owncast: Federation: Pleroma receives "Unhandled Activity" error when going live

(I’m also going to open an issue with Pleroma, I’m unsure if the error is in Owncast or Pleroma)

I noticed that when Owncast goes live and sends out the ActivityPub message to Pleroma instances, Pleroma responds “OK” to the update - but the update never shows in the user’s timeline.

With debug logging enabled, Pleroma reports “Unhandled Activity”.

What’s interesting is, if I modify owncast to not include the Preview image - the activity is published correctly to users. So, I think this has something to do with the image attachment.

I tried modifying the code to do things like:

• Use the Document type instead of Image type (seems to be what mastodon does)
• add a mediaType attribute
• add width and height attributes

None of these seem to work. The only other thing that’s striking to me is - I noticed most implementations seem to send the attachment property as an array, but it looks like the go-fed package serializes that to an object when there’s only a single instance. From reading the activitypub specs, I’m 99% sure that’s legal - which is why I’m going to also open a Pleroma bug and make sure it isn’t a problem there.

So to recap, sending an ActivityPub JSON like this works with Pleroma (and Mastodon):

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    "http://joinmastodon.org/ns"
  ],
  "actor": "https://owncast.localhost.localdomain/federation/user/streamer",
  "audience": "https://www.w3.org/ns/activitystreams#Public",
  "id": "https://owncast.localhost.localdomain/federation/h7S-lqU7gz",
  "object": {
    "attributedTo": "https://owncast.localhost.localdomain/federation/user/streamer",
    "audience": "https://www.w3.org/ns/activitystreams#Public",
    "content": "<p>I've gone live!</p><p></p><p><a class=\"hashtag\" href=\"https://directory.owncast.online/tags/owncast\">#owncast</a> <a class=\"hashtag\" href=\"https://directory.owncast.online/tags/streaming\">#streaming</a></p><a href=\"https://owncast.localhost.localdomain\">https://owncast.localhost.localdomain</a>",
    "id": "https://owncast.localhost.localdomain/federation/2nI-lqUnR",
    "published": "2022-04-17T15:30:21Z",
    "tag": [
      {
        "href": "https://directory.owncast.online/tags/owncast",
        "name": "#owncast",
        "type": "Hashtag"
      },
      {
        "href": "https://directory.owncast.online/tags/streaming",
        "name": "#streaming",
        "type": "Hashtag"
      },
      {
        "href": "https://directory.owncast.online/tags/owncast",
        "name": "#owncast",
        "type": "Hashtag"
      }
    ],
    "to": "https://www.w3.org/ns/activitystreams#Public",
    "type": "Note"
  },
  "to": "https://www.w3.org/ns/activitystreams#Public",
  "type": "Create"
}

However, sending one like this (with an image) does not work with Pleroma, but does work with Mastodon:

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    "http://joinmastodon.org/ns"
  ],
  "actor": "https://owncast.localhost.localdomain/federation/user/streamer",
  "audience": "https://www.w3.org/ns/activitystreams#Public",
  "id": "https://owncast.localhost.localdomain/federation/KCfHuqU7Rz",
  "object": {
    "attachment": {
      "content": "Live stream preview",
      "type": "Image",
      "url": "https://owncast.localhost.localdomain/preview.gif?us=KjfNX387gm"
    },
    "attributedTo": "https://owncast.localhost.localdomain/federation/user/streamer",
    "audience": "https://www.w3.org/ns/activitystreams#Public",
    "content": "<p>I've gone live!</p><p></p><p><a class=\"hashtag\" href=\"https://directory.owncast.online/tags/owncast\">#owncast</a> <a class=\"hashtag\" href=\"https://directory.owncast.online/tags/streaming\">#streaming</a></p><a href=\"https://owncast.localhost.localdomain\">https://owncast.localhost.localdomain</a>",
    "id": "https://owncast.localhost.localdomain/federation/KjBNuq8ng",
    "published": "2022-04-17T15:42:03Z",
    "tag": [
      {
        "href": "https://directory.owncast.online/tags/owncast",
        "name": "#owncast",
        "type": "Hashtag"
      },
      {
        "href": "https://directory.owncast.online/tags/streaming",
        "name": "#streaming",
        "type": "Hashtag"
      },
      {
        "href": "https://directory.owncast.online/tags/owncast",
        "name": "#owncast",
        "type": "Hashtag"
      }
    ],
    "to": "https://www.w3.org/ns/activitystreams#Public",
    "type": "Note"
  },
  "to": "https://www.w3.org/ns/activitystreams#Public",
  "type": "Create"
}