Add scheduler-primitives

The main benefit is whatever scheduler is in use, we now have a single API to
receive TXs to sign (which is of value to the TX signer crate we'll inevitably
build).

processor/scheduler/primitives/src/lib.rs

@@ -0,0 +1,48 @@
+#![cfg_attr(docsrs, feature(doc_auto_cfg))]
+#![doc = include_str!("../README.md")]
+#![deny(missing_docs)]
+
+use core::marker::PhantomData;
+use std::io;
+
+use group::GroupEncoding;
+
+use serai_db::DbTxn;
+
+/// A signable transaction.
+pub trait SignableTransaction: 'static + Sized + Send + Sync {
+  /// Read a `SignableTransaction`.
+  fn read(reader: &mut impl io::Read) -> io::Result<Self>;
+  /// Write a `SignableTransaction`.
+  fn write(&self, writer: &mut impl io::Write) -> io::Result<()>;
+}
+
+mod db {
+  use serai_db::{Get, DbTxn, create_db, db_channel};
+
+  db_channel! {
+    SchedulerPrimitives {
+      TransactionsToSign: (key: &[u8]) -> Vec<u8>,
+    }
+  }
+}
+
+/// The transactions to sign, as scheduled by a Scheduler.
+pub struct TransactionsToSign<T>(PhantomData<T>);
+impl<T: SignableTransaction> TransactionsToSign<T> {
+  /// Send a transaction to sign.
+  pub fn send(txn: &mut impl DbTxn, key: &impl GroupEncoding, tx: &T) {
+    let mut buf = Vec::with_capacity(128);
+    tx.write(&mut buf).unwrap();
+    db::TransactionsToSign::send(txn, key.to_bytes().as_ref(), &buf);
+  }
+
+  /// Try to receive a transaction to sign.
+  pub fn try_recv(txn: &mut impl DbTxn, key: &impl GroupEncoding) -> Option<T> {
+    let tx = db::TransactionsToSign::try_recv(txn, key.to_bytes().as_ref())?;
+    let mut tx = tx.as_slice();
+    let res = T::read(&mut tx).unwrap();
+    assert!(tx.is_empty());
+    Some(res)
+  }
+}