From 1a82024dd868970143a85e3148d3504dfd8dce61 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <eelco.dolstra@logicblox.com>
Date: Mon, 29 Oct 2012 21:01:36 +0100
Subject: [PATCH] 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 | 11 +++++++++
 modules/system/boot/systemd.nix              | 25 ++++++++++++++++----
 modules/testing/test-instrumentation.nix     | 12 ++++++----
 tests/nat.nix                                |  3 ++-
 4 files changed, 42 insertions(+), 9 deletions(-)

diff --git a/modules/system/boot/systemd-unit-options.nix b/modules/system/boot/systemd-unit-options.nix
index 239c837c676..5707fb6f87e 100644
--- a/modules/system/boot/systemd-unit-options.nix
+++ b/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;
diff --git a/modules/system/boot/systemd.nix b/modules/system/boot/systemd.nix
index dc0c2ba4504..aa0b679ea2f 100644
--- a/modules/system/boot/systemd.nix
+++ b/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;
diff --git a/modules/testing/test-instrumentation.nix b/modules/testing/test-instrumentation.nix
index efce3153c88..222a10a0043 100644
--- a/modules/testing/test-instrumentation.nix
+++ b/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
diff --git a/tests/nat.nix b/tests/nat.nix
index e2f01e71b26..7b03739a9bc 100644
--- a/tests/nat.nix
+++ b/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");
     '';