activitypub: Implement an instance-wide actor

An instance-wide actor is required for outgoing signed requests that are
done on behalf of the instance, rather than on behalf of other actors.
Such things include updating profile information, or fetching public
keys.

Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>

tests/integration/api_activitypub_actor_test.go

@@ -0,0 +1,50 @@
+// Copyright 2024 The Forgejo Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package integration
+
+import (
+	"net/http"
+	"net/url"
+	"testing"
+
+	"code.gitea.io/gitea/modules/setting"
+	"code.gitea.io/gitea/modules/test"
+	"code.gitea.io/gitea/routers"
+
+	ap "github.com/go-ap/activitypub"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestActivityPubActor(t *testing.T) {
+	defer test.MockVariableValue(&setting.Federation.Enabled, true)()
+	defer test.MockVariableValue(&testWebRoutes, routers.NormalRoutes())()
+
+	onGiteaRun(t, func(*testing.T, *url.URL) {
+		req := NewRequest(t, "GET", "/api/v1/activitypub/actor")
+		resp := MakeRequest(t, req, http.StatusOK)
+		body := resp.Body.Bytes()
+		assert.Contains(t, string(body), "@context")
+
+		var actor ap.Actor
+		err := actor.UnmarshalJSON(body)
+		require.NoError(t, err)
+
+		assert.Equal(t, ap.ApplicationType, actor.Type)
+		assert.Equal(t, setting.Domain, actor.PreferredUsername.String())
+		keyID := actor.GetID().String()
+		assert.Regexp(t, "activitypub/actor$", keyID)
+		assert.Regexp(t, "activitypub/actor/outbox$", actor.Outbox.GetID().String())
+		assert.Regexp(t, "activitypub/actor/inbox$", actor.Inbox.GetID().String())
+
+		pubKey := actor.PublicKey
+		assert.NotNil(t, pubKey)
+		publicKeyID := keyID + "#main-key"
+		assert.Equal(t, pubKey.ID.String(), publicKeyID)
+
+		pubKeyPem := pubKey.PublicKeyPem
+		assert.NotNil(t, pubKeyPem)
+		assert.Regexp(t, "^-----BEGIN PUBLIC KEY-----", pubKeyPem)
+	})
+}