Handle the combination of DKG removals with re-attempts

With a DKG removal comes a reduction in the amount of participants which was
ignored by re-attempts.

Now, we determine n/i based on the parties removed, and deterministically
obtain the context of who was removd.

coordinator/src/tributary/mod.rs

@@ -1,3 +1,7 @@
+use ciphersuite::{group::GroupEncoding, Ciphersuite, Ristretto};
+
+use serai_client::validator_sets::primitives::ValidatorSet;
+
 use tributary::{
   ReadWrite,
   transaction::{TransactionError, TransactionKind, Transaction as TransactionTrait},
@@ -20,6 +24,58 @@ pub use handle::*;
 
 pub mod scanner;
 
+pub fn removed_as_of_dkg_attempt(
+  getter: &impl Get,
+  genesis: [u8; 32],
+  attempt: u32,
+) -> Option<Vec<<Ristretto as Ciphersuite>::G>> {
+  if attempt == 0 {
+    Some(vec![])
+  } else {
+    FatalSlashesAsOfDkgAttempt::get(getter, genesis, attempt).map(|keys| {
+      keys.iter().map(|key| <Ristretto as Ciphersuite>::G::from_bytes(key).unwrap()).collect()
+    })
+  }
+}
+
+pub fn latest_removed(getter: &impl Get, genesis: [u8; 32]) -> Vec<<Ristretto as Ciphersuite>::G> {
+  #[allow(clippy::unwrap_or_default)]
+  FatalSlashes::get(getter, genesis)
+    .unwrap_or(vec![])
+    .iter()
+    .map(|key| <Ristretto as Ciphersuite>::G::from_bytes(key).unwrap())
+    .collect()
+}
+
+pub fn removed_as_of_set_keys(
+  getter: &impl Get,
+  set: ValidatorSet,
+  genesis: [u8; 32],
+) -> Option<Vec<<Ristretto as Ciphersuite>::G>> {
+  // SeraiDkgCompleted has the key placed on-chain.
+  // This key can be uniquely mapped to an attempt so long as one participant was honest, which we
+  // assume as a presumably honest participant.
+  // Resolve from generated key to attempt to fatally slashed as of attempt.
+
+  // This expect will trigger if this is prematurely called and Substrate has tracked the keys yet
+  // we haven't locally synced and handled the Tributary
+  // All callers of this, at the time of writing, ensure the Tributary has sufficiently synced
+  // making the panic with context more desirable than the None
+  let attempt = KeyToDkgAttempt::get(getter, SeraiDkgCompleted::get(getter, set)?)
+    .expect("key completed on-chain didn't have an attempt related");
+  removed_as_of_dkg_attempt(getter, genesis, attempt)
+}
+
+pub fn removed_as_of_fatal_slash(
+  getter: &impl Get,
+  genesis: [u8; 32],
+  fatally_slashed: [u8; 32],
+) -> Option<Vec<<Ristretto as Ciphersuite>::G>> {
+  FatalSlashesAsOfFatalSlash::get(getter, genesis, fatally_slashed).map(|keys| {
+    keys.iter().map(|key| <Ristretto as Ciphersuite>::G::from_bytes(key).unwrap()).collect()
+  })
+}
+
 pub async fn publish_signed_transaction<D: Db, P: crate::P2p>(
   txn: &mut D::Transaction<'_>,
   tributary: &Tributary<D, Transaction, P>,