mysqlBackup service: let it work with default settings

* Grants enough privileges to the configured user so that it can run
  mysqldump.

* Adds a nixos test.

* Use systemd timers instead of a cronjob (by @fadenb).

* Creates a new user for backups by default, instead of using mysql
  user.

* Ensures that backup user has write permissions on backup location.

* Write backup to a temporary file before renaming so that a failed
  backup won't overwrite the previous backup, and so that the backup
  location will never contain a partial backup.

Breaking changes:

 * Renamed period to calendar to reflect the change in how to
   configure the backup time.

 * A failed backup will no longer result in cron sending an e-mail --
   users' monitoring systems must be updated.

Resolves #24728

2 files changed, 43 insertions, 0 deletions

diff --git a/nixos/tests/mysql-backup.nix b/nixos/tests/mysql-backup.nix
new file mode 100644
index 000000000000..f5bcc460cba7
--- /dev/null
+++ b/nixos/tests/mysql-backup.nix
@@ -0,0 +1,42 @@
+# Test whether mysqlBackup option works
+import ./make-test.nix ({ pkgs, ... } : {
+  name = "mysql-backup";
+  meta = with pkgs.stdenv.lib.maintainers; {
+    maintainers = [ rvl ];
+  };
+
+  nodes = {
+    master = { config, pkgs, ... }: {
+      services.mysql = {
+        enable = true;
+        initialDatabases = [ { name = "testdb"; schema = ./testdb.sql; } ];
+        package = pkgs.mysql;
+      };
+
+      services.mysqlBackup = {
+        enable = true;
+        databases = [ "doesnotexist" "testdb" ];
+      };
+    };
+  };
+
+  testScript =
+    '' startAll;
+
+       # Need to have mysql started so that it can be populated with data.
+       $master->waitForUnit("mysql.service");
+
+       # Wait for testdb to be populated.
+       $master->sleep(10);
+
+       # Do a backup and wait for it to finish.
+       $master->startJob("mysql-backup.service");
+       $master->waitForJob("mysql-backup.service");
+
+       # Check that data appears in backup
+       $master->succeed("${pkgs.gzip}/bin/zcat /var/backup/mysql/testdb.gz | grep hello");
+
+       # Check that a failed backup is logged
+       $master->succeed("journalctl -u mysql-backup.service | grep 'fail.*doesnotexist' > /dev/null");
+    '';
+})
diff --git a/nixos/tests/testdb.sql b/nixos/tests/testdb.sql
index 4fb28fea3df9..3c68c49ae82c 100644
--- a/nixos/tests/testdb.sql
+++ b/nixos/tests/testdb.sql
@@ -8,3 +8,4 @@ insert into tests values (1, 'a');
 insert into tests values (2, 'b');
 insert into tests values (3, 'c');
 insert into tests values (4, 'd');
+insert into tests values (5, 'hello');