refactor(tvix): move castore into tvix-castore crate r/6629

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>

7 files changed, 1478 insertions, 0 deletions

diff --git a/tvix/castore/src/blobservice/from_addr.rs b/tvix/castore/src/blobservice/from_addr.rs
new file mode 100644
index 0000000000..2e0a30697d
--- /dev/null
+++ b/tvix/castore/src/blobservice/from_addr.rs
@@ -0,0 +1,30 @@
+use std::sync::Arc;
+use url::Url;
+
+use super::{BlobService, GRPCBlobService, MemoryBlobService, SledBlobService};
+
+/// Constructs a new instance of a [BlobService] from an URI.
+///
+/// The following schemes are supported by the following services:
+/// - `memory://` ([MemoryBlobService])
+/// - `sled://` ([SledBlobService])
+/// - `grpc+*://` ([GRPCBlobService])
+///
+/// See their `from_url` methods for more details about their syntax.
+pub fn from_addr(uri: &str) -> Result<Arc<dyn BlobService>, crate::Error> {
+    let url = Url::parse(uri)
+        .map_err(|e| crate::Error::StorageError(format!("unable to parse url: {}", e)))?;
+
+    Ok(if url.scheme() == "memory" {
+        Arc::new(MemoryBlobService::from_url(&url)?)
+    } else if url.scheme() == "sled" {
+        Arc::new(SledBlobService::from_url(&url)?)
+    } else if url.scheme().starts_with("grpc+") {
+        Arc::new(GRPCBlobService::from_url(&url)?)
+    } else {
+        Err(crate::Error::StorageError(format!(
+            "unknown scheme: {}",
+            url.scheme()
+        )))?
+    })
+}
diff --git a/tvix/castore/src/blobservice/grpc.rs b/tvix/castore/src/blobservice/grpc.rs
new file mode 100644
index 0000000000..db9d4a9c00
--- /dev/null
+++ b/tvix/castore/src/blobservice/grpc.rs
@@ -0,0 +1,426 @@
+use super::{naive_seeker::NaiveSeeker, BlobReader, BlobService, BlobWriter};
+use crate::{proto, B3Digest};
+use futures::sink::SinkExt;
+use futures::TryFutureExt;
+use std::{
+    collections::VecDeque,
+    io::{self},
+    pin::pin,
+    task::Poll,
+};
+use tokio::io::AsyncWriteExt;
+use tokio::{net::UnixStream, task::JoinHandle};
+use tokio_stream::{wrappers::ReceiverStream, StreamExt};
+use tokio_util::{
+    io::{CopyToBytes, SinkWriter},
+    sync::{PollSendError, PollSender},
+};
+use tonic::{async_trait, transport::Channel, Code, Status};
+use tracing::instrument;
+
+/// Connects to a (remote) tvix-store BlobService over gRPC.
+#[derive(Clone)]
+pub struct GRPCBlobService {
+    /// The internal reference to a gRPC client.
+    /// Cloning it is cheap, and it internally handles concurrent requests.
+    grpc_client: proto::blob_service_client::BlobServiceClient<Channel>,
+}
+
+impl GRPCBlobService {
+    /// construct a [GRPCBlobService] from a [proto::blob_service_client::BlobServiceClient].
+    /// panics if called outside the context of a tokio runtime.
+    pub fn from_client(
+        grpc_client: proto::blob_service_client::BlobServiceClient<Channel>,
+    ) -> Self {
+        Self { grpc_client }
+    }
+}
+
+#[async_trait]
+impl BlobService for GRPCBlobService {
+    /// Constructs a [GRPCBlobService] from the passed [url::Url]:
+    /// - scheme has to match `grpc+*://`.
+    ///   That's normally grpc+unix for unix sockets, and grpc+http(s) for the HTTP counterparts.
+    /// - In the case of unix sockets, there must be a path, but may not be a host.
+    /// - In the case of non-unix sockets, there must be a host, but no path.
+    fn from_url(url: &url::Url) -> Result<Self, crate::Error> {
+        // Start checking for the scheme to start with grpc+.
+        match url.scheme().strip_prefix("grpc+") {
+            None => Err(crate::Error::StorageError("invalid scheme".to_string())),
+            Some(rest) => {
+                if rest == "unix" {
+                    if url.host_str().is_some() {
+                        return Err(crate::Error::StorageError(
+                            "host may not be set".to_string(),
+                        ));
+                    }
+                    let path = url.path().to_string();
+                    let channel = tonic::transport::Endpoint::try_from("http://[::]:50051") // doesn't matter
+                        .unwrap()
+                        .connect_with_connector_lazy(tower::service_fn(
+                            move |_: tonic::transport::Uri| UnixStream::connect(path.clone()),
+                        ));
+                    let grpc_client = proto::blob_service_client::BlobServiceClient::new(channel);
+                    Ok(Self::from_client(grpc_client))
+                } else {
+                    // ensure path is empty, not supported with gRPC.
