This requirement was originally added together with splicing the
inbox owner into the non b* addressing fields to make bcc transports
work in https://git.pleroma.social/pleroma/pleroma/-/merge_requests/390.
Later on this was relaxed to always allow deliveries devoid of any
addressing at all in f6cb963df2
and always allow deliveries from actors the owner is following in
750b369d04 to fix interop issues with
Mastodon and Honk respectively.
The justification for both the filtering and splicing comes from
one sentence in AP spec’s inbox section:
> In general, the owner of an inbox is likely
> to be able to access all of their inbox contents.
While this may provide plausible justification for splicing the owner
into cc, it is less clear how this requires or justifies the set of
filtering rules employed here.
Surveying a few other implementations no similar
filtering or splicing appears to be employed.
Furthermore, spec-compliant servers will strip bto/bcc _before_
delivery to remote servers, meaning any compliant bcc transport
out there will NOT contain any explicit addressing of the inbox owner.
Thus the addressing requirement directly opposes
the goal of the original patch.
Currently the requirement for the owner to be addressed once again
is causing interop issues. It turns out to be the root cause of
a long-standing (2+ years) bug preventing meaningful federation.
Bridgy sends e.g. Follow activities and Accepts for Follows directly
to the affected user’s personal inbox while solely addressing
the public scope in the to field. Notably follow relations never
getting established prevented the "accept if followed" allow rule
to ever come into effect.
To make matters worse non-addressed messages simply lead to a
vague "internal server error" response being sent back
which likely slowed down locating the issue.
Furthermore additional issues wrt to signatures cropped up after
the 500-response issues wa first reported, but they seem to have
already been fixed in the meantime, possibly with the signature
handling overhaul in Akkoma.
Given it repeatedly caused issues, does not appear to align with common
practice in the wider fedi ecosystem and apparently contradicts its
original intention, simply remove the requirement.
This is confirmed to fix bridgy interop.
The addressing splicing actually should also add the inbox owner to bto
or bcc instead of cc, but for now this is not changed and in practice
bto/bcc delivery appears to be basically unused anyway.
2b885288fa