A sender's digest of its followers here is compared and mended

FEP-8fcf, received. When a delivery's Collection-Synchronization header digests the sender's followers on PrivaPub
otherwise than the personas following it, a job reads the list the header names (on the sender's origin, signed by the
instance actor): a follow the list leaves out ends, only when the list is the one the digest describes; a request it
lists is taken as accepted; a persona it lists that follows nothing there sends Undo{Follow}, as Mastodon does. Each
claiming delivery is compared once.

Checked live (scenarios/followsync.sh, now 15 checks): PrivaPub ends a follow Mastodon lost and undoes one only
Mastodon remembered.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

tools/pasture/scenarios/followsync.sh

@@ -1,7 +1,9 @@
-# Followers synchronisation (FEP-8fcf, owner decision 2026-10-06): alice's followers-only post tells Mastodon, in a signed
-# header, a digest of her followers there; when Mastodon's view differs it reads her roll-call and mends itself. Both ways:
-# a follow PrivaPub lost (Mastodon drops it), and a follow Mastodon lost (it sends the Undo PrivaPub then applies). The
-# roll-call lists Mastodon's accounts only, and only to a signed request. Needs the mastodon peer.
+# Followers synchronisation (FEP-8fcf, owner decision 2026-10-06), both ways. Sent: alice's followers-only post tells
+# Mastodon, in a signed header, a digest of her followers there; when Mastodon's view differs it reads her roll-call and
+# mends itself: a follow PrivaPub lost (Mastodon drops it), and a follow Mastodon lost (it sends the Undo PrivaPub
+# applies). The roll-call answers only a signed request. Received: mastouser's followers-only post tells PrivaPub a digest
+# of its followers here (alice and bob); PrivaPub reads Mastodon's list and ends a follow Mastodon lost, and undoes one
+# only Mastodon remembers. Needs the mastodon peer.
 M=https://mastodon.test:6443
 mcurl() { curl -sk --resolve mastodon.test:6443:127.0.0.1 "$@"; }
 . "$here/peers/mastodon.sh"
@@ -49,3 +51,29 @@ m_rails "a = Account.find_local(\"mastouser\"); t = Account.find_by(uri: \"$alic
 [ "$(m_follows)" = "False" ] && ok "Mastodon forgets the follow" || ko "Mastodon still has the follow"
 quiet_post "for my followers again $run"
 until_true 60 '[ "$(alice_followers)" = "0" ]' && ok "Mastodon reads the roll-call and undoes the follow PrivaPub had" || ko "PrivaPub still counts mastouser"
+
+echo "  received: mastouser's followers here"
+BT=$(privapub_token bob_sync)
+BH="Authorization: Bearer $BT"
+m_on_p=$(curl -s -H "$AH" "$P/api/v2/search?q=mastouser@mastodon.test&resolve=true&type=accounts" | j "print(d['accounts'][0]['id'])")
+p_follows() { curl -s -H "$1" "$P/api/v1/accounts/relationships?id[]=$m_on_p" | j "print(d[0]['following'])"; }
+m_followed_by() { m_rails "puts Follow.exists?(account: Account.find_by(uri: \"${P_BASE:-https://privapub.test}/peasants/$1\"), target_account: Account.find_local(\"mastouser\")) ? \"True\" : \"False\""; }
+m_quiet() { mcurl -s -o /dev/null -X POST -H "$MH" "$M/api/v1/statuses" -d "status=$1&visibility=private"; }
+for who in "$AH" "$BH"; do
+	[ "$(p_follows "$who")" = "True" ] || curl -s -o /dev/null -X POST -H "$who" "$P/api/v1/accounts/$m_on_p/follow"
+done
+until_true 45 '[ "$(p_follows "$AH")" = "True" ] && [ "$(p_follows "$BH")" = "True" ] && [ "$(m_followed_by alice_sync)" = "True" ] && [ "$(m_followed_by bob_sync)" = "True" ]' \
+	&& ok "alice and bob follow mastouser" || ko "alice and bob never both followed mastouser"
+
+m_rails "Follow.where(account: Account.find_by(uri: \"https://privapub.test/peasants/alice_sync\"), target_account: Account.find_local(\"mastouser\")).destroy_all" >/dev/null
+[ "$(m_followed_by alice_sync)" = "False" ] && ok "Mastodon forgets alice's follow" || ko "Mastodon still has alice's follow"
+m_quiet "for my followers $run"
+until_true 60 '[ "$(p_follows "$AH")" = "False" ]' && ok "PrivaPub reads mastouser's list and ends alice's follow" || ko "alice still follows mastouser on PrivaPub"
+[ "$(p_follows "$BH")" = "True" ] && ok "bob's follow stays" || ko "bob's follow ended too"
+
+curl -s -o /dev/null -X POST -H "$AH" "$P/api/v1/accounts/$m_on_p/follow"
+until_true 45 '[ "$(p_follows "$AH")" = "True" ] && [ "$(m_followed_by alice_sync)" = "True" ]' && ok "alice follows mastouser again" || ko "alice could not follow mastouser again"
+p_mongo "db.Following.deleteMany({AvatarId:'$alice_id', TargetActorURI:'$mastouser_uri'})" >/dev/null
+[ "$(p_follows "$AH")" = "False" ] && ok "PrivaPub forgets alice's follow" || ko "PrivaPub still has alice's follow"
+m_quiet "for my followers again $run"
+until_true 60 '[ "$(m_followed_by alice_sync)" = "False" ]' && ok "PrivaPub undoes the follow only Mastodon remembered" || ko "Mastodon still has alice following mastouser"