coredns: fix externalPlugins by 'go mod vendor' after 'go generate'

- also add a nixos test that covers the external plugins case

nixos/tests/all-tests.nix

@@ -407,6 +407,7 @@ in
   containers-tmpfs = runTest ./containers-tmpfs.nix;
   containers-unified-hierarchy = runTest ./containers-unified-hierarchy.nix;
   convos = runTest ./convos.nix;
+  coredns = runTest ./coredns.nix;
   corerad = runTest ./corerad.nix;
   corteza = runTest ./corteza.nix;
   cosmic = runTest {

nixos/tests/coredns.nix

@@ -0,0 +1,42 @@
+{ pkgs, ... }:
+{
+  name = "coredns";
+  meta = with pkgs.lib.maintainers; {
+    maintainers = [ johanot ];
+  };
+
+  nodes.machine =
+    { pkgs, ... }:
+    {
+      environment.systemPackages = [ pkgs.dnsutils ];
+      services.coredns = {
+        enable = true;
+        config = ''
+          .:10053 {
+                ipecho {
+                  domain test.nixos.org
+                  ttl 2629800
+                }
+              }
+        '';
+        package = pkgs.coredns.override {
+          externalPlugins = [
+            {
+              name = "ipecho";
+              repo = "github.com/Eun/coredns-ipecho";
+              version = "224170ebca45cc59c6b071d280a18f42d1ff130c";
+              position = "start-of-file";
+            }
+          ];
+          vendorHash = "sha256-dNxHpXkiqz7B/JhZdxfZluIHFVXILlSm3XtB+v/EoMY=";
+        };
+      };
+    };
+
+  testScript = ''
+    machine.start()
+    machine.wait_for_unit("coredns.service")
+    machine.wait_for_open_port(10053)
+    machine.succeed("dig @127.0.0.1 -p 10053 127.0.0.2.test.nixos.org A +short | grep 127.0.0.2")
+  '';
+}

pkgs/by-name/co/coredns/package.nix

@@ -78,6 +78,7 @@ buildGoModule (finalAttrs: {
     for src in ${toString (attrsToSources externalPlugins)}; do go get $src; done
     go mod vendor
     CC= GOOS= GOARCH= go generate
+    go mod vendor
     go mod tidy
   '';
 
@@ -134,6 +135,7 @@ buildGoModule (finalAttrs: {
   '';
 
   passthru.tests = {
+    coredns-external-plugins = nixosTests.coredns;
     kubernetes-single-node = nixosTests.kubernetes.dns-single-node;
     kubernetes-multi-node = nixosTests.kubernetes.dns-multi-node;
   };