Add lock capabilities during snapshot (#2063)

* Add service which locks database tables

Add an additional service which helds a lock on all database tables.
This allows to safely snapshot the database files.

* Lock database during snapshot operation

Fixes: #1353

* Update config.json

* Update CHANGELOG.md

Co-authored-by: Pascal Vizeli <pvizeli@syshack.ch>

mariadb/rootfs/etc/services.d/mariadb-lock/notification-fd

@@ -0,0 +1 @@
+3

mariadb/rootfs/etc/services.d/mariadb-lock/run

@@ -0,0 +1,28 @@
+#!/usr/bin/with-contenv bashio
+# ==============================================================================
+# Start MariaDB client to lock tables (for snapshots)
+# ==============================================================================
+
+bashio::log.info "Lock tables using mariadb client..."
+
+exec 4> >(mariadb)
+MARIADB_PID=$!
+
+echo "FLUSH TABLES WITH READ LOCK;" >&4
+
+# Notify s6 about successful service start
+echo "" >&3
+
+bashio::log.info "MariaDB tables locked."
+
+# Register stop
+function stop_mariadb_client() {
+    echo "UNLOCK TABLES;" >&4
+    bashio::log.info "MariaDB tables unlocked."
+    exec 4>&-
+    # Successful exit, avoid wait exit status to propagate
+    exit 0
+}
+trap "stop_mariadb_client" SIGTERM SIGHUP
+
+wait "${MARIADB_PID}"