+                    if !url.path().is_empty() {
+                        return Err(crate::Error::StorageError(
+                            "path may not be set".to_string(),
+                        ));
+                    }
+
+                    // clone the uri, and drop the grpc+ from the scheme.
+                    // Recreate a new uri with the `grpc+` prefix dropped from the scheme.
+                    // We can't use `url.set_scheme(rest)`, as it disallows
+                    // setting something http(s) that previously wasn't.
+                    let url = {
+                        let url_str = url.to_string();
+                        let s_stripped = url_str.strip_prefix("grpc+").unwrap();
+                        url::Url::parse(s_stripped).unwrap()
+                    };
+                    let channel = tonic::transport::Endpoint::try_from(url.to_string())
+                        .unwrap()
+                        .connect_lazy();
+
+                    let grpc_client = proto::blob_service_client::BlobServiceClient::new(channel);
+                    Ok(Self::from_client(grpc_client))
+                }
+            }
+        }
+    }
+
+    #[instrument(skip(self, digest), fields(blob.digest=%digest))]
+    async fn has(&self, digest: &B3Digest) -> Result<bool, crate::Error> {
+        let mut grpc_client = self.grpc_client.clone();
+        let resp = grpc_client
+            .stat(proto::StatBlobRequest {
+                digest: digest.clone().into(),
+            })
+            .await;
+
+        match resp {
+            Ok(_blob_meta) => Ok(true),
+            Err(e) if e.code() == Code::NotFound => Ok(false),
+            Err(e) => Err(crate::Error::StorageError(e.to_string())),
+        }
+    }
+
+    // On success, this returns a Ok(Some(io::Read)), which can be used to read
+    // the contents of the Blob, identified by the digest.
+    async fn open_read(
+        &self,
+        digest: &B3Digest,
+    ) -> Result<Option<Box<dyn BlobReader>>, crate::Error> {
+        // Get a new handle to the gRPC client, and copy the digest.
+        let mut grpc_client = self.grpc_client.clone();
+
+        // Get a stream of [proto::BlobChunk], or return an error if the blob
+        // doesn't exist.
+        let resp = grpc_client
+            .read(proto::ReadBlobRequest {
+                digest: digest.clone().into(),
+            })
+            .await;
+
+        // This runs the task to completion, which on success will return a stream.
+        // On reading from it, we receive individual [proto::BlobChunk], so we
+        // massage this to a stream of bytes,
+        // then create an [AsyncRead], which we'll turn into a [io::Read],
+        // that's returned from the function.
+        match resp {
+            Ok(stream) => {
+                // map the stream of proto::BlobChunk to bytes.
+                let data_stream = stream.into_inner().map(|x| {
+                    x.map(|x| VecDeque::from(x.data.to_vec()))
+                        .map_err(|e| std::io::Error::new(std::io::ErrorKind::InvalidInput, e))
+                });
+
+                // Use StreamReader::new to convert to an AsyncRead.
+                let data_reader = tokio_util::io::StreamReader::new(data_stream);
+
+                Ok(Some(Box::new(NaiveSeeker::new(data_reader))))
+            }
+            Err(e) if e.code() == Code::NotFound => Ok(None),
+            Err(e) => Err(crate::Error::StorageError(e.to_string())),
+        }
+    }
+
+    /// Returns a BlobWriter, that'll internally wrap each write in a
+    // [proto::BlobChunk], which is send to the gRPC server.
+    async fn open_write(&self) -> Box<dyn BlobWriter> {
+        let mut grpc_client = self.grpc_client.clone();
+
+        // set up an mpsc channel passing around Bytes.
+        let (tx, rx) = tokio::sync::mpsc::channel::<bytes::Bytes>(10);
+
+        // bytes arriving on the RX side are wrapped inside a
+        // [proto::BlobChunk], and a [ReceiverStream] is constructed.
+        let blobchunk_stream = ReceiverStream::new(rx).map(|x| proto::BlobChunk { data: x });
+
+        // That receiver stream is used as a stream in the gRPC BlobService.put rpc call.
+        let task: JoinHandle<Result<_, Status>> =
+            tokio::spawn(async move { Ok(grpc_client.put(blobchunk_stream).await?.into_inner()) });
+
+        // The tx part of the channel is converted to a sink of byte chunks.
+
+        // We need to make this a function pointer, not a closure.
+        fn convert_error(_: PollSendError<bytes::Bytes>) -> io::Error {
+            io::Error::from(io::ErrorKind::BrokenPipe)
+        }
+
+        let sink = PollSender::new(tx)
+            .sink_map_err(convert_error as fn(PollSendError<bytes::Bytes>) -> io::Error);
+        // We need to explicitly cast here, otherwise rustc does error with "expected fn pointer, found fn item"
+
+        // … which is turned into an [tokio::io::AsyncWrite].
