where cloud-init misidentifies the location of grub
    fix_1336855