Add a new primitive of a DB-backed channel

The coordinator already had one of these, albeit implemented much worse than
the one now properly introduced. It had to either be sending or receiving,
whereas the new one can do both at the same time.

This replaces said instance and enables pleasant patterns when implementing the
processor/coordinator.

coordinator/src/substrate/db.rs

@@ -1,5 +1,3 @@
-use std::sync::{OnceLock, MutexGuard, Mutex};
-
 use scale::{Encode, Decode};
 
 pub use serai_db::*;
@@ -10,11 +8,10 @@ use serai_client::{
 };
 
 create_db! {
-  NewSubstrateDb {
+  SubstrateDb {
     CosignTriggered: () -> (),
     IntendedCosign: () -> (u64, Option<u64>),
     BlockHasEvents: (block: u64) -> u8,
-    CosignTransactions: (network: NetworkId) -> Vec<(Session, u64, [u8; 32])>,
   }
 }
 
@@ -29,48 +26,15 @@ impl IntendedCosign {
   }
 }
 
-// This guarantees:
-// 1) Appended transactions are appended
-// 2) Taking cosigns does not clear any TXs which weren't taken
-// 3) Taking does actually clear the set
-static COSIGN_LOCK: OnceLock<Mutex<()>> = OnceLock::new();
-pub struct CosignTxn<T: DbTxn>(T, MutexGuard<'static, ()>);
-impl<T: DbTxn> CosignTxn<T> {
-  pub fn new(txn: T) -> Self {
-    Self(txn, COSIGN_LOCK.get_or_init(|| Mutex::new(())).lock().unwrap())
-  }
-  pub fn commit(self) {
-    self.0.commit();
+db_channel! {
+  SubstrateDb {
+    CosignTransactions: (network: NetworkId) -> (Session, u64, [u8; 32]),
   }
 }
 impl CosignTransactions {
   // Append a cosign transaction.
-  pub fn append_cosign<T: DbTxn>(
-    txn: &mut CosignTxn<T>,
-    set: ValidatorSet,
-    number: u64,
-    hash: [u8; 32],
-  ) {
-    #[allow(clippy::unwrap_or_default)]
-    let mut txs = CosignTransactions::get(&txn.0, set.network).unwrap_or(vec![]);
-    txs.push((set.session, number, hash));
-    CosignTransactions::set(&mut txn.0, set.network, &txs);
-  }
-  // Peek at the next cosign transaction.
-  pub fn peek_cosign(getter: &impl Get, network: NetworkId) -> Option<(Session, u64, [u8; 32])> {
-    let mut to_cosign = CosignTransactions::get(getter, network)?;
-    if to_cosign.is_empty() {
-      None?
-    }
-    Some(to_cosign.swap_remove(0))
-  }
-  // Take the next transaction, panicking if it doesn't exist.
-  pub fn take_cosign(mut txn: impl DbTxn, network: NetworkId) {
-    let _lock = COSIGN_LOCK.get_or_init(|| Mutex::new(())).lock().unwrap();
-    let mut txs = CosignTransactions::get(&txn, network).unwrap();
-    txs.remove(0);
-    CosignTransactions::set(&mut txn, network, &txs);
-    txn.commit();
+  pub fn append_cosign(txn: &mut impl DbTxn, set: ValidatorSet, number: u64, hash: [u8; 32]) {
+    CosignTransactions::send(txn, set.network, &(set.session, number, hash))
   }
 }