From fc2fa3cda56d61d5d81b9a0d2b478aa76d5d7101 Mon Sep 17 00:00:00 2001
From: Richard Marko <srk@48.io>
Date: Wed, 24 Feb 2021 14:00:06 +0100
Subject: [PATCH 1/2] nixos/nixos-containers: default boot.enableContainers to
 true

Related to #85746 which addresses documentation issue,
digging deeper for a reason why this was disabled
was simply because it wasn't working which is not the case anymore.
---
 nixos/modules/virtualisation/nixos-containers.nix | 11 +++--------
 1 file changed, 3 insertions(+), 8 deletions(-)

diff --git a/nixos/modules/virtualisation/nixos-containers.nix b/nixos/modules/virtualisation/nixos-containers.nix
index 3754fe6dac6..c1ff1bb5d47 100644
--- a/nixos/modules/virtualisation/nixos-containers.nix
+++ b/nixos/modules/virtualisation/nixos-containers.nix
@@ -439,21 +439,16 @@ in
       default = false;
       description = ''
         Whether this NixOS machine is a lightweight container running
-        in another NixOS system. If set to true, support for nested
-        containers is disabled by default, but can be reenabled by
-        setting <option>boot.enableContainers</option> to true.
+        in another NixOS system.
       '';
     };
 
     boot.enableContainers = mkOption {
       type = types.bool;
-      default = !config.boot.isContainer;
+      default = true;
       description = ''
         Whether to enable support for NixOS containers. Defaults to true
-        (at no cost if containers are not actually used), but only if the
-        system is not itself a lightweight container of a host.
-        To enable support for nested containers, this option has to be
-        explicitly set to true (in the outer container).
+        (at no cost if containers are not actually used).
       '';
     };
 

From d23ba22076f64c9daffedde66376b11796046b40 Mon Sep 17 00:00:00 2001
From: Richard Marko <srk@48.io>
Date: Wed, 24 Feb 2021 14:02:57 +0100
Subject: [PATCH 2/2] nixosTests.containers-nested: init

---
 nixos/tests/all-tests.nix         |  1 +
 nixos/tests/containers-nested.nix | 30 ++++++++++++++++++++++++++++++
 2 files changed, 31 insertions(+)
 create mode 100644 nixos/tests/containers-nested.nix

diff --git a/nixos/tests/all-tests.nix b/nixos/tests/all-tests.nix
index 7d676e15fa9..3784dc6c886 100644
--- a/nixos/tests/all-tests.nix
+++ b/nixos/tests/all-tests.nix
@@ -74,6 +74,7 @@ in
   containers-ip = handleTest ./containers-ip.nix {};
   containers-macvlans = handleTest ./containers-macvlans.nix {};
   containers-names = handleTest ./containers-names.nix {};
+  containers-nested = handleTest ./containers-nested.nix {};
   containers-physical_interfaces = handleTest ./containers-physical_interfaces.nix {};
   containers-portforward = handleTest ./containers-portforward.nix {};
   containers-reloadable = handleTest ./containers-reloadable.nix {};
diff --git a/nixos/tests/containers-nested.nix b/nixos/tests/containers-nested.nix
new file mode 100644
index 00000000000..a653361494f
--- /dev/null
+++ b/nixos/tests/containers-nested.nix
@@ -0,0 +1,30 @@
+# Test for NixOS' container nesting.
+
+import ./make-test-python.nix ({ pkgs, ... }: {
+  name = "nested";
+
+  meta = with pkgs.lib.maintainers; { maintainers = [ sorki ]; };
+
+  machine = { lib, ... }:
+    let
+      makeNested = subConf: {
+        containers.nested = {
+          autoStart = true;
+          privateNetwork = true;
+          config = subConf;
+        };
+      };
+    in makeNested (makeNested { });
+
+  testScript = ''
+    machine.start()
+    machine.wait_for_unit("container@nested.service")
+    machine.succeed("systemd-run --pty --machine=nested -- machinectl list | grep nested")
+    print(
+        machine.succeed(
+            "systemd-run --pty --machine=nested -- systemd-run --pty --machine=nested -- systemctl status"
+        )
+    )
+  '';
+})
+