walletrpc: add output leases RPCs

lnrpc/walletrpc/walletkit.proto

@@ -10,6 +10,22 @@ option go_package = "github.com/lightningnetwork/lnd/lnrpc/walletrpc";
 // WalletKit is a service that gives access to the core functionalities of the
 // daemon's wallet.
 service WalletKit {
+    /*
+    LeaseOutput locks an output to the given ID, preventing it from being
+    available for any future coin selection attempts. The absolute time of the
+    lock's expiration is returned. The expiration of the lock can be extended by
+    successive invocations of this RPC. Outputs can be unlocked before their
+    expiration through `ReleaseOutput`.
+    */
+    rpc LeaseOutput (LeaseOutputRequest) returns (LeaseOutputResponse);
+
+    /*
+    ReleaseOutput unlocks an output, allowing it to be available for coin
+    selection if it remains unspent. The ID should match the one used to
+    originally lock the output.
+    */
+    rpc ReleaseOutput (ReleaseOutputRequest) returns (ReleaseOutputResponse);
+
     /*
     DeriveNextKey attempts to derive the *next* key within the key family
     (account in BIP43) specified. This method should return the next external
@@ -108,6 +124,35 @@ service WalletKit {
         returns (LabelTransactionResponse);
 }
 
+message LeaseOutputRequest {
+    /*
+    An ID of 32 random bytes that must be unique for each distinct application
+    using this RPC which will be used to bound the output lease to.
+    */
+    bytes id = 1;
+
+    // The identifying outpoint of the output being leased.
+    lnrpc.OutPoint outpoint = 2;
+}
+
+message LeaseOutputResponse {
+    /*
+    The absolute expiration of the output lease represented as a unix timestamp.
+    */
+    uint64 expiration = 1;
+}
+
+message ReleaseOutputRequest {
+    // The unique ID that was used to lock the output.
+    bytes id = 1;
+
+    // The identifying outpoint of the output being released.
+    lnrpc.OutPoint outpoint = 2;
+}
+
+message ReleaseOutputResponse {
+}
+
 message KeyReq {
     /*
     Is the key finger print of the root pubkey that this request is targeting.