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/Cargo.toml

@@ -0,0 +1,25 @@
+[package]
+name = "serai-processor-scheduler-primitives"
+version = "0.1.0"
+description = "Primitives for schedulers for the Serai processor"
+license = "AGPL-3.0-only"
+repository = "https://github.com/serai-dex/serai/tree/develop/processor/scheduler/primitives"
+authors = ["Luke Parker <lukeparker5132@gmail.com>"]
+keywords = []
+edition = "2021"
+publish = false
+
+[package.metadata.docs.rs]
+all-features = true
+rustdoc-args = ["--cfg", "docsrs"]
+
+[lints]
+workspace = true
+
+[dependencies]
+group = { version = "0.13", default-features = false }
+
+scale = { package = "parity-scale-codec", version = "3", default-features = false, features = ["std"] }
+borsh = { version = "1", default-features = false, features = ["std", "derive", "de_strict_order"] }
+
+serai-db = { path = "../../../common/db" }

processor/scheduler/primitives/LICENSE

@@ -0,0 +1,15 @@
+AGPL-3.0-only license
+
+Copyright (c) 2024 Luke Parker
+
+This program is free software: you can redistribute it and/or modify
+it under the terms of the GNU Affero General Public License Version 3 as
+published by the Free Software Foundation.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+GNU Affero General Public License for more details.
+
+You should have received a copy of the GNU Affero General Public License
+along with this program. If not, see <http://www.gnu.org/licenses/>.

processor/scheduler/primitives/README.md

@@ -0,0 +1,3 @@
+# Scheduler Primitives
+
+Primitives for schedulers.

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)
+  }
+}