r/serterm: Deploy serial terminal multiplexer

Using `tmux`, we can spawn a bunch of `picocom` processes for the serial
ports connected to other server's console ports.  The
_serial-terminal-server_ service manages the `tmux` server process,
while the individual _serial-terminal-server-window@.service_ units
create a window in the `tmux` session.

The serial terminal server runs as a dedicated user.  The SSH server is
configured to force this user to connect to the `tmux` session.  This
should help ensure the serial consoles are accessible, even if the
Active Directory server is unavailable.

roles/serterm/files/add-window.sh

@@ -0,0 +1,6 @@
+#!/bin/sh
+# vim: set sw=4 ts=4 sts=4 et :
+
+port="$1"
+
+exec tmux new-window -a -t 'serial:{end}' -n "${port##*/}" /usr/local/libexec/serterm/connect-serial /dev/"${port}"

roles/serterm/files/connect-serial.sh

@@ -0,0 +1,8 @@
+#!/bin/sh
+# vim: set sw=4 ts=4 sts=4 et :
+
+: "${LOG_DIR:=/var/log/serial}"
+
+port="$1"
+
+exec picocom -b 115200 -g "${LOG_DIR%/}"/"${port##*/}".log "${port}"

roles/serterm/files/remove-window.sh

@@ -0,0 +1,6 @@
+#!/bin/sh
+# vim: set sw=4 ts=4 sts=4 et :
+
+port="$1"
+
+exec tmux kill-window -t "${port##*/}"

roles/serterm/files/serial-terminal-server-window@.service

@@ -0,0 +1,16 @@
+# vim: set ft=systemd :
+[Unit]
+Description=Add serial terminal window for %I
+Requires=serial-terminal-server.service
+After=serial-terminal-server.service
+
+[Service]
+Type=oneshot
+ExecStart=/usr/local/libexec/serterm/add-window %I
+ExecStop=/usr/local/libexec/serterm/remove-window %I
+RemainAfterExit=yes
+User=serterm
+Group=serterm
+
+[Install]
+WantedBy=serial-terminal-server.service

roles/serterm/files/serial-terminal-server.service

@@ -0,0 +1,14 @@
+# vim: set ft=systemd :
+[Unit]
+Description=Serial Terminal
+
+[Service]
+Type=forking
+ExecStart=/usr/local/libexec/serterm/start-server
+User=serterm
+Group=serterm
+Restart=always
+RestartSec=1s
+
+[Install]
+WantedBy=multi-user.target

roles/serterm/files/start-server.sh

@@ -0,0 +1,16 @@
+#!/bin/sh
+# vim: set sw=4 ts=4 sts=4 et :
+
+if [ -z "${TMUX}" ]; then
+    exec tmux new-session -s serial -d "$0"
+fi
+
+tmux \
+    set-option -t serial:0 -w remain-on-exit on \; \
+    rename-window -t serial:0 info
+
+cat <<'EOF'
+This window must remain open to keep the tmux server/session running.
+If all windows are closed, tmux will exit and will not be able to
+receive commands to add new windows for hotplugged devices.
+EOF

roles/serterm/files/start-session.sh

@@ -0,0 +1,18 @@
+#!/bin/sh
+# vim: set sw=4 ts=4 sts=4 et :
+
+if [ -z "${TMUX}" ]; then
+    exec tmux new-session -s serial -d "$0"
+fi
+
+tmux \
+    set-option -t serial:0 -w remain-on-exit on \; \
+    rename-window -t serial:0 info
+
+cat <<'EOF'
+This window must remain open to keep the tmux server/session running.
+If all windows are closed, tmux will exit and will not be able to
+receive commands to add new windows for hotplugged devices.
+
+Use `respawn-window` to restart all serial terminal windows.
+EOF

roles/serterm/files/start-terminal-server.sh

@@ -0,0 +1,18 @@
+#!/bin/sh
+# vim: set sw=4 ts=4 sts=4 et :
+
+if [ -z "${TMUX}" ]; then
+    exec tmux new-session -s serial -d "$0"
+fi
+
+tmux \
+    set-option -t serial:0 -w remain-on-exit on \; \
+    rename-window -t serial:0 info
+
+cat <<'EOF'
+This window must remain open to keep the tmux server/session running.
+If all windows are closed, tmux will exit and will not be able to
+receive commands to add new windows for hotplugged devices.
+
+Use `respawn-window` to restart all serial terminal windows.
+EOF