In the tests, don't start agetty on /dev/ttyS0

Running agetty on ttyS0 interferes with the backdoor, which uses ttyS0
as its standard error.  After agetty starts, writes to the stderr file
descriptor will return EIO (though doing "exec 2>/proc/self/fd/2" will
miracuously fix this).

http://hydra.nixos.org/build/3252782

modules/system/boot/systemd-unit-options.nix

@@ -6,6 +6,17 @@ rec {
 
   unitOptions = {
 
+    enable = mkOption {
+      default = true;
+      types = types.bool;
+      description = ''
+        If set to false, this unit will be a symlink to
+        /dev/null. This is primarily useful to prevent specific
+        template instances (e.g. <literal>serial-getty@ttyS0</literal>)
+        from being started.
+      '';
+    };
+
     description = mkOption {
       default = "";
       types = types.uniq types.string;

modules/system/boot/systemd.nix

@@ -10,7 +10,14 @@ let
   systemd = pkgs.systemd;
 
   makeUnit = name: unit:
-    pkgs.writeTextFile { name = "unit"; inherit (unit) text; destination = "/${name}"; };
+    pkgs.runCommand "unit" { inherit (unit) text; }
+      (if unit.enable then  ''
+        mkdir -p $out
+        echo -n "$text" > $out/${name}
+      '' else ''
+        mkdir -p $out
+        ln -s /dev/null $out/${name}
+      '');
 
   upstreamUnits =
     [ # Targets.
@@ -205,7 +212,7 @@ let
         as));
 
   targetToUnit = name: def:
-    { inherit (def) wantedBy;
+    { inherit (def) wantedBy enable;
       text =
         ''
           [Unit]
@@ -214,7 +221,7 @@ let
     };
 
   serviceToUnit = name: def:
-    { inherit (def) wantedBy;
+    { inherit (def) wantedBy enable;
       text =
         ''
           [Unit]
@@ -258,7 +265,7 @@ let
     };
 
   socketToUnit = name: def:
-    { inherit (def) wantedBy;
+    { inherit (def) wantedBy enable;
       text =
         ''
           [Unit]
@@ -333,6 +340,16 @@ in
           types = types.uniq types.string;
           description = "Text of this systemd unit.";
         };
+        enable = mkOption {
+          default = true;
+          types = types.bool;
+          description = ''
+            If set to false, this unit will be a symlink to
+            /dev/null. This is primarily useful to prevent specific
+            template instances (e.g. <literal>serial-getty@ttyS0</literal>)
+            from being started.
+          '';
+        };
         wantedBy = mkOption {
           default = [];
           types = types.listOf types.string;

modules/testing/test-instrumentation.nix

@@ -13,9 +13,8 @@ let kernel = config.boot.kernelPackages.kernel; in
 
     boot.systemd.services.backdoor =
       { wantedBy = [ "multi-user.target" ];
-        requires = [ "dev-hvc0.device" ];
-        after = [ "dev-hvc0.device" ];
-
+        requires = [ "dev-hvc0.device" "dev-ttyS0.device" ];
+        after = [ "dev-hvc0.device" "dev-ttyS0.device" ];
         script =
           ''
             export USER=root
@@ -27,10 +26,15 @@ let kernel = config.boot.kernelPackages.kernel; in
             echo "connecting to host..." >&2
             stty -F /dev/hvc0 raw -echo # prevent nl -> cr/nl conversion
             echo
-            PS1= /bin/sh
+            PS1= exec /bin/sh
           '';
       };
 
+    # Prevent agetty from being instantiated on ttyS0, since it
+    # interferes with the backdoor (writes to ttyS0 will randomly fail
+    # with EIO).
+    boot.systemd.services."serial-getty@ttyS0".enable = false;
+
     boot.initrd.postDeviceCommands =
       ''
         # Using acpi_pm as a clock source causes the guest clock to

tests/nat.nix

@@ -40,6 +40,7 @@
       startAll;
 
       # The router should have access to the server.
+      $server->waitForUnit("network.target");
       $server->waitForUnit("httpd");
       $router->waitForUnit("network.target");
       $router->succeed("curl --fail http://server/ >&2");
@@ -68,7 +69,7 @@
       $client->fail("ping -c 1 server >&2");
 
       # And make sure that restarting the NAT job works.
-      $router->succeed("start nat");
+      $router->succeed("systemctl start nat");
       $client->succeed("curl --fail http://server/ >&2");
       $client->succeed("ping -c 1 server >&2");
     '';