Adds a test for nss preload mechanism

This tests for the fix implemented in #5224

Signed-off-by: Arthur Gautier <baloo@superbaloo.net>

flake.nix

@@ -444,6 +444,12 @@
           inherit (self) overlay;
         };
 
+        tests.nssPreload = (import ./tests/nss-preload.nix rec {
+          system = "x86_64-linux";
+          inherit nixpkgs;
+          inherit (self) overlay;
+        });
+
         tests.githubFlakes = (import ./tests/github-flakes.nix rec {
           system = "x86_64-linux";
           inherit nixpkgs;

tests/nss-preload.nix

@@ -0,0 +1,123 @@
+{ nixpkgs, system, overlay }:
+
+with import (nixpkgs + "/nixos/lib/testing-python.nix") {
+  inherit system;
+  extraConfigurations = [ { nixpkgs.overlays = [ overlay ]; } ];
+};
+
+makeTest (
+
+rec {
+  name = "nss-preload";
+
+  nodes = {
+    http_dns = { lib, pkgs, config, ... }: {
+      networking.firewall.enable = false;
+      networking.interfaces.eth1.ipv6.addresses = lib.mkForce [
+        { address = "fd21::1"; prefixLength = 64; }
+      ];
+      networking.interfaces.eth1.ipv4.addresses = lib.mkForce [
+        { address = "192.168.0.1"; prefixLength = 24; }
+      ];
+
+      services.unbound = {
+        enable = true;
+        enableRootTrustAnchor = false;
+        settings = {
+          server = {
+            interface = [ "192.168.0.1" "fd21::1" "::1" "127.0.0.1" ];
+            access-control = [ "192.168.0.0/24 allow" "fd21::/64 allow" "::1 allow" "127.0.0.0/8 allow" ];
+            local-data = [
+              ''"example.com. IN A 192.168.0.1"''
+              ''"example.com. IN AAAA fd21::1"''
+              ''"tarballs.nixos.org. IN A 192.168.0.1"''
+              ''"tarballs.nixos.org. IN AAAA fd21::1"''
+            ];
+          };
+        };
+      };
+
+      services.nginx = {
+        enable = true;
+        virtualHosts."example.com" = {
+          root = pkgs.runCommand "testdir" {} ''
+            mkdir "$out"
+            echo hello world > "$out/index.html"
+          '';
+        };
+      };
+    };
+
+    # client consumes a remote resolver
+    client = { lib, nodes, pkgs, ... }: {
+      networking.useDHCP = false;
+      networking.nameservers = [
+        (lib.head nodes.http_dns.config.networking.interfaces.eth1.ipv6.addresses).address
+        (lib.head nodes.http_dns.config.networking.interfaces.eth1.ipv4.addresses).address
+      ];
+      networking.interfaces.eth1.ipv6.addresses = [
+        { address = "fd21::10"; prefixLength = 64; }
+      ];
+      networking.interfaces.eth1.ipv4.addresses = [
+        { address = "192.168.0.10"; prefixLength = 24; }
+      ];
+
+      nix.sandboxPaths = lib.mkForce [];
+      nix.binaryCaches = lib.mkForce [];
+      nix.useSandbox = lib.mkForce true;
+    };
+  };
+
+  nix-fetch = pkgs.writeText "fetch.nix" ''
+    derivation {
+        # This derivation is an copy from what is available over at
+        # nix.git:corepkgs/fetchurl.nix
+        builder = "builtin:fetchurl";
+
+        # We're going to fetch data from the http_dns instance created before
+        # we expect the content to be the same as the content available there.
+        # ```
+        # $ nix-hash --type sha256 --to-base32 $(echo "hello world" | sha256sum | cut -d " " -f 1)
+        # 0ix4jahrkll5zg01wandq78jw3ab30q4nscph67rniqg5x7r0j59
+        # ```
+        outputHash = "0ix4jahrkll5zg01wandq78jw3ab30q4nscph67rniqg5x7r0j59";
+        outputHashAlgo = "sha256";
+        outputHashMode = "flat";
+
+        name = "example.com";
+        url = "http://example.com";
+
+        unpack = false;
+        executable = false;
+
+        system = "builtin";
+
+        preferLocalBuild = true;
+
+        impureEnvVars = [
+            "http_proxy" "https_proxy" "ftp_proxy" "all_proxy" "no_proxy"
+        ];
+
+        urls = [ "http://example.com" ];
+      }
+  '';
+
+  testScript = { nodes, ... }: ''
+    http_dns.wait_for_unit("nginx")
+    http_dns.wait_for_open_port(80)
+    http_dns.wait_for_unit("unbound")
+    http_dns.wait_for_open_port(53)
+
+    client.start()
+    client.wait_for_unit('multi-user.target')
+
+    with subtest("can fetch data from a remote server outside sandbox"):
+        client.succeed("nix --version >&2")
+        client.succeed("curl -vvv http://example.com/index.html >&2")
+
+    with subtest("nix-build can lookup dns and fetch data"):
+        client.succeed("""
+          nix-build ${nix-fetch} >&2
+          """)
+  '';
+})