Add a message queue

This is intended to be a reliable transport between the processors and
coordinator. Since it'll be intranet only, it's written as never fail.

Primarily needs testing and a proper ID.

message-queue/src/queue.rs

@@ -0,0 +1,57 @@
+use serai_db::{DbTxn, Db};
+
+use crate::messages::*;
+
+#[derive(Clone, Debug)]
+pub(crate) struct Queue<D: Db>(pub(crate) D, pub(crate) Service);
+impl<D: Db> Queue<D> {
+  fn key(domain: &'static [u8], key: impl AsRef<[u8]>) -> Vec<u8> {
+    [&[u8::try_from(domain.len()).unwrap()], domain, key.as_ref()].concat()
+  }
+
+  fn message_count_key(&self) -> Vec<u8> {
+    Self::key(b"message_count", serde_json::to_vec(&self.1).unwrap())
+  }
+  pub(crate) fn message_count(&self) -> u64 {
+    self
+      .0
+      .get(self.message_count_key())
+      .map(|bytes| u64::from_le_bytes(bytes.try_into().unwrap()))
+      .unwrap_or(0)
+  }
+
+  fn last_acknowledged_key(&self) -> Vec<u8> {
+    Self::key(b"last_acknowledged", serde_json::to_vec(&self.1).unwrap())
+  }
+  pub(crate) fn last_acknowledged(&self) -> Option<u64> {
+    self
+      .0
+      .get(self.last_acknowledged_key())
+      .map(|bytes| u64::from_le_bytes(bytes.try_into().unwrap()))
+  }
+
+  fn message_key(&self, id: u64) -> Vec<u8> {
+    Self::key(b"message", serde_json::to_vec(&(self.1, id)).unwrap())
+  }
+  pub(crate) fn queue_message(&mut self, msg: QueuedMessage) {
+    let id = self.message_count();
+    let msg_key = self.message_key(id);
+    let msg_count_key = self.message_count_key();
+
+    let mut txn = self.0.txn();
+    txn.put(msg_key, serde_json::to_vec(&msg).unwrap());
+    txn.put(msg_count_key, (id + 1).to_le_bytes());
+    txn.commit();
+  }
+
+  pub(crate) fn get_message(&self, id: u64) -> Option<QueuedMessage> {
+    self.0.get(self.message_key(id)).map(|bytes| serde_json::from_slice(&bytes).unwrap())
+  }
+
+  pub(crate) fn ack_message(&mut self, id: u64) {
+    let ack_key = self.last_acknowledged_key();
+    let mut txn = self.0.txn();
+    txn.put(ack_key, id.to_le_bytes());
+    txn.commit();
+  }
+}