issuegen: split out if_snippets_gen, add path unit

Split out `if_snippets_gen` from the `issuegen` script. This makes
the udev rule `90-console-login-helper-messages-if_snippets_gen.rules`
responsible only for writing snippets with information about network
interfaces to the runtime issue snippets directory
(`/run/console-login-helper-messages/issue.d`) read by `issuegen`
to generate the final `/run/console-login-helper-messages/40_console-login-helper-messages.issue`
file read by the serial terminal (agetty).

This is beneficial as it helps avoid situations where multiple
invocations of `issuegen` are executed concurrently (leading to
bugs like coreos#35). This is also a first step towards making the shown
information more configurable (coreos#15).

Preparation also for coreos#7, reducing the number of places in the code
that `issuegen` is called directly, reducing potential downstream
patching needed to pass in a `--legacy` option to the script
(coreos#7 (comment)).

Add `console-login-helper-messages.path`, so that `issuegen` is run
whenever snippets are written to `/run/console-login-helper-messages/issue.d`.
This should be the main way of triggering a run of `issuegen` (first
trigger the path, which starts the service, which will run
`issuegen`). The only other time the `issuegen` service is invoked
is on boot (if the unit is enabled). Note, enablement of the path
unit is required upon installation.

usr/lib/systemd/system/console-login-helper-messages-issuegen.path

@@ -0,0 +1,9 @@
+[Unit]
+Description=Monitor console-login-helper-messages runtime issue snippets directory for changes
+
+[Path]
+PathChanged=/run/console-login-helper-messages/issue.d
+Unit=console-login-helper-messages-issuegen.service
+
+[Install]
+WantedBy=multi-user.target

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

@@ -1,12 +1,32 @@
 [Unit]
-Description=Generate /run/issue.d/console-login-helper-messages.issue
+Description=Generate console-login-helper-messages issue snippet
 Before=systemd-user-sessions.service
 Wants=network-online.target
 After=network-online.target sshd-keygen.target
 
+# The issue file generated by `issuegen` shown at the serial console should be
+# kept up to date, no matter the number of successive start requests within a
+# given time interval. However, it is unlikely that the configured defaults on
+# a system for start request rate limiting of this unit (through
+# StartLimitIntervalSec= and StartLimitBurst=) would be exceeded. For example,
+# typical scenarios causing this service to be started would be through the
+# udev rule `90-console-login-helper-messages-if_snippets_gen`, which are
+# likely to involve low numbers of start requests (one for every time a network
+# device is connected or disconnected). This unit therefore does not set any
+# rate-limiting parameters. However, if this unit is often failing with
+# `start-limit-hit`, try disabling rate limiting for this unit by uncommenting
+# the following:
+#StartLimitIntervalSec=0
+
 [Service]
 Type=oneshot
-RemainAfterExit=yes
+# This service expects to be started repeatedly after exiting (whenever a new
+# runtime issue snippet is dropped into the directory watched by
+# `console-login-helper-messages-issuegen.path`). `RemainAfterExit` is thus
+# explicitly set to `no`, so that the service can re-execute (otherwise,
+# `start` on this service after it has exited once before does not have any
+# effect).
+RemainAfterExit=no
 ExecStart=/usr/libexec/console-login-helper-messages/issuegen
 
 [Install]

usr/lib/udev/rules.d/90-console-login-helper-messages-if_snippets_gen.rules

@@ -0,0 +1,2 @@
+ACTION=="add", SUBSYSTEM=="net", ENV{INTERFACE}=="e*", RUN+="/usr/libexec/console-login-helper-messages/if_snippets_gen add $env{INTERFACE}"
+ACTION=="remove", SUBSYSTEM=="net", ENV{INTERFACE}=="e*", RUN+="/usr/libexec/console-login-helper-messages/if_snippets_gen remove $env{INTERFACE}"

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

@@ -0,0 +1,24 @@
+#!/bin/bash
+
+# Generate snippets for network devices, invoked by a udev rule.
+
+# Copyright (c) 2019 Fedora CoreOS Authors. All rights reserved.
+# Copyright (c) 2013 The CoreOS Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+# Modified from the CoreOS repository:
+#  * https://github.com/coreos/init/blob/master/scripts/issuegen
+
+set -e
+
+. "$(dirname ${0})/issue_defs"
+
+# Add/remove data from udev rules.
+case "$1" in
+	add)
+		echo "${2}: \\4{${2}} \\6{${2}}" > "${RUN_SNIPPETS}/22_${2}"
+		;;
+	remove)
+		rm -f "${RUN_SNIPPETS}/22_${2}"
+		;;
+esac

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

@@ -0,0 +1,12 @@
+#!/bin/bash
+
+PKG_NAME=console-login-helper-messages
+ISSUE_SNIPPETS_PATH=${PKG_NAME}/issue.d
+ETC_SNIPPETS="/etc/${ISSUE_SNIPPETS_PATH}"
+RUN_SNIPPETS="/run/${ISSUE_SNIPPETS_PATH}"
+USR_LIB_SNIPPETS="/usr/lib/${ISSUE_SNIPPETS_PATH}"
+
+
+# Make sure the `${RUN_SNIPPETS}` directory is created upfront,
+# as it may be written to by callers of this script.
+mkdir -p "${RUN_SNIPPETS}"

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

@@ -1,7 +1,7 @@
 #!/bin/bash
 
-# Get updated system information, including SSH keys and network devices,
-# and generate an issue to display before login.
+# Get updated system information, including SSH keys, and generate an
+# issue to display before login.
 
 # Copyright (c) 2019 Fedora CoreOS Authors. All rights reserved.
 # Copyright (c) 2013 The CoreOS Authors. All rights reserved.
@@ -13,15 +13,7 @@
 
 set -e
 
-PKG_NAME=console-login-helper-messages
-ISSUE_SNIPPETS_PATH=${PKG_NAME}/issue.d
-ETC_SNIPPETS="/etc/${ISSUE_SNIPPETS_PATH}"
-RUN_SNIPPETS="/run/${ISSUE_SNIPPETS_PATH}"
-USR_LIB_SNIPPETS="/usr/lib/${ISSUE_SNIPPETS_PATH}"
-
-# Parts of this script write to the `${RUN_SNIPPETS}` directory,
-# make sure it is created upfront.
-mkdir -p "${RUN_SNIPPETS}"
+. "$(dirname ${0})/issue_defs"
 
 
 # Provide key fingerprints via issue.
@@ -33,17 +25,6 @@ for KEY_FILE in $(find "${SSH_DIR}" -name 'ssh_host_*_key') ; do
 done | awk '{print "SSH host key: " $2 " " $4}' > "${RUN_SNIPPETS}/21_ssh_host_keys.issue"
 
 
-# Add/remove data from udev rules.
-case "$1" in
-	add)
-		echo "${2}: \\4{${2}} \\6{${2}}" > "${RUN_SNIPPETS}/22_${2}.issue"
-		;;
-	remove)
-		rm -f "${RUN_SNIPPETS}/22_${2}.issue"
-		;;
-esac
-
-
 # Generate a final issue message from compiling the snippets.
 # Pick 40 as a prefix as other files can order around it easily.
 generated="/run/${PKG_NAME}/40_${PKG_NAME}.issue"