+        let writer = SinkWriter::new(CopyToBytes::new(sink));
+
+        Box::new(GRPCBlobWriter {
+            task_and_writer: Some((task, writer)),
+            digest: None,
+        })
+    }
+}
+
+pub struct GRPCBlobWriter<W: tokio::io::AsyncWrite> {
+    /// The task containing the put request, and the inner writer, if we're still writing.
+    task_and_writer: Option<(JoinHandle<Result<proto::PutBlobResponse, Status>>, W)>,
+
+    /// The digest that has been returned, if we successfully closed.
+    digest: Option<B3Digest>,
+}
+
+#[async_trait]
+impl<W: tokio::io::AsyncWrite + Send + Sync + Unpin + 'static> BlobWriter for GRPCBlobWriter<W> {
+    async fn close(&mut self) -> Result<B3Digest, crate::Error> {
+        if self.task_and_writer.is_none() {
+            // if we're already closed, return the b3 digest, which must exist.
+            // If it doesn't, we already closed and failed once, and didn't handle the error.
+            match &self.digest {
+                Some(digest) => Ok(digest.clone()),
+                None => Err(crate::Error::StorageError(
+                    "previously closed with error".to_string(),
+                )),
+            }
+        } else {
+            let (task, mut writer) = self.task_and_writer.take().unwrap();
+
+            // invoke shutdown, so the inner writer closes its internal tx side of
+            // the channel.
+            writer
+                .shutdown()
+                .map_err(|e| crate::Error::StorageError(e.to_string()))
+                .await?;
+
+            // block on the RPC call to return.
+            // This ensures all chunks are sent out, and have been received by the
+            // backend.
+
+            match task.await? {
+                Ok(resp) => {
+                    // return the digest from the response, and store it in self.digest for subsequent closes.
+                    let digest: B3Digest = resp.digest.try_into().map_err(|_| {
+                        crate::Error::StorageError(
+                            "invalid root digest length in response".to_string(),
+                        )
+                    })?;
+                    self.digest = Some(digest.clone());
+                    Ok(digest)
+                }
+                Err(e) => Err(crate::Error::StorageError(e.to_string())),
+            }
+        }
+    }
+}
+
+impl<W: tokio::io::AsyncWrite + Unpin> tokio::io::AsyncWrite for GRPCBlobWriter<W> {
+    fn poll_write(
+        mut self: std::pin::Pin<&mut Self>,
+        cx: &mut std::task::Context<'_>,
+        buf: &[u8],
+    ) -> std::task::Poll<Result<usize, io::Error>> {
+        match &mut self.task_and_writer {
+            None => Poll::Ready(Err(io::Error::new(
+                io::ErrorKind::NotConnected,
+                "already closed",
+            ))),
+            Some((_, ref mut writer)) => {
+                let pinned_writer = pin!(writer);
+                pinned_writer.poll_write(cx, buf)
+            }
+        }
+    }
+
+    fn poll_flush(
+        mut self: std::pin::Pin<&mut Self>,
+        cx: &mut std::task::Context<'_>,
+    ) -> std::task::Poll<Result<(), io::Error>> {
+        match &mut self.task_and_writer {
+            None => Poll::Ready(Err(io::Error::new(
+                io::ErrorKind::NotConnected,
+                "already closed",
+            ))),
+            Some((_, ref mut writer)) => {
+                let pinned_writer = pin!(writer);
+                pinned_writer.poll_flush(cx)
+            }
+        }
+    }
+
+    fn poll_shutdown(
+        self: std::pin::Pin<&mut Self>,
+        _cx: &mut std::task::Context<'_>,
+    ) -> std::task::Poll<Result<(), io::Error>> {
+        // TODO(raitobezarius): this might not be a graceful shutdown of the
+        // channel inside the gRPC connection.
+        Poll::Ready(Ok(()))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::sync::Arc;
+    use std::thread;
+
+    use tempfile::TempDir;
+    use tokio::net::UnixListener;
+    use tokio::time;
+    use tokio_stream::wrappers::UnixListenerStream;
+
+    use crate::blobservice::MemoryBlobService;
+    use crate::fixtures;
+    use crate::proto::GRPCBlobServiceWrapper;
+
+    use super::BlobService;
+    use super::GRPCBlobService;
+
+    /// This uses the wrong scheme
+    #[test]
+    fn test_invalid_scheme() {
+        let url = url::Url::parse("http://foo.example/test").expect("must parse");
+
+        assert!(GRPCBlobService::from_url(&url).is_err());
+    }
+
+    /// This uses the correct scheme for a unix socket.
