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

1 files changed, 8 insertions, 4 deletions

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