Allow combining different stores (#2489)

* Create skeleton of a combinator to use different stores based on the root key.

* Implement the remaining functions

linera-storage/src/db_storage.rs

@@ -21,6 +21,7 @@ use linera_execution::{
     WasmRuntime,
 };
 use linera_views::{
+    backends::dual::{DualStoreRootKeyAssignment, StoreInUse},
     batch::Batch,
     common::KeyValueStore,
     context::ViewContext,
@@ -227,6 +228,20 @@ enum BaseKey {
     BlobState(BlobId),
 }
 
+/// An implementation of [`DualStoreRootKeyAssignment`] that stores the
+/// chain states into the first store.
+pub struct ChainStatesFirstAssignment;
+
+impl DualStoreRootKeyAssignment for ChainStatesFirstAssignment {
+    fn assigned_store(root_key: &[u8]) -> Result<StoreInUse, bcs::Error> {
+        let store = match bcs::from_bytes(root_key)? {
+            BaseKey::ChainState(_) => StoreInUse::First,
+            _ => StoreInUse::Second,
+        };
+        Ok(store)
+    }
+}
+
 /// A `Clock` implementation using the system clock.
 #[derive(Clone)]
 pub struct WallClock;

linera-storage/src/lib.rs

@@ -37,7 +37,7 @@ use linera_views::{
 
 #[cfg(with_testing)]
 pub use crate::db_storage::TestClock;
-pub use crate::db_storage::{DbStorage, WallClock};
+pub use crate::db_storage::{ChainStatesFirstAssignment, DbStorage, WallClock};
 #[cfg(with_metrics)]
 pub use crate::db_storage::{
     READ_CERTIFICATE_COUNTER, READ_HASHED_CERTIFICATE_VALUE_COUNTER, WRITE_CERTIFICATE_COUNTER,