Add script to send mail in case btrfs issues were detected

This can be very useful for smaller setups where the admin still would
like to receive an email in case a disk in a btrfs RAID array fails.

Partially resolves #88

btrfs-errmail.service

@@ -0,0 +1,7 @@
+[Unit]
+Description=Check for btrfs issues and send an email if any were found
+Documentation=man:btrfs
+
+[Service]
+Type=simple
+ExecStart=/usr/share/btrfsmaintenance/btrfs-errmail.sh

btrfs-errmail.sh

@@ -0,0 +1,53 @@
+#!/bin/bash
+#
+# Copyright (c) 2022 Matthias Klumpp <matthias@tenstral.net>
+
+umask 022
+PATH=/sbin:/bin:/usr/sbin:/usr/bin
+export PATH
+
+if [ -f /etc/sysconfig/btrfsmaintenance ] ; then
+    . /etc/sysconfig/btrfsmaintenance
+fi
+
+if [ -f /etc/default/btrfsmaintenance ] ; then
+    . /etc/default/btrfsmaintenance
+fi
+
+. $(dirname $(realpath "$0"))/btrfsmaintenance-functions
+
+if [ -z "$BTRFS_MAILADDR" ]
+then
+      # no email set, nothing to do for us
+      exit 0
+fi
+
+BTRFS_STATS_MOUNTPOINTS=$(expand_auto_mountpoint "auto")
+OIFS="$IFS"
+IFS=:
+for MM in $BTRFS_STATS_MOUNTPOINTS; do
+	if ! is_btrfs "$MM"; then
+		echo "Path $MM is not btrfs, skipping"
+		continue
+	fi
+	devstats=$(btrfs device stats --check $MM 2>&1)
+	if [ $? -ne 0 ]; then
+		mail_body="$(sendmail -t <<EOF
+To: $BTRFS_MAILADDR
+Subject: Btrfs device issue on $MM @ $HOSTNAME
+
+This is an automatically generated mail message from btrfs-errmail
+running on $HOSTNAME
+
+An issue has been detected on the btrfs device mounted as $MM.
+
+Faithfully yours, etc.
+
+P.S. The 'btrfs device stats' output is:
+$devstats
+EOF
+)"
+        fi
+done
+
+exit 0

btrfs-errmail.timer

@@ -0,0 +1,10 @@
+[Unit]
+Description=Check for btrfs issues and send mail if any were found
+Documentation=man:btrfs
+
+[Timer]
+OnCalendar=hourly
+Persistent=true
+
+[Install]
+WantedBy=timers.target

sysconfig.btrfsmaintenance

@@ -152,3 +152,13 @@ BTRFS_TRIM_MOUNTPOINTS="/"
 # the timer for these tasks(s) elapsed while the system was suspended
 # or powered off.
 BTRFS_ALLOW_CONCURRENCY="false"
+
+## Path:	System/File systems/btrfs
+## Description:	Mail address to send issue reports to
+## Type: 	string
+## Default:	""
+#
+# If this is set to an email address or an username like "root", btrfs device stats
+# data will be checked every hour and an email will be sent to the given address
+# if any issues(like data corruption or read issues) were found.
+BTRFS_MAILADDR=""