dracut: add 99emergency-timeout

If Ignition fails, instead of immediately dropping to an emergency
shell, output a message indicating that Ignition failed and point
to ignition-validate. Offer to drop to an emergency shell if the
user presses Enter. If the user doesn't respond in 5 minutes,
reboot to try again. While waiting for the timeout, keep updating
the prompt with the time remaining until reboot.

Brought forward from the `99emergency-timeout` dracut module
present in Container Linux, with minor modifications:
https://github.com/coreos/bootengine/tree/c691205c0046e3828a8536d553aea7307ca3abee/dracut/99emergency-timeout

Closes: #35

dracut/99emergency-timeout/module-setup.sh

@@ -0,0 +1,11 @@
+#!/bin/bash
+# -*- mode: shell-script; indent-tabs-mode: nil; sh-basic-offset: 4; -*-
+# ex: ts=8 sw=4 sts=4 et filetype=sh
+
+install() {
+    inst_multiple \
+        cut \
+        date
+
+    inst_hook emergency 99 "${moddir}/timeout.sh"
+}

dracut/99emergency-timeout/timeout.sh

@@ -0,0 +1,91 @@
+# Before starting the emergency shell, prompt the user to press Enter.
+# If they don't, reboot the system.
+#
+# Assumes /bin/sh is bash.
+
+# _wait_for_journalctl_to_stop will block until either:
+# - no messages have appeared in journalctl for the past 5 seconds
+# - 15 seconds have elapsed
+_wait_for_journalctl_to_stop() {
+    local time_since_last_log=0
+
+    local time_started="$(date '+%s')"
+    local now="$(date '+%s')"
+
+    while [ ${time_since_last_log} -lt 5 -a $((now-time_started)) -lt 15 ]; do
+        sleep 1
+
+        local last_log_timestamp="$(journalctl -e -n 1 -q -o short-unix | cut -d '.' -f 1)"
+        local now="$(date '+%s')"
+
+        local time_since_last_log=$((now-last_log_timestamp))
+    done
+}
+
+_prompt_for_timeout() {
+    local timeout=300
+    local interval=15
+
+    if [[ -e /.emergency-shell-confirmed ]]; then
+        return
+    fi
+    ignition_units="ignition-disks.service ignition-files.service"
+    if systemctl show $ignition_units | grep -q "^ActiveState=failed$"; then
+        # Ignition has failed, suppress kernel logs so that Ignition logs stay
+        # on the screen
+        dmesg --console-off
+
+        # There's a couple straggler systemd messages. Wait until it's been 5
+        # seconds since something was written to the journal.
+        _wait_for_journalctl_to_stop
+
+        # Print Ignition logs
+        cat <<EOF
+-------------------------------------------------------------------------------
+Ignition has failed. Please ensure your config is valid. Note that only Ignition spec
+v2.x.x configs are accepted.
+A CLI validation tool to check this called ignition-validate can be downloaded from GitHub:
+    https://github.com/coreos/ignition/releases
+Note that the v0.x Ignition releases have the correct validator for config spec v2.x.x.
+Here are the Ignition logs:
+EOF
+        journalctl -t ignition --no-pager --no-hostname -o cat
+    fi
+
+    # Regularly prompt with time remaining.  This ensures the prompt doesn't
+    # get lost among kernel and systemd messages, and makes it clear what's
+    # going on if the user just connected a serial console.
+    while [[ $timeout > 0 ]]; do
+        local m=$(( $timeout / 60 ))
+        local s=$(( $timeout % 60 ))
+        local m_label="minutes"
+        if [[ $m = 1 ]]; then
+            m_label="minute"
+        fi
+
+        if [[ $s != 0 ]]; then
+            echo -n -e "Press Enter for emergency shell or wait $m $m_label $s seconds for reboot.      \r"
+        else
+            echo -n -e "Press Enter for emergency shell or wait $m $m_label for reboot.                 \r"
+        fi
+
+        local anything
+        if read -t $interval anything; then
+            > /.emergency-shell-confirmed
+            return
+        fi
+        timeout=$(( $timeout - $interval ))
+    done
+
+    echo -e "\nRebooting."
+    # This is not very nice, but since reboot.target likely conflicts with
+    # the existing goal target wrt the desired state of shutdown.target,
+    # there doesn't seem to be a better option.
+    systemctl reboot --force
+    exit 0
+}
+
+# If we're invoked from a dracut breakpoint rather than
+# dracut-emergency.service, we won't have a controlling terminal and stdio
+# won't be connected to it. Explicitly read/write /dev/console.
+_prompt_for_timeout < /dev/console > /dev/console