nixosTests.earlyoom: add a proper OOM test

nixos/modules/services/system/earlyoom.nix

@@ -4,8 +4,6 @@ let
   cfg = config.services.earlyoom;
 
   inherit (lib)
-    concatStringsSep
-    escapeShellArg
     literalExpression
     mkDefault
     mkEnableOption

nixos/tests/earlyoom.nix

@@ -1,16 +1,34 @@
 import ./make-test-python.nix ({ lib, ... }: {
   name = "earlyoom";
   meta = {
-    maintainers = with lib.maintainers; [ ncfavier AndersonTorres ];
+    maintainers = with lib.maintainers; [ ncfavier AndersonTorres oxalica ];
   };
 
-  nodes.machine = {
+  nodes.machine = { pkgs, ... }: {
+    # Limit VM resource usage.
+    virtualisation.memorySize = 1024;
+
     services.earlyoom = {
       enable = true;
+      # Use SIGKILL, or `tail` will catch SIGTERM and exit successfully.
+      freeMemKillThreshold = 90;
+    };
+
+    systemd.services.testbloat = {
+      description = "Create a lot of memory pressure";
+      serviceConfig = {
+        ExecStart = "${pkgs.coreutils}/bin/tail /dev/zero";
+      };
     };
   };
 
   testScript = ''
     machine.wait_for_unit("earlyoom.service")
+
+    with subtest("earlyoom should kill the bad service"):
+        machine.fail("systemctl start --wait testbloat.service")
+        assert machine.get_unit_info("testbloat.service")["Result"] == "signal"
+        output = machine.succeed('journalctl -u earlyoom.service -b0')
+        assert 'low memory! at or below SIGKILL limits' in output
   '';
 })