nixos/tests: fix flaky cntr test

The cntr sometimes hangs until the 10-hour hydra limit. This behaviour
appears to be an edge-case related to the type of TTY in which the cntr
command runs during test execution. We can work around this by running
the command as a background job.

I additionally added a wait_for_open_port to fix nondeterministic test
failures I observed after fixing the hanging issue.

1 files changed, 16 insertions, 4 deletions

diff --git a/nixos/tests/cntr.nix b/nixos/tests/cntr.nix
index 66847075620..e4e13545b87 100644
--- a/nixos/tests/cntr.nix
+++ b/nixos/tests/cntr.nix
@@ -28,10 +28,16 @@ let
       testScript = ''
         start_all()
         ${backend}.wait_for_unit("${backend}-nginx.service")
-        result = ${backend}.wait_until_succeeds(
-            "cntr attach -t ${backend} nginx sh -- -c 'curl localhost | grep Hello'"
+        ${backend}.wait_for_open_port(8181)
+        # For some reason, the cntr command hangs when run without the &.
+        # As such, we have to do some messy things to ensure we check the exitcode and output in a race-condition-safe manner
+        ${backend}.execute(
+            "(cntr attach -t ${backend} nginx sh -- -c 'curl localhost | grep Hello' > /tmp/result; echo $? > /tmp/exitcode; touch /tmp/done) &"
         )
-        assert "Hello" in result
+
+        ${backend}.wait_for_file("/tmp/done")
+        assert "0" == ${backend}.succeed("cat /tmp/exitcode").strip(), "non-zero exit code"
+        assert "Hello" in ${backend}.succeed("cat /tmp/result"), "no greeting in output"
       '';
     };
 
@@ -54,7 +60,13 @@ let
     testScript = ''
       machine.start()
       machine.wait_for_unit("container@test.service")
-      machine.succeed("cntr attach test sh -- -c 'ping -c5 172.16.0.1'")
+      # I haven't observed the same hanging behaviour in this version as in the OCI version which necessetates this messy invocation, but it's probably better to be safe than sorry and use it here as well
+      machine.execute(
+          "(cntr attach test sh -- -c 'ping -c5 172.16.0.1'; echo $? > /tmp/exitcode; touch /tmp/done) &"
+      )
+
+      machine.wait_for_file("/tmp/done")
+      assert "0" == machine.succeed("cat /tmp/exitcode").strip(), "non-zero exit code"
     '';
   };
 in {