+    /// The fact that /path/to/somewhere doesn't exist yet is no problem, because we connect lazily.
+    #[tokio::test]
+    async fn test_valid_unix_path() {
+        let url = url::Url::parse("grpc+unix:///path/to/somewhere").expect("must parse");
+
+        assert!(GRPCBlobService::from_url(&url).is_ok());
+    }
+
+    /// This uses the correct scheme for a unix socket,
+    /// but sets a host, which is unsupported.
+    #[tokio::test]
+    async fn test_invalid_unix_path_with_domain() {
+        let url =
+            url::Url::parse("grpc+unix://host.example/path/to/somewhere").expect("must parse");
+
+        assert!(GRPCBlobService::from_url(&url).is_err());
+    }
+
+    /// This uses the correct scheme for a HTTP server.
+    /// The fact that nothing is listening there is no problem, because we connect lazily.
+    #[tokio::test]
+    async fn test_valid_http() {
+        let url = url::Url::parse("grpc+http://localhost").expect("must parse");
+
+        assert!(GRPCBlobService::from_url(&url).is_ok());
+    }
+
+    /// This uses the correct scheme for a HTTPS server.
+    /// The fact that nothing is listening there is no problem, because we connect lazily.
+    #[tokio::test]
+    async fn test_valid_https() {
+        let url = url::Url::parse("grpc+https://localhost").expect("must parse");
+
+        assert!(GRPCBlobService::from_url(&url).is_ok());
+    }
+
+    /// This uses the correct scheme, but also specifies
+    /// an additional path, which is not supported for gRPC.
+    /// The fact that nothing is listening there is no problem, because we connect lazily.
+    #[tokio::test]
+    async fn test_invalid_http_with_path() {
+        let url = url::Url::parse("grpc+https://localhost/some-path").expect("must parse");
+
+        assert!(GRPCBlobService::from_url(&url).is_err());
+    }
+
+    /// This uses the correct scheme for a unix socket, and provides a server on the other side.
+    /// This is not a tokio::test, because spawn two separate tokio runtimes and
+    // want to have explicit control.
+    #[test]
+    fn test_valid_unix_path_ping_pong() {
+        let tmpdir = TempDir::new().unwrap();
+        let path = tmpdir.path().join("daemon");
+
+        let path_clone = path.clone();
+
+        // Spin up a server, in a thread far away, which spawns its own tokio runtime,
+        // and blocks on the task.
+        thread::spawn(move || {
+            // Create the runtime
+            let rt = tokio::runtime::Runtime::new().unwrap();
+
+            let task = rt.spawn(async {
+                let uds = UnixListener::bind(path_clone).unwrap();
+                let uds_stream = UnixListenerStream::new(uds);
+
+                // spin up a new server
+                let mut server = tonic::transport::Server::builder();
+                let router =
+                    server.add_service(crate::proto::blob_service_server::BlobServiceServer::new(
+                        GRPCBlobServiceWrapper::from(
+                            Arc::new(MemoryBlobService::default()) as Arc<dyn BlobService>
+                        ),
+                    ));
+                router.serve_with_incoming(uds_stream).await
+            });
+
+            rt.block_on(task).unwrap().unwrap();
+        });
+
+        // Now create another tokio runtime which we'll use in the main test code.
+        let rt = tokio::runtime::Runtime::new().unwrap();
+
+        let task = rt.spawn(async move {
+            // wait for the socket to be created
+            {
+                let mut socket_created = false;
+                // TODO: exponential backoff urgently
+                for _try in 1..20 {
+                    if path.exists() {
+                        socket_created = true;
+                        break;
+                    }
+                    tokio::time::sleep(time::Duration::from_millis(20)).await;
+                }
+
+                assert!(
+                    socket_created,
+                    "expected socket path to eventually get created, but never happened"
+                );
+            }
+
+            // prepare a client
+            let client = {
+                let mut url =
+                    url::Url::parse("grpc+unix:///path/to/somewhere").expect("must parse");
+                url.set_path(path.to_str().unwrap());
+                GRPCBlobService::from_url(&url).expect("must succeed")
+            };
+
+            let has = client
+                .has(&fixtures::BLOB_A_DIGEST)
+                .await
+                .expect("must not be err");
+
+            assert!(!has);
+        });
+        rt.block_on(task).unwrap()
+    }
+}
diff --git a/tvix/castore/src/blobservice/memory.rs b/tvix/castore/src/blobservice/memory.rs
new file mode 100644
index 0000000000..383127344a
--- /dev/null
+++ b/tvix/castore/src/blobservice/memory.rs
@@ -0,0 +1,196 @@
+use std::io::{self, Cursor, Write};
+use std::task::Poll;
+use std::{
+    collections::HashMap,
+    sync::{Arc, RwLock},
+};
+use tonic::async_trait;
+use tracing::instrument;
+
+use super::{BlobReader, BlobService, BlobWriter};
+use crate::{B3Digest, Error};
+
+#[derive(Clone, Default)]
+pub struct MemoryBlobService {
+    db: Arc<RwLock<HashMap<B3Digest, Vec<u8>>>>,
+}
+
+#[async_trait]
+impl BlobService for MemoryBlobService {
+    /// Constructs a [MemoryBlobService] from the passed [url::Url]:
+    /// - scheme has to be `memory://`
+    /// - there may not be a host.
