CommonFixes: more predictable context generation

`context` fields for objects and activities can now be generated based
on the object/activity `inReplyTo` field or its ActivityPub ID, as a
fallback method in cases where `context` fields are missing for incoming
activities and objects.

lib/pleroma/web/activity_pub/object_validators/common_fixes.ex

@@ -22,7 +22,10 @@ defmodule Pleroma.Web.ActivityPub.ObjectValidators.CommonFixes do
   end
 
   def fix_object_defaults(data) do
-    context = Utils.maybe_create_context(data["context"] || data["conversation"])
+    context =
+      Utils.maybe_create_context(
+        data["context"] || data["conversation"] || data["inReplyTo"] || data["id"]
+      )
 
     %User{follower_address: follower_collection} = User.get_cached_by_ap_id(data["attributedTo"])
 

test/fixtures/tesla_mock/mk.absturztau.be-93e7nm8wqg-activity.json

@@ -0,0 +1 @@
+{"@context":["https://www.w3.org/ns/activitystreams","https://w3id.org/security/v1",{"manuallyApprovesFollowers":"as:manuallyApprovesFollowers","sensitive":"as:sensitive","Hashtag":"as:Hashtag","quoteUrl":"as:quoteUrl","toot":"http://joinmastodon.org/ns#","Emoji":"toot:Emoji","featured":"toot:featured","discoverable":"toot:discoverable","schema":"http://schema.org#","PropertyValue":"schema:PropertyValue","value":"schema:value","misskey":"https://misskey-hub.net/ns#","_misskey_content":"misskey:_misskey_content","_misskey_quote":"misskey:_misskey_quote","_misskey_reaction":"misskey:_misskey_reaction","_misskey_votes":"misskey:_misskey_votes","_misskey_talk":"misskey:_misskey_talk","isCat":"misskey:isCat","vcard":"http://www.w3.org/2006/vcard/ns#"}],"id":"https://mk.absturztau.be/notes/93e7nm8wqg/activity","actor":"https://mk.absturztau.be/users/8ozbzjs3o8","type":"Create","published":"2022-08-01T11:06:49.568Z","object":{"id":"https://mk.absturztau.be/notes/93e7nm8wqg","type":"Note","attributedTo":"https://mk.absturztau.be/users/8ozbzjs3o8","summary":null,"content":"<p><span>meow</span></p>","_misskey_content":"meow","published":"2022-08-01T11:06:49.568Z","to":["https://www.w3.org/ns/activitystreams#Public"],"cc":["https://mk.absturztau.be/users/8ozbzjs3o8/followers"],"inReplyTo":null,"attachment":[],"sensitive":false,"tag":[]},"to":["https://www.w3.org/ns/activitystreams#Public"],"cc":["https://mk.absturztau.be/users/8ozbzjs3o8/followers"]}

test/pleroma/web/activity_pub/transmogrifier/note_handling_test.exs

@@ -783,4 +783,42 @@ defmodule Pleroma.Web.ActivityPub.Transmogrifier.NoteHandlingTest do
       } = Transmogrifier.fix_quote_url(note)
     end
   end
+
+  test "the standalone note uses its own ID when context is missing" do
+    insert(:user, ap_id: "https://mk.absturztau.be/users/8ozbzjs3o8")
+
+    activity =
+      "test/fixtures/tesla_mock/mk.absturztau.be-93e7nm8wqg-activity.json"
+      |> File.read!()
+      |> Jason.decode!()
+
+    {:ok, %Activity{} = modified} = Transmogrifier.handle_incoming(activity)
+    object = Object.normalize(modified, fetch: false)
+
+    assert object.data["context"] == object.data["id"]
+    assert modified.data["context"] == object.data["id"]
+  end
+
+  test "the reply note uses its parent's ID when context is missing and reply is unreachable" do
+    insert(:user, ap_id: "https://mk.absturztau.be/users/8ozbzjs3o8")
+
+    activity =
+      "test/fixtures/tesla_mock/mk.absturztau.be-93e7nm8wqg-activity.json"
+      |> File.read!()
+      |> Jason.decode!()
+
+    object =
+      activity["object"]
+      |> Map.put("inReplyTo", "https://404.site/object/went-to-buy-milk")
+
+    activity =
+      activity
+      |> Map.put("object", object)
+
+    {:ok, %Activity{} = modified} = Transmogrifier.handle_incoming(activity)
+    object = Object.normalize(modified, fetch: false)
+
+    assert object.data["context"] == object.data["inReplyTo"]
+    assert modified.data["context"] == object.data["inReplyTo"]
+  end
 end

test/support/http_request_mock.ex

@@ -1085,6 +1085,14 @@ defmodule HttpRequestMock do
      }}
   end
 
+  def get("https://404.site" <> _, _, _, _) do
+    {:ok,
+     %Tesla.Env{
+       status: 404,
+       body: ""
+     }}
+  end
+
   def get(
         "https://zetsubou.xn--q9jyb4c/.well-known/webfinger?resource=acct:lain@zetsubou.xn--q9jyb4c",
         _,
@@ -1399,6 +1407,15 @@ defmodule HttpRequestMock do
      }}
   end
 
+  def get("https://mk.absturztau.be/notes/93e7nm8wqg/activity", _, _, _) do
+    {:ok,
+     %Tesla.Env{
+       status: 200,
+       body: File.read!("test/fixtures/tesla_mock/mk.absturztau.be-93e7nm8wqg-activity.json"),
+       headers: activitypub_object_headers()
+     }}
+  end
+
   def get("https://p.helene.moe/objects/fd5910ac-d9dc-412e-8d1d-914b203296c4", _, _, _) do
     {:ok,
      %Tesla.Env{