Expose tier storage configuration across the FFI boundary

Add UniFFI-facing store abstractions and builder APIs so foreign-language
callers can configure backup and ephemeral stores when constructing nodes
with custom storage.

This introduces `FfiDynStoreTrait` as an FFI-safe equivalent of
`DynStoreTrait`, along with a Rust-side adapter that bridges foreign store
implementations into the internal dynamic store abstraction used by the
builder.

As part of this change:
- add UniFFI bindings for custom primary, backup, and ephemeral stores
- expose `Builder::set_backup_store`, `Builder::set_ephemeral_store`, and
  `Builder::build_with_store` on the FFI surface
- route FFI-backed builder construction through the native dyn-store path
- move FFI IO-related types into a dedicated module
- preserve per-key write ordering across the FFI boundary
- route Rust-side sync access through the async mutation path to avoid
  runtime-sensitive locking behavior

Author: enigbe

bindings/ldk_node.udl

@@ -32,6 +32,28 @@ interface LogWriter {
 	void log(LogRecord record);
 };
 
+[Trait, WithForeign]
+interface FfiDynStoreTrait {
+	[Throws=IOError, Async]
+	bytes read_async(string primary_namespace, string secondary_namespace, string key);
+	[Throws=IOError, Async]
+	void write_async(string primary_namespace, string secondary_namespace, string key, bytes buf);
+	[Throws=IOError, Async]
+	void remove_async(string primary_namespace, string secondary_namespace, string key, boolean lazy);
+	[Throws=IOError, Async]
+	sequence<string> list_async(string primary_namespace, string secondary_namespace);
+
+	[Throws=IOError]
+	bytes read(string primary_namespace, string secondary_namespace, string key);
+	[Throws=IOError]
+	void write(string primary_namespace, string secondary_namespace, string key, bytes buf);
+	[Throws=IOError]
+	void remove(string primary_namespace, string secondary_namespace, string key, boolean lazy);
+	[Throws=IOError]
+	sequence<string> list(string primary_namespace, string secondary_namespace);
+};
+
+
 interface Builder {
 	constructor();
 	[Name=from_config]
@@ -58,6 +80,8 @@ interface Builder {
 	void set_tor_config(TorConfig tor_config);
 	[Throws=BuildError]
 	void set_node_alias(string node_alias);
+	void set_backup_store(FfiDynStoreTrait backup_store);
+	void set_ephemeral_store(FfiDynStoreTrait ephemeral_store);
 	[Throws=BuildError]
 	void set_async_payments_role(AsyncPaymentsRole? role);
 	void set_wallet_recovery_mode();
@@ -73,6 +97,8 @@ interface Builder {
 	Node build_with_vss_store_and_fixed_headers(NodeEntropy node_entropy, string vss_url, string store_id, record<string, string> fixed_headers);
 	[Throws=BuildError]
 	Node build_with_vss_store_and_header_provider(NodeEntropy node_entropy, string vss_url, string store_id, VssHeaderProvider header_provider);
+	[Throws=BuildError]
+	Node build_with_store(NodeEntropy node_entropy, FfiDynStoreTrait store);
 };
 
 interface Node {
@@ -227,6 +253,8 @@ enum NodeError {
 	"InvalidLnurl",
 };
 
+typedef enum IOError;
+
 typedef dictionary NodeStatus;
 
 [Remote]

src/builder.rs

@@ -52,6 +52,8 @@ use crate::connection::ConnectionManager;
 use crate::entropy::NodeEntropy;
 use crate::event::EventQueue;
 use crate::fee_estimator::OnchainFeeEstimator;
+#[cfg(feature = "uniffi")]
+use crate::ffi::{FfiDynStore, FfiDynStoreTrait};
 use crate::gossip::GossipSource;
 use crate::io::sqlite_store::SqliteStore;
 use crate::io::tier_store::TierStore;
@@ -1152,6 +1154,32 @@ impl ArcedNodeBuilder {
 		self.inner.write().unwrap().set_wallet_recovery_mode();
 	}
 
+	/// Configures the backup store for local disaster recovery.
+	///
+	/// When building with tiered storage, this store receives a second durable
+	/// copy of data written to the primary store.
+	///
+	/// Writes and removals for primary-backed data only succeed once both the
+	/// primary and backup stores complete successfully.
+	///
+	/// If not set, durable data will be stored only in the primary store.
+	pub fn set_backup_store(&self, backup_store: Arc<dyn FfiDynStoreTrait>) {
+		let store: Arc<DynStore> = Arc::new(FfiDynStore::from_store(backup_store));
+		self.inner.write().unwrap().set_backup_store(store);
+	}
+
+	/// Configures the ephemeral store for non-critical, frequently-accessed data.
+	///
+	/// When building with tiered storage, this store is used for ephemeral data like
+	/// the network graph and scorer data to reduce latency for reads. Data stored here
+	/// can be rebuilt if lost.
+	///
+	/// If not set, non-critical data will be stored in the primary store.
+	pub fn set_ephemeral_store(&self, ephemeral_store: Arc<dyn FfiDynStoreTrait>) {
+		let store: Arc<DynStore> = Arc::new(FfiDynStore::from_store(ephemeral_store));
+		self.inner.write().unwrap().set_ephemeral_store(store);
+	}
+
 	/// Builds a [`Node`] instance with a [`SqliteStore`] backend and according to the options
 	/// previously configured.
 	pub fn build(&self, node_entropy: Arc<NodeEntropy>) -> Result<Arc<Node>, BuildError> {
@@ -1280,12 +1308,19 @@ impl ArcedNodeBuilder {
 	}
 
 	/// Builds a [`Node`] instance according to the options previously configured.
-	// Note that the generics here don't actually work for Uniffi, but we don't currently expose
-	// this so its not needed.
-	pub fn build_with_store<S: SyncAndAsyncKVStore + Send + Sync + 'static>(
-		&self, node_entropy: Arc<NodeEntropy>, kv_store: S,
+	///
+	/// The provided `kv_store` will be used as the primary storage backend. Optionally,
+	/// an ephemeral store for frequently-accessed non-critical data (e.g., network graph, scorer)
+	/// and a backup store for local disaster recovery can be configured via
+	/// [`set_ephemeral_store`] and [`set_backup_store`].
+	///
+	/// [`set_ephemeral_store`]: Self::set_ephemeral_store
+	/// [`set_backup_store`]: Self::set_backup_store
+	pub fn build_with_store(
+		&self, node_entropy: Arc<NodeEntropy>, kv_store: Arc<dyn FfiDynStoreTrait>,
 	) -> Result<Arc<Node>, BuildError> {
-		self.inner.read().unwrap().build_with_store(*node_entropy, kv_store).map(Arc::new)
+		let store: Arc<DynStore> = Arc::new(FfiDynStore::from_store(kv_store));
+		self.inner.read().unwrap().build_with_dynstore(*node_entropy, store).map(Arc::new)
 	}
 }