+    /// - there may not be a path.
+    fn from_url(url: &url::Url) -> Result<Self, Error> {
+        if url.scheme() != "memory" {
+            return Err(crate::Error::StorageError("invalid scheme".to_string()));
+        }
+
+        if url.has_host() || !url.path().is_empty() {
+            return Err(crate::Error::StorageError("invalid url".to_string()));
+        }
+
+        Ok(Self::default())
+    }
+
+    #[instrument(skip(self, digest), fields(blob.digest=%digest))]
+    async fn has(&self, digest: &B3Digest) -> Result<bool, Error> {
+        let db = self.db.read().unwrap();
+        Ok(db.contains_key(digest))
+    }
+
+    async fn open_read(&self, digest: &B3Digest) -> Result<Option<Box<dyn BlobReader>>, Error> {
+        let db = self.db.read().unwrap();
+
+        match db.get(digest).map(|x| Cursor::new(x.clone())) {
+            Some(result) => Ok(Some(Box::new(result))),
+            None => Ok(None),
+        }
+    }
+
+    #[instrument(skip(self))]
+    async fn open_write(&self) -> Box<dyn BlobWriter> {
+        Box::new(MemoryBlobWriter::new(self.db.clone()))
+    }
+}
+
+pub struct MemoryBlobWriter {
+    db: Arc<RwLock<HashMap<B3Digest, Vec<u8>>>>,
+
+    /// Contains the buffer Vec and hasher, or None if already closed
+    writers: Option<(Vec<u8>, blake3::Hasher)>,
+
+    /// The digest that has been returned, if we successfully closed.
+    digest: Option<B3Digest>,
+}
+
+impl MemoryBlobWriter {
+    fn new(db: Arc<RwLock<HashMap<B3Digest, Vec<u8>>>>) -> Self {
+        Self {
+            db,
+            writers: Some((Vec::new(), blake3::Hasher::new())),
+            digest: None,
+        }
+    }
+}
+impl tokio::io::AsyncWrite for MemoryBlobWriter {
+    fn poll_write(
+        mut self: std::pin::Pin<&mut Self>,
+        _cx: &mut std::task::Context<'_>,
+        b: &[u8],
+    ) -> std::task::Poll<Result<usize, io::Error>> {
+        Poll::Ready(match &mut self.writers {
+            None => Err(io::Error::new(
+                io::ErrorKind::NotConnected,
+                "already closed",
+            )),
+            Some((ref mut buf, ref mut hasher)) => {
+                let bytes_written = buf.write(b)?;
+                hasher.write(&b[..bytes_written])
+            }
+        })
+    }
+
+    fn poll_flush(
+        self: std::pin::Pin<&mut Self>,
+        _cx: &mut std::task::Context<'_>,
+    ) -> std::task::Poll<Result<(), io::Error>> {
+        Poll::Ready(match self.writers {
+            None => Err(io::Error::new(
+                io::ErrorKind::NotConnected,
+                "already closed",
+            )),
+            Some(_) => Ok(()),
+        })
+    }
+
+    fn poll_shutdown(
+        self: std::pin::Pin<&mut Self>,
+        _cx: &mut std::task::Context<'_>,
+    ) -> std::task::Poll<Result<(), io::Error>> {
+        // shutdown is "instantaneous", we only write to memory.
+        Poll::Ready(Ok(()))
+    }
+}
+
+#[async_trait]
+impl BlobWriter for MemoryBlobWriter {
+    async fn close(&mut self) -> Result<B3Digest, Error> {
+        if self.writers.is_none() {
+            match &self.digest {
+                Some(digest) => Ok(digest.clone()),
+                None => Err(crate::Error::StorageError(
+                    "previously closed with error".to_string(),
+                )),
+            }
+        } else {
+            let (buf, hasher) = self.writers.take().unwrap();
+
+            // We know self.hasher is doing blake3 hashing, so this won't fail.
+            let digest: B3Digest = hasher.finalize().as_bytes().into();
+
+            // Only insert if the blob doesn't already exist.
+            let db = self.db.read()?;
+            if !db.contains_key(&digest) {
+                // drop the read lock, so we can open for writing.
+                drop(db);
+
+                // open the database for writing.
+                let mut db = self.db.write()?;
+
+                // and put buf in there. This will move buf out.
