feat(tvix/store/directorysvc): add put_multiple_start

This provides a handle to upload multiple proto::Directory as part of
the same closure.

Change-Id: I9213dde257a260c8622239918ea541064b270484
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8356
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: flokli <flokli@flokli.de>

tvix/store/src/directoryservice/mod.rs

@@ -14,6 +14,7 @@ pub use self::utils::DirectoryTraverser;
 /// digest.
 pub trait DirectoryService {
     type DirectoriesIterator: Iterator<Item = Result<proto::Directory, Error>> + Send;
+    type DirectoryPutter: DirectoryPutter;
 
     /// Get looks up a single Directory message by its digest.
     /// In case the directory is not found, Ok(None) is returned.
@@ -27,4 +28,25 @@ pub trait DirectoryService {
     /// and we'd be able to add a default implementation for it here, but
     /// we can't have that yet.
     fn get_recursive(&self, root_directory_digest: &[u8; 32]) -> Self::DirectoriesIterator;
+
+    /// Allows persisting a closure of [proto::Directory], which is a graph of
+    /// connected Directory messages.
+    fn put_multiple_start(&self) -> Self::DirectoryPutter;
+}
+
+/// Provides a handle to put a closure of connected [proto::Directory] elements.
+///
+/// The consumer can periodically call [put], starting from the leaves. Once
+/// the root is reached, [close] can be called to retrieve the root digest (or
+/// an error).
+pub trait DirectoryPutter {
+    /// Put a individual [proto::Directory] into the store.
+    /// Error semantics and behaviour is up to the specific implementation of
+    /// this trait.
+    /// Due to bursting, the returned error might refer to an object previously
+    /// sent via `put`.
+    fn put(&mut self, directory: proto::Directory) -> Result<(), Error>;
+
+    /// Close the stream, and wait for any errors.
+    fn close(&mut self) -> Result<[u8; 32], Error>;
 }