nixos/tests/gitea: test gpg commit signing key

`/api/v1/signing-key.gpg` spawns a `gpg` process,
which is great to test if `gpg` is available
and can be invoked from in the unit.

Which is somewhat relevant, since `gpg` was
missing from the unit's `$PATH` until recently.

And even after adding `gpg` to the unit's `$PATH`,
configuring commit signing for a instance
resulted in http/500s nonetheless.

That's due to `@memlock` being present in
`SystemCallFilter=~` and `gpg` trying to
use `mlock` (probably to prevent secrets
in the memory to swap), resulting in an
immediate `SIGKILL` of any spawned `gpg` processes.

nixos/tests/gitea.nix

@@ -8,6 +8,21 @@ with import ../lib/testing-python.nix { inherit system pkgs; };
 with pkgs.lib;
 
 let
+  ## gpg --faked-system-time='20230301T010000!' --quick-generate-key snakeoil ed25519 sign
+  signingPrivateKey = ''
+    -----BEGIN PGP PRIVATE KEY BLOCK-----
+
+    lFgEY/6jkBYJKwYBBAHaRw8BAQdADXiZRV8RJUyC9g0LH04wLMaJL9WTc+szbMi7
+    5fw4yP8AAQCl8EwGfzSLm/P6fCBfA3I9znFb3MEHGCCJhJ6VtKYyRw7ktAhzbmFr
+    ZW9pbIiUBBMWCgA8FiEE+wUM6VW/NLtAdSixTWQt6LZ4x50FAmP+o5ACGwMFCQPC
+    ZwAECwkIBwQVCgkIBRYCAwEAAh4FAheAAAoJEE1kLei2eMedFTgBAKQs1oGFZrCI
+    TZP42hmBTKxGAI1wg7VSdDEWTZxut/2JAQDGgo2sa4VHMfj0aqYGxrIwfP2B7JHO
+    GCqGCRf9O/hzBA==
+    =9Uy3
+    -----END PGP PRIVATE KEY BLOCK-----
+  '';
+  signingPrivateKeyId = "4D642DE8B678C79D";
+
   supportedDbTypes = [ "mysql" "postgres" "sqlite3" ];
   makeGiteaTest = type: nameValuePair type (makeTest {
     name = "${giteaPackage.pname}-${type}";
@@ -21,8 +36,9 @@ let
           database = { inherit type; };
           package = giteaPackage;
           settings.service.DISABLE_REGISTRATION = true;
+          settings."repository.signing".SIGNING_KEY = signingPrivateKeyId;
         };
-        environment.systemPackages = [ giteaPackage pkgs.jq ];
+        environment.systemPackages = [ giteaPackage pkgs.gnupg pkgs.jq ];
         services.openssh.enable = true;
       };
       client1 = { config, pkgs, ... }: {
@@ -58,6 +74,13 @@ let
       server.wait_for_open_port(3000)
       server.succeed("curl --fail http://localhost:3000/")
 
+      server.succeed(
+          "su -l gitea -c 'gpg --homedir /var/lib/gitea/data/home/.gnupg "
+          + "--import ${toString (pkgs.writeText "gitea.key" signingPrivateKey)}'"
+      )
+
+      assert "BEGIN PGP PUBLIC KEY BLOCK" in server.succeed("curl http://localhost:3000/api/v1/signing-key.gpg")
+
       server.succeed(
           "curl --fail http://localhost:3000/user/sign_up | grep 'Registration is disabled. "
           + "Please contact your site administrator.'"