[Bug]: activity ids on outbox change on every request.

[lullis]

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Summary

As discussed on the Lemmy dev group: If the ids of the activity objects change on every request, then it's virtually impossible for clients to pull data from the outbox in a reliable manner.

Steps to Reproduce

1. make multiple requests to any community outbox, using 'application/ld+json' as the accept header.
2. Verify that the ids of the activities (create/announce/like) change on every request.

Technical Details

The solution to fix is to use a deterministic function to generate the id, instead of the random UUIDs that are being currently provided. I'd suggest a hash of ":".

Example using md5 as the hash function to generate the ids of Announce and Create for an object https://lemmy.example.com/post/123

Create activity: md5("create:https://lemmy.example.com/post/123") -> 911ab771355e05ef91ca61dd4e9460f1 -> ap id -> https://lemmy.example.com/activities/create/911ab771-355e-05ef-91ca-61dd4e9460f1

Announce: "md5("announce:https://lemmy.example.com/activities/create/911ab771-355e-05ef-91ca-61dd4e9460f1") -> 8993d7d8484786c0960e9e169a756a82 -> ap id: https://lemmy.example.com/activities/announce/create/8993d7d8-4847-86c0-960e-9e169a756a82.

Given that these are not cryptographic hashes, you can even use a faster hash function like murmur3. murmur3 also generates 128 bits digests, which means that you can format it like an UUID and there would be no difference in the URL format.

Version

0.19.15

Lemmy Instance URL

No response

[Nutomic]

This is relatively easy to fix by changing generate_activity_id(). First add a parameter object_id: Option<&str> and if it is Some, generate a hash and use that instead of Uuid. If it is None keep using Uuid like before. Then change the corresponding calls for create post etc to pass comment.ap_id to generate_activity_id.