TT#37401 Add safety measures, create tmp files before moving to output file

In this way, if the file fails to be generated (e.g. because of a broken .yml
file) the old file is preserved intact.

It doesn't protect from all failures (in particular, it is ineffective against
wrong content due to programming errors), but it's safer than having an empty
file or truncated as a result of the parsing/generation being stopped due to
incorrect files or external events.

This is added in the context of creating new /etc/hosts and
/etc/network/interfaces files because we found the problem at this point, and
having those files empty completely breaks the system.

Change-Id: I01e8ef9d6971506aa3f8d9da174ce890ea83393c

helper/build_config

@@ -76,9 +76,16 @@ umask 0077
 
 TT_WRAPPER="${HELPER}/tt2-wrapper"
 
+# generate in a temporary location, then move if successful
+tt_tmp_output_file_base="$(basename "${output_file}")"
+tt_tmp_output_file="$(mktemp ngcpcfg."${tt_tmp_output_file_base}".XXXXXXXXXX)"
+
 log_debug "Output file ${output_file} based on ${input_file}"
-log_debug "Executing: $TT_WRAPPER ${input_file} > ${output_file}"
-if "$TT_WRAPPER" "${input_file}" > "${output_file}" 2>/dev/null ; then
+log_debug "Executing: $TT_WRAPPER ${input_file} > ${tt_tmp_output_file}"
+log_debug "      and: cat ${tt_tmp_output_file} > ${output_file}"
+if "$TT_WRAPPER" "${input_file}" > "${tt_tmp_output_file}" 2>/dev/null &&
+   cat "${tt_tmp_output_file}" > "${output_file}" 2>/dev/null ; then
+  rm -f "${tt_tmp_output_file}"
   log_info "Generating ${output_file}: OK"
   RC=0
 else
@@ -89,8 +96,9 @@ else
   for f in "${input_file}" ${host_conf:-} ${local_conf:-} "$NGCPCTL_CONFIG" "${NETWORK_CONFIG:-}" ${EXTRA_CONFIG_FILES:-} "$CONSTANTS_CONFIG" ; do
     [ -r "$f" ] && log_info "$f"
   done
-  log_info "Running /usr/share/ngcp-ngcpcfg/helper/tt2-wrapper <file> should provide more details."
-
+  log_info "Running /usr/share/ngcp-ngcpcfg/helper/tt2-wrapper <file>"
+  log_info "or inspecting temporary ${tt_tmp_output_file}"
+  log_info "should provide more details."
 fi
 
 if [ -L "$output_file" ] ; then