[FC-40738] systemd: init ScalableService component

src/batou_ext/resources/scalable-service.nix

@@ -0,0 +1,18 @@
+{ lib, ... }:
+
+let
+  numRunningInstances = lib.toInt "{{ component.running_instances }}";
+  baseName = "{{ component.base_name }}";
+in {
+  systemd.targets."${baseName}" = {
+    wantedBy = [ "multi-user.target" ];
+    # If this target is reached (i.e. started), also start
+    # {{ component.running_instances }} messenger services.
+    wants = lib.genList (n: "${baseName}@${toString n}.service") numRunningInstances;
+  };
+
+  systemd.services."${baseName}@" = {
+    # If the target stops or gets restarted, also stop/restart this unit.
+    partOf = [ "${baseName}.target" ];
+  };
+}

src/batou_ext/systemd.py

@@ -0,0 +1,83 @@
+import pkg_resources
+from batou.component import Attribute, Component
+from batou.lib.file import File
+
+
+class ScalableService(Component):
+    """
+    Configure a systemd unit that can have multiple equal instances, such
+    as multiple consumers for a message queue.
+
+    Usage::
+
+        self += ScalableService("message-consumer", running_instances=5)
+        self.unit_identifier = self._.unit_identifier
+        self += File("/etc/local/nixos/message-consumer-settings.nix")
+        self += Rebuild()
+
+    With a `message-consumer-settings.nix` looking like this:
+
+        {
+          systemd.services."{{ component.unit_identifier }}" = {
+            serviceConfig = {
+              /* ExecStart etc. */
+            };
+          };
+        }
+
+    This creates a systemd target called `message-consumer.target`
+    and a template service called `[email protected]`. When
+    the configuration gets activated, five instances of this service
+    are started, namely `[email protected]` to
+    `[email protected]`. When the machine gets rebooted,
+    these five instances will be started automatically.
+
+    The number of instances running by default can be changed with the
+    `running_instances` attribute.
+
+    Please note that the unit is completely empty and thus invalid. A rebuild
+    must not happen before `message-consumer-settings.nix` in the example above
+    is added that fully configures the service.
+
+    If needed, more services from this template can be started like this:
+
+        $ systemctl start message-consumer@{5..23}
+
+    This starts 19 additional units.
+
+    When running
+
+        $ systemctl restart message-consumer.target
+
+    all 24 running units will be restarted.
+
+    When running
+
+        $ systemctl stop message-consumer.target
+
+    all 24 running units will be stopped.
+
+    However, when running
+
+        $ systemctl start message-consumer.target
+
+    after that, only 5 units will get back up. The same applies to an
+    additional deployment or a reboot of the VM.
+
+    Hence, starting additional services is a temporary measure to quickly
+    get more workers up. The change must be persisted in the deployment
+    after that.
+    """
+
+    namevar = "base_name"
+    running_instances = Attribute(int, 4)
+
+    def configure(self):
+        self += File(
+            f"/etc/local/nixos/scale-{self.base_name}.nix",
+            content=pkg_resources.resource_string(
+                "batou_ext", "resources/scalable-service.nix"
+            ),
+        )
+
+        self.unit_identifier = f"{self.base_name}@"