+                db.insert(digest.clone(), buf);
+            }
+
+            self.digest = Some(digest.clone());
+
+            Ok(digest)
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::BlobService;
+    use super::MemoryBlobService;
+
+    /// This uses a wrong scheme.
+    #[test]
+    fn test_invalid_scheme() {
+        let url = url::Url::parse("http://foo.example/test").expect("must parse");
+
+        assert!(MemoryBlobService::from_url(&url).is_err());
+    }
+
+    /// This correctly sets the scheme, and doesn't set a path.
+    #[test]
+    fn test_valid_scheme() {
+        let url = url::Url::parse("memory://").expect("must parse");
+
+        assert!(MemoryBlobService::from_url(&url).is_ok());
+    }
+
+    /// This sets the host to `foo`
+    #[test]
+    fn test_invalid_host() {
+        let url = url::Url::parse("memory://foo").expect("must parse");
+
+        assert!(MemoryBlobService::from_url(&url).is_err());
+    }
+
+    /// This has the path "/", which is invalid.
+    #[test]
+    fn test_invalid_has_path() {
+        let url = url::Url::parse("memory:///").expect("must parse");
+
+        assert!(MemoryBlobService::from_url(&url).is_err());
+    }
+
+    /// This has the path "/foo", which is invalid.
+    #[test]
+    fn test_invalid_path2() {
+        let url = url::Url::parse("memory:///foo").expect("must parse");
+
+        assert!(MemoryBlobService::from_url(&url).is_err());
+    }
+}
diff --git a/tvix/castore/src/blobservice/mod.rs b/tvix/castore/src/blobservice/mod.rs
new file mode 100644
index 0000000000..5ecf25ac13
--- /dev/null
+++ b/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>> {}
diff --git a/tvix/castore/src/blobservice/naive_seeker.rs b/tvix/castore/src/blobservice/naive_seeker.rs
new file mode 100644
index 0000000000..e65a82c7f4
--- /dev/null
+++ b/tvix/castore/src/blobservice/naive_seeker.rs
@@ -0,0 +1,269 @@
+use super::BlobReader;
+use pin_project_lite::pin_project;
+use std::io;
+use std::task::Poll;
+use tokio::io::AsyncRead;
+use tracing::{debug, instrument};
+
+pin_project! {
+    /// This implements [tokio::io::AsyncSeek] for and [tokio::io::AsyncRead] by
+    /// simply skipping over some bytes, keeping track of the position.
+    /// It fails whenever you try to seek backwards.
+    ///
+    /// ## Pinning concerns:
+    ///
+    /// [NaiveSeeker] is itself pinned by callers, and we do not need to concern
+    /// ourselves regarding that.
+    ///
+    /// Though, its fields as per
+    /// <https://doc.rust-lang.org/std/pin/#pinning-is-not-structural-for-field>
+    /// can be pinned or unpinned.
+    ///
+    /// So we need to go over each field and choose our policy carefully.
+    ///
+    /// The obvious cases are the bookkeeping integers we keep in the structure,
+    /// those are private and not shared to anyone, we never build a
+    /// `Pin<&mut X>` out of them at any point, therefore, we can safely never
+    /// mark them as pinned. Of course, it is expected that no developer here
+    /// attempt to `pin!(self.pos)` to pin them because it makes no sense. If
+    /// they have to become pinned, they should be marked `#[pin]` and we need
+    /// to discuss it.
+    ///
+    /// So the bookkeeping integers are in the right state with respect to their
+    /// pinning status. The projection should offer direct access.
+    ///
+    /// On the `r` field, i.e. a `BufReader<R>`, given that
+    /// <https://docs.rs/tokio/latest/tokio/io/struct.BufReader.html#impl-Unpin-for-BufReader%3CR%3E>
+    /// is available, even a `Pin<&mut BufReader<R>>` can be safely moved.
+    ///
+    /// The only care we should have regards the internal reader itself, i.e.
+    /// the `R` instance, see that Tokio decided to `#[pin]` it too:
+    /// <https://docs.rs/tokio/latest/src/tokio/io/util/buf_reader.rs.html#29>
+    ///
+    /// In general, there's no `Unpin` instance for `R: tokio::io::AsyncRead`
+    /// (see <https://docs.rs/tokio/latest/tokio/io/trait.AsyncRead.html>).
+    ///
+    /// Therefore, we could keep it unpinned and pin it in every call site
+    /// whenever we need to call `poll_*` which can be confusing to the non-
+    /// expert developer and we have a fair share amount of situations where the
+    /// [BufReader] instance is naked, i.e. in its `&mut BufReader<R>`
+    /// form, this is annoying because it could lead to expose the naked `R`
+    /// internal instance somehow and would produce a risk of making it move
+    /// unexpectedly.
