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/blobservice/mod.rs

@@ -0,0 +1,62 @@
+use std::io;
+use tonic::async_trait;
+
+use crate::{B3Digest, Error};
+
+mod from_addr;
+mod grpc;
+mod memory;
+mod naive_seeker;
+mod sled;
+
+#[cfg(test)]
+mod tests;
+
+pub use self::from_addr::from_addr;
+pub use self::grpc::GRPCBlobService;
+pub use self::memory::MemoryBlobService;
+pub use self::sled::SledBlobService;
+
+/// The base trait all BlobService services need to implement.
+/// It provides functions to check whether a given blob exists,
+/// a way to get a [io::Read] to a blob, and a method to initiate writing a new
+/// Blob, which will return something implmenting io::Write, and providing a
+/// close funtion, to finalize a blob and get its digest.
+#[async_trait]
+pub trait BlobService: 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;
+
+    /// Check if the service has the blob, by its content hash.
+    async fn has(&self, digest: &B3Digest) -> Result<bool, Error>;
+
+    /// Request a blob from the store, by its content hash.
+    async fn open_read(&self, digest: &B3Digest) -> Result<Option<Box<dyn BlobReader>>, Error>;
+
+    /// Insert a new blob into the store. Returns a [BlobWriter], which
+    /// implements [io::Write] and a [BlobWriter::close].
+    async fn open_write(&self) -> Box<dyn BlobWriter>;
+}
+
+/// A [tokio::io::AsyncWrite] that you need to close() afterwards, and get back
+/// the digest of the written blob.
+#[async_trait]
+pub trait BlobWriter: tokio::io::AsyncWrite + Send + Sync + Unpin + 'static {
+    /// Signal there's no more data to be written, and return the digest of the
+    /// contents written.
+    ///
+    /// Closing a already-closed BlobWriter is a no-op.
+    async fn close(&mut self) -> Result<B3Digest, Error>;
+}
+
+/// A [tokio::io::AsyncRead] that also allows seeking.
+pub trait BlobReader:
+    tokio::io::AsyncRead + tokio::io::AsyncSeek + tokio::io::AsyncBufRead + Send + Unpin + 'static
+{
+}
+
+/// A [`io::Cursor<Vec<u8>>`] can be used as a BlobReader.
+impl BlobReader for io::Cursor<Vec<u8>> {}