Support multiple batches per block by the coordinator

Also corrects an assumption block hash == batch ID.

coordinator/src/db.rs

@@ -1,3 +1,6 @@
+use scale::Encode;
+use serai_client::primitives::{NetworkId, BlockHash};
+
 pub use serai_db::*;
 
 use crate::tributary::TributarySpec;
@@ -42,6 +45,42 @@ impl<'a, D: Db> MainDb<'a, D> {
     txn.commit();
   }
 
+  fn batches_in_block_key(network: NetworkId, block: [u8; 32]) -> Vec<u8> {
+    Self::main_key(b"batches_in_block", (network, block).encode())
+  }
+  pub fn batches_in_block<G: Get>(
+    getter: &G,
+    network: NetworkId,
+    block: [u8; 32],
+  ) -> Vec<[u8; 32]> {
+    getter
+      .get(Self::batches_in_block_key(network, block))
+      .expect("asking for batches in block for block without batches")
+      .chunks(32)
+      .map(|id| id.try_into().unwrap())
+      .collect()
+  }
+  pub fn add_batch_to_block(
+    txn: &mut D::Transaction<'_>,
+    network: NetworkId,
+    block: BlockHash,
+    id: [u8; 32],
+  ) {
+    let key = Self::batches_in_block_key(network, block.0);
+    let Some(mut existing) = txn.get(&key) else {
+      txn.put(&key, block.0);
+      return;
+    };
+
+    if existing.chunks(32).any(|existing_id| existing_id == id) {
+      // TODO: Is this an invariant?
+      return;
+    }
+
+    existing.extend(block.0);
+    txn.put(&key, existing);
+  }
+
   fn first_preprocess_key(id: [u8; 32]) -> Vec<u8> {
     Self::main_key(b"first_preprocess", id)
   }