+    ///
+    /// We choose the path of the least resistance as we have no reason to have
+    /// access to the raw `BufReader<R>` instance, we just `#[pin]` it too and
+    /// enjoy its `poll_*` safe APIs and push the unpinning concerns to the
+    /// internal implementations themselves, which studied the question longer
+    /// than us.
+    pub struct NaiveSeeker<R: tokio::io::AsyncRead> {
+        #[pin]
+        r: tokio::io::BufReader<R>,
+        pos: u64,
+        bytes_to_skip: u64,
+    }
+}
+
+impl<R: tokio::io::AsyncRead> NaiveSeeker<R> {
+    pub fn new(r: R) -> Self {
+        NaiveSeeker {
+            r: tokio::io::BufReader::new(r),
+            pos: 0,
+            bytes_to_skip: 0,
+        }
+    }
+}
+
+impl<R: tokio::io::AsyncRead> tokio::io::AsyncRead for NaiveSeeker<R> {
+    fn poll_read(
+        self: std::pin::Pin<&mut Self>,
+        cx: &mut std::task::Context<'_>,
+        buf: &mut tokio::io::ReadBuf<'_>,
+    ) -> Poll<std::io::Result<()>> {
+        // The amount of data read can be determined by the increase
+        // in the length of the slice returned by `ReadBuf::filled`.
+        let filled_before = buf.filled().len();
+        let this = self.project();
+        let pos: &mut u64 = this.pos;
+
+        match this.r.poll_read(cx, buf) {
+            Poll::Ready(a) => {
+                let bytes_read = buf.filled().len() - filled_before;
+                *pos += bytes_read as u64;
+
+                Poll::Ready(a)
+            }
+            Poll::Pending => Poll::Pending,
+        }
+    }
+}
+
+impl<R: tokio::io::AsyncRead> tokio::io::AsyncBufRead for NaiveSeeker<R> {
+    fn poll_fill_buf(
+        self: std::pin::Pin<&mut Self>,
+        cx: &mut std::task::Context<'_>,
+    ) -> Poll<io::Result<&[u8]>> {
+        self.project().r.poll_fill_buf(cx)
+    }
+
+    fn consume(self: std::pin::Pin<&mut Self>, amt: usize) {
+        let this = self.project();
+        this.r.consume(amt);
+        let pos: &mut u64 = this.pos;
+        *pos += amt as u64;
+    }
+}
+
+impl<R: tokio::io::AsyncRead> tokio::io::AsyncSeek for NaiveSeeker<R> {
+    #[instrument(skip(self))]
+    fn start_seek(
+        self: std::pin::Pin<&mut Self>,
+        position: std::io::SeekFrom,
+    ) -> std::io::Result<()> {
+        let absolute_offset: u64 = match position {
+            io::SeekFrom::Start(start_offset) => {
+                if start_offset < self.pos {
+                    return Err(io::Error::new(
+                        io::ErrorKind::Unsupported,
+                        format!("can't seek backwards ({} -> {})", self.pos, start_offset),
+                    ));
+                } else {
+                    start_offset
+                }
+            }
+            // we don't know the total size, can't support this.
+            io::SeekFrom::End(_end_offset) => {
+                return Err(io::Error::new(
+                    io::ErrorKind::Unsupported,
+                    "can't seek from end",
+                ));
+            }
+            io::SeekFrom::Current(relative_offset) => {
+                if relative_offset < 0 {
+                    return Err(io::Error::new(
+                        io::ErrorKind::Unsupported,
+                        "can't seek backwards relative to current position",
+                    ));
+                } else {
+                    self.pos + relative_offset as u64
+                }
+            }
+        };
+
+        debug!(absolute_offset=?absolute_offset, "seek");
+
+        // we already know absolute_offset is larger than self.pos
+        debug_assert!(
+            absolute_offset >= self.pos,
+            "absolute_offset {} is larger than self.pos {}",
+            absolute_offset,
+            self.pos
+        );
+
+        // calculate bytes to skip
+        *self.project().bytes_to_skip = absolute_offset - self.pos;
+
+        Ok(())
+    }
+
+    #[instrument(skip(self))]
+    fn poll_complete(
+        mut self: std::pin::Pin<&mut Self>,
+        cx: &mut std::task::Context<'_>,
+    ) -> Poll<std::io::Result<u64>> {
+        if self.bytes_to_skip == 0 {
+            // return the new position (from the start of the stream)
+            return Poll::Ready(Ok(self.pos));
+        }
+
+        // discard some bytes, until pos is where we want it to be.
+        // We create a buffer that we'll discard later on.
+        let mut buf = [0; 1024];
+
+        // Loop until we've reached the desired seek position. This is done by issuing repeated
+        // `poll_read` calls. If the data is not available yet, we will yield back to the executor
+        // and wait to be polled again.
