tests: Move NixOS tests to tests/nixos

This will allow contributors to find them more easily.

tests/nixos/remote-builds.nix

@@ -0,0 +1,112 @@
+# Test Nix's remote build feature.
+
+{ nixpkgs, system, overlay }:
+
+with import (nixpkgs + "/nixos/lib/testing-python.nix") {
+  inherit system;
+  extraConfigurations = [ { nixpkgs.overlays = [ overlay ]; } ];
+};
+
+makeTest (
+
+let
+
+  # The configuration of the remote builders.
+  builder =
+    { config, pkgs, ... }:
+    { services.openssh.enable = true;
+      virtualisation.writableStore = true;
+      nix.settings.sandbox = true;
+    };
+
+  # Trivial Nix expression to build remotely.
+  expr = config: nr: pkgs.writeText "expr.nix"
+    ''
+      let utils = builtins.storePath ${config.system.build.extraUtils}; in
+      derivation {
+        name = "hello-${toString nr}";
+        system = "i686-linux";
+        PATH = "''${utils}/bin";
+        builder = "''${utils}/bin/sh";
+        args = [ "-c" "if [ ${toString nr} = 5 ]; then echo FAIL; exit 1; fi; echo Hello; mkdir $out $foo; cat /proc/sys/kernel/hostname > $out/host; ln -s $out $foo/bar; sleep 10" ];
+        outputs = [ "out" "foo" ];
+      }
+    '';
+
+in
+
+{
+  name = "remote-builds";
+
+  nodes =
+    { builder1 = builder;
+      builder2 = builder;
+
+      client =
+        { config, lib, pkgs, ... }:
+        { nix.settings.max-jobs = 0; # force remote building
+          nix.distributedBuilds = true;
+          nix.buildMachines =
+            [ { hostName = "builder1";
+                sshUser = "root";
+                sshKey = "/root/.ssh/id_ed25519";
+                system = "i686-linux";
+                maxJobs = 1;
+              }
+              { hostName = "builder2";
+                sshUser = "root";
+                sshKey = "/root/.ssh/id_ed25519";
+                system = "i686-linux";
+                maxJobs = 1;
+              }
+            ];
+          virtualisation.writableStore = true;
+          virtualisation.additionalPaths = [ config.system.build.extraUtils ];
+          nix.settings.substituters = lib.mkForce [ ];
+          programs.ssh.extraConfig = "ConnectTimeout 30";
+        };
+    };
+
+  testScript = { nodes }: ''
+    # fmt: off
+    import subprocess
+
+    start_all()
+
+    # Create an SSH key on the client.
+    subprocess.run([
+      "${pkgs.openssh}/bin/ssh-keygen", "-t", "ed25519", "-f", "key", "-N", ""
+    ], capture_output=True, check=True)
+    client.succeed("mkdir -p -m 700 /root/.ssh")
+    client.copy_from_host("key", "/root/.ssh/id_ed25519")
+    client.succeed("chmod 600 /root/.ssh/id_ed25519")
+
+    # Install the SSH key on the builders.
+    client.wait_for_unit("network.target")
+    for builder in [builder1, builder2]:
+      builder.succeed("mkdir -p -m 700 /root/.ssh")
+      builder.copy_from_host("key.pub", "/root/.ssh/authorized_keys")
+      builder.wait_for_unit("sshd")
+      client.succeed(f"ssh -o StrictHostKeyChecking=no {builder.name} 'echo hello world'")
+
+    # Perform a build and check that it was performed on the builder.
+    out = client.succeed(
+      "nix-build ${expr nodes.client.config 1} 2> build-output",
+      "grep -q Hello build-output"
+    )
+    builder1.succeed(f"test -e {out}")
+
+    # And a parallel build.
+    paths = client.succeed(r'nix-store -r $(nix-instantiate ${expr nodes.client.config 2})\!out $(nix-instantiate ${expr nodes.client.config 3})\!out')
+    out1, out2 = paths.split()
+    builder1.succeed(f"test -e {out1} -o -e {out2}")
+    builder2.succeed(f"test -e {out1} -o -e {out2}")
+
+    # And a failing build.
+    client.fail("nix-build ${expr nodes.client.config 5}")
+
+    # Test whether the build hook automatically skips unavailable builders.
+    builder1.block()
+    client.succeed("nix-build ${expr nodes.client.config 4}")
+  '';
+})