motdgen: do not share a staged file, use mktemp and mv

With the `staged` file shared, there would be potential for two or
more processes executing `motdgen` to write to it resulting in
corrupted output, or the error in the `cat` command due to missing
file reported in coreos#35 (comment). Currently, this is not a problem with
motdgen, but could be if `motdgen` were invoked by something like the
udev rules that invoke `issuegen`. A bug reported in `issuegen` for
this reason is: coreos#35

Instead, write the intermediate output to a unique tempfile and mv the
tempfile to the final output location. If the final output is on the
same filesystem as the tempfile, this operation should be atomic. This
ensures only valid output is written to the issue file shown to the
terminal.

Additionally, perform code tidyups similar to those done for
`issuegen` in coreos#40.

usr/lib/console-login-helper-messages/libutil

@@ -0,0 +1,23 @@
+#!/usr/bin/bash
+
+# Collection of util functions and common definitions for
+# console-login-helper-messages scripts.
+
+PKG_NAME=console-login-helper-messages
+TEMPFILE_TEMPLATE=$PKG_NAME.XXXXXXXXXX.tmp
+# Use same filesystem, under /run, as where snippets are generated,
+# so that rename operations through `mv` are atomic.
+TEMPFILE_DIR=/run/$PKG_NAME
+
+# Write stdin to a tempfile, and rename the tempfile to the path
+# given as an argument. When called from multiple processes on the
+# same generated file path, this avoids interleaving writes to the
+# generated file by using `mv` to overwrite the file.
+write_via_tempfile() {
+    generated_file=$1
+    staged_file=$(mktemp --tmpdir=$TEMPFILE_DIR $TEMPFILE_TEMPLATE)
+    cat > "${staged_file}"
+    # Atomically rename staged to generated file (see TEMPFILE_DIR
+    # above).
+    mv "$staged_file" "$generated_file"
+}

usr/lib/systemd/system/console-login-helper-messages-motdgen.service

@@ -1,5 +1,5 @@
 [Unit]
-Description=Generate /run/motd.d/console-login-helper-messages.motd
+Description=Generate console-login-helper-messages motd snippet
 Documentation=https://github.com/coreos/console-login-helper-messages
 Before=systemd-user-sessions.service
 

usr/libexec/console-login-helper-messages/motdgen

@@ -9,35 +9,32 @@
 # Get updated system information and generate a motd
 # to display this at login.
 
+. /usr/lib/console-login-helper-messages/libutil
+
 set -e
 
 PKG_NAME=console-login-helper-messages
-MOTD_DIR_PUBLIC=motd.d
-# Should only be read by this script.
-MOTD_DIR_PRIVATE="${PKG_NAME}/motd.d"
+MOTD_SNIPPETS_PATH="${PKG_NAME}/motd.d"
+ETC_SNIPPETS="/etc/${MOTD_SNIPPETS_PATH}"
+RUN_SNIPPETS="/run/${MOTD_SNIPPETS_PATH}"
+USR_LIB_SNIPPETS="/usr/lib/${MOTD_SNIPPETS_PATH}"
 
-staged="/run/${PKG_NAME}/40_${PKG_NAME}.motd.staged"
-# Pick 40 as an index as other files can order around it easily.
-generated="/run/motd.d/40_${PKG_NAME}.motd"
+# Parts of this script write to the `${RUN_SNIPPETS}` directory,
+# make sure it is created upfront.
+mkdir -p "${RUN_SNIPPETS}"
 
-mkdir -p "/run/${MOTD_DIR_PRIVATE}"
-mkdir -p "/run/${MOTD_DIR_PUBLIC}"
-rm -f "${generated}"
 
+# Write distro release information to MOTD, in distro-specific color.
 source /usr/lib/os-release
-echo -e "\e[${ANSI_COLOR}m${PRETTY_NAME}\e[39m" > "/run/${MOTD_DIR_PRIVATE}/21_os_release.motd"
-
-# Generate a motd from files found in the private (package-specific) directories,
-# and place the motd in a public directory.
-if [[ -d "/etc/${MOTD_DIR_PRIVATE}" ]]; then
-	cat /etc/${MOTD_DIR_PRIVATE}/* 2>/dev/null >> "${staged}" || true
-fi
-if [[ -d /run/"${MOTD_DIR_PRIVATE}" ]]; then
-	cat /run/${MOTD_DIR_PRIVATE}/* 2>/dev/null >> "${staged}" || true
-fi
-if [[ -d /usr/lib/"${MOTD_DIR_PRIVATE}" ]]; then
-	cat /usr/lib/${MOTD_DIR_PRIVATE}/* 2>/dev/null >> "${staged}" || true
-fi
-
-cat "${staged}" > "${generated}"
-rm -rf "${staged}"
+OS_RELEASE_OUTDIR="${RUN_SNIPPETS}"
+echo -e "\e[${ANSI_COLOR}m${PRETTY_NAME}\e[39m" | write_via_tempfile "${OS_RELEASE_OUTDIR}/21_os_release.motd"
+
+
+# Generate a final motd from compiling the snippets.
+# Pick 40 as a prefix as other files can order around it easily.
+generated_file="/run/motd.d/40_${PKG_NAME}.motd"
+staged_file=$(mktemp --tmpdir=$TEMPFILE_DIR $TEMPFILE_TEMPLATE)
+cat $ETC_SNIPPETS/*.motd 2>/dev/null >> "$staged_file" || :
+cat $RUN_SNIPPETS/*.motd 2>/dev/null >> "$staged_file" || :
+cat $USR_LIB_SNIPPETS/*.motd 2>/dev/null >> "$staged_file" || :
+mv "$staged_file" "$generated_file"