add fingerprint auto-shutoff on lid close to docs

arch.md

@@ -192,10 +192,101 @@ sudo ufw enable
 
     ```conf
     # fingerprint auth
-    auth       sufficient                  pam_unix.so try_first_pass likeauth nullok
     auth      sufficient pam_fprintd.so
     ```
 
+    sudo vim /etc/pam.d/system-auth and at the top of the file:
+
+    ```conf
+    # fingerprint auth
+    auth      sufficient pam_fprintd.so
+    ```
+
+**Turn Off Fingerprint When Laptop Lid Closed**
+
+To disable fingerprint authentication when the laptop lid is closed, and
+re-enable when it is reopened, we will use acpid to bind to the button/lid.*
+event to a custom script that will stop and mask the fprintd service on lid
+close, and unmask and start the fprintd service on lid open.
+
+We also check that the HDMI cable is connected by testing the contents of
+/sys/class/drm/card0-HDMI-A-1/status.
+
+Follow the steps below:
+
+1. `pacman -S acpid` and then `systemctl enable --now acpid`
+1. Create a .locks file in your home dir: `mkdir ~/.locks`
+2. Create file /etc/acpi/laptop-lid.sh with the following contents:
+
+    ```bash
+    #!/bin/bash
+
+    lock=/home/ducoterra/.locks/fprint-disabled.lock
+
+    if grep -Fq closed /proc/acpi/button/lid/LID0/state # &&
+        # This is used to detect if a display is connected.
+        # For USB C displayport use: 
+        # grep -Fxq connected /sys/class/drm/card1-DP-2/status
+        # For hdmi use:
+        # grep -Fxq connected /sys/class/drm/card0-HDMI-A-1/status
+    then
+        touch "$lock"
+        systemctl stop fprintd
+        systemctl mask fprintd
+    elif [ -f "$lock" ]
+    then
+        systemctl unmask fprintd
+        systemctl start fprintd
+        rm -f "$lock"
+    fi
+    ```
+
+3. Make the file executable with
+
+    `chmod +x /etc/acpi/laptop-lid.sh`
+
+4. Create file /etc/acpi/events/laptop-lid with the following contents:
+
+    ```bash
+    event=button/lid.*
+    action=/etc/acpi/laptop-lid.sh
+    ```
+
+5. Restart the acpid service with:
+
+    `systemctl restart acpid`
+
+Now the fingerprint will be used only when the lid is open.
+
+In order to restore the correct state of the fprintd service if you
+disconnect/reconnect while the laptop is off, you may call the above script
+from a systemd init file. The steps to do this are the following:
+
+1. Create a file named /etc/systemd/system/laptop-lid.service with the following contents:
+
+    ```bash
+    [Unit]
+    Description=Laptop Lid
+    After=suspend.target
+
+    [Service]
+    ExecStart=/etc/acpi/laptop-lid.sh
+
+    [Install]
+    WantedBy=multi-user.target
+    WantedBy=suspend.target
+    ```
+
+2. Reload the systemd config files with
+
+    `sudo systemctl daemon-reload`
+
+3. Start and enable the service with
+
+    `sudo systemctl enable --now laptop-lid.service`
+
+Now the status should be correct even after connecting/disconnecting when the computer is off.
+
 ### AppImage Support
 
 fuse is required to run most appimages.