nixos: tarsnap - make systemd timer persistent

A machine may not always be active (or online!) when a backup timer
triggers, meaning backups can be missed - now we properly set the
tarsnap timer's Persistent option so systemd will run the command even
when the machine wasn't online at that exact time.

However, we also need to make sure that we can contact the tarsnap
server reliably before we start the backup. So, we attempt to ping the
access endpoint in a loop with a sleep, before continuing.

This fixes #8823.

Signed-off-by: Austin Seipp <[e-mail]>

nixos/modules/services/backup/tarsnap.nix

···
 
     systemd.services."tarsnap@" = {
       description = "Tarsnap archive '%i'";
-      requires    = [ "network.target" ];
+      requires    = [ "network-online.target" ];
+      after       = [ "network-online.target" ];
 
-      path = [ pkgs.tarsnap pkgs.coreutils ];
+      path = [ pkgs.iputils pkgs.tarsnap pkgs.coreutils ];
+
+      # In order for the persistent tarsnap timer to work reliably, we have to
+      # make sure that the tarsnap server is reachable after systemd starts up
+      # the service - therefore we sleep in a loop until we can ping the
+      # endpoint.
+      preStart = "while ! ping -q -c 1 betatest-server.tarsnap.com &> /dev/null; do sleep 3; done";
       scriptArgs = "%i";
       script = ''
         mkdir -p -m 0755 ${dirOf cfg.cachedir}
···
         IOSchedulingClass = "idle";
         NoNewPrivileges = "true";
         CapabilityBoundingSet = "CAP_DAC_READ_SEARCH";
+        PermissionsStartOnly = "true";
       };
     };
 
+    # Note: the timer must be Persistent=true, so that systemd will start it even
+    # if e.g. your laptop was asleep while the latest interval occurred.
     systemd.timers = mapAttrs' (name: cfg: nameValuePair "tarsnap@${name}"
       { timerConfig.OnCalendar = cfg.period;
+        timerConfig.Persistent = "true";
         wantedBy = [ "timers.target" ];
       }) cfg.archives;