refactor(tvix): move castore into tvix-castore crate

This splits the pure content-addressed layers from tvix-store into a
`castore` crate, and only leaves PathInfo related things, as well as the
CLI entrypoint in the tvix-store crate.

Notable changes:
 - `fixtures` and `utils` had to be moved out of the `test` cfg, so they
   can be imported from tvix-store.
 - Some ad-hoc fixtures in the test were moved to proper fixtures in the
   same step.
 - The protos are now created by a (more static) recipe in the protos/
   directory.

The (now two) golang targets are commented out, as it's not possible to
update them properly in the same CL. This will be done by a followup CL
once this is merged (and whitby deployed)

Bug: https://b.tvl.fyi/issues/301

Change-Id: I8d675d4bf1fb697eb7d479747c1b1e3635718107
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9370
Reviewed-by: tazjin <tazjin@tvl.su>
Reviewed-by: flokli <flokli@flokli.de>
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: Connor Brewster <cbrewster@hey.com>

tvix/castore/src/directoryservice/mod.rs

@@ -0,0 +1,76 @@
+use crate::{proto, B3Digest, Error};
+use futures::Stream;
+use std::pin::Pin;
+use tonic::async_trait;
+
+mod from_addr;
+mod grpc;
+mod memory;
+mod sled;
+mod traverse;
+mod utils;
+
+pub use self::from_addr::from_addr;
+pub use self::grpc::GRPCDirectoryService;
+pub use self::memory::MemoryDirectoryService;
+pub use self::sled::SledDirectoryService;
+pub use self::traverse::descend_to;
+
+/// The base trait all Directory services need to implement.
+/// This is a simple get and put of [crate::proto::Directory], returning their
+/// digest.
+#[async_trait]
+pub trait DirectoryService: Send + Sync {
+    /// Create a new instance by passing in a connection URL.
+    /// TODO: check if we want to make this async, instead of lazily connecting
+    fn from_url(url: &url::Url) -> Result<Self, Error>
+    where
+        Self: Sized;
+
+    /// Get looks up a single Directory message by its digest.
+    /// In case the directory is not found, Ok(None) is returned.
+    async fn get(&self, digest: &B3Digest) -> Result<Option<proto::Directory>, Error>;
+    /// Get uploads a single Directory message, and returns the calculated
+    /// digest, or an error.
+    async fn put(&self, directory: proto::Directory) -> Result<B3Digest, Error>;
+
+    /// Looks up a closure of [proto::Directory].
+    /// Ideally this would be a `impl Stream<Item = Result<proto::Directory, Error>>`,
+    /// and we'd be able to add a default implementation for it here, but
+    /// we can't have that yet.
+    ///
+    /// This returns a pinned, boxed stream. The pinning allows for it to be polled easily,
+    /// and the box allows different underlying stream implementations to be returned since
+    /// Rust doesn't support this as a generic in traits yet. This is the same thing that
+    /// [async_trait] generates, but for streams instead of futures.
+    fn get_recursive(
+        &self,
+        root_directory_digest: &B3Digest,
+    ) -> Pin<Box<dyn Stream<Item = Result<proto::Directory, Error>> + Send>>;
+
+    /// Allows persisting a closure of [proto::Directory], which is a graph of
+    /// connected Directory messages.
+    fn put_multiple_start(&self) -> Box<dyn DirectoryPutter>;
+}
+
+/// Provides a handle to put a closure of connected [proto::Directory] elements.
+///
+/// The consumer can periodically call [DirectoryPutter::put], starting from the
+/// leaves. Once the root is reached, [DirectoryPutter::close] can be called to
+/// retrieve the root digest (or an error).
+#[async_trait]
+pub trait DirectoryPutter: Send {
+    /// 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`.
+    async fn put(&mut self, directory: proto::Directory) -> Result<(), Error>;
+
+    /// Close the stream, and wait for any errors.
+    async fn close(&mut self) -> Result<B3Digest, Error>;
+
+    /// Return whether the stream is closed or not.
+    /// Used from some [DirectoryService] implementations only.
+    fn is_closed(&self) -> bool;
+}