+        loop {
+            // calculate the length we want to skip at most, which is either a max
+            // buffer size, or the number of remaining bytes to read, whatever is
+            // smaller.
+            let bytes_to_skip = std::cmp::min(self.bytes_to_skip as usize, buf.len());
+
+            let mut read_buf = tokio::io::ReadBuf::new(&mut buf[..bytes_to_skip]);
+
+            match self.as_mut().poll_read(cx, &mut read_buf) {
+                Poll::Ready(_a) => {
+                    let bytes_read = read_buf.filled().len() as u64;
+
+                    if bytes_read == 0 {
+                        return Poll::Ready(Err(io::Error::new(
+                            io::ErrorKind::UnexpectedEof,
+                            format!(
+                                "tried to skip {} bytes, but only was able to skip {} until reaching EOF",
+                                bytes_to_skip, bytes_read
+                            ),
+                        )));
+                    }
+
+                    // calculate bytes to skip
+                    let bytes_to_skip = self.bytes_to_skip - bytes_read;
+
+                    *self.as_mut().project().bytes_to_skip = bytes_to_skip;
+
+                    if bytes_to_skip == 0 {
+                        return Poll::Ready(Ok(self.pos));
+                    }
+                }
+                Poll::Pending => return Poll::Pending,
+            };
+        }
+    }
+}
+
+impl<R: tokio::io::AsyncRead + Send + Unpin + 'static> BlobReader for NaiveSeeker<R> {}
+
+#[cfg(test)]
+mod tests {
+    use super::NaiveSeeker;
+    use std::io::{Cursor, SeekFrom};
+    use tokio::io::{AsyncReadExt, AsyncSeekExt};
+
+    /// This seek requires multiple `poll_read` as we use a 1024 bytes internal
+    /// buffer when doing the seek.
+    /// This ensures we don't hang indefinitely.
+    #[tokio::test]
+    async fn seek() {
+        let buf = vec![0u8; 4096];
+        let reader = Cursor::new(&buf);
+        let mut seeker = NaiveSeeker::new(reader);
+        seeker.seek(SeekFrom::Start(4000)).await.unwrap();
+    }
+
+    #[tokio::test]
+    async fn seek_read() {
+        let mut buf = vec![0u8; 2048];
+        buf.extend_from_slice(&[1u8; 2048]);
+        buf.extend_from_slice(&[2u8; 2048]);
+
+        let reader = Cursor::new(&buf);
+        let mut seeker = NaiveSeeker::new(reader);
+
+        let mut read_buf = vec![0u8; 1024];
+        seeker.read_exact(&mut read_buf).await.expect("must read");
+        assert_eq!(read_buf.as_slice(), &[0u8; 1024]);
+
+        seeker
+            .seek(SeekFrom::Current(1024))
+            .await
+            .expect("must seek");
+        seeker.read_exact(&mut read_buf).await.expect("must read");
+        assert_eq!(read_buf.as_slice(), &[1u8; 1024]);
+
+        seeker
+            .seek(SeekFrom::Start(2 * 2048))
+            .await
+            .expect("must seek");
+        seeker.read_exact(&mut read_buf).await.expect("must read");
+        assert_eq!(read_buf.as_slice(), &[2u8; 1024]);
+    }
+}
diff --git a/tvix/castore/src/blobservice/sled.rs b/tvix/castore/src/blobservice/sled.rs
new file mode 100644
index 0000000000..209f0b76fc
--- /dev/null
+++ b/tvix/castore/src/blobservice/sled.rs
@@ -0,0 +1,249 @@
+use super::{BlobReader, BlobService, BlobWriter};
+use crate::{B3Digest, Error};
+use std::{
+    io::{self, Cursor, Write},
+    path::PathBuf,
+    task::Poll,
+};
+use tonic::async_trait;
+use tracing::instrument;
+
+#[derive(Clone)]
+pub struct SledBlobService {
+    db: sled::Db,
+}
+
+impl SledBlobService {
+    pub fn new(p: PathBuf) -> Result<Self, sled::Error> {
+        let config = sled::Config::default().use_compression(true).path(p);
+        let db = config.open()?;
+
+        Ok(Self { db })
+    }
+
+    pub fn new_temporary() -> Result<Self, sled::Error> {
+        let config = sled::Config::default().temporary(true);
+        let db = config.open()?;
+
+        Ok(Self { db })
+    }
+}
+
+#[async_trait]
+impl BlobService for SledBlobService {
+    /// Constructs a [SledBlobService] from the passed [url::Url]:
+    /// - scheme has to be `sled://`
+    /// - there may not be a host.
+    /// - a path to the sled needs to be provided (which may not be `/`).
+    fn from_url(url: &url::Url) -> Result<Self, Error> {
+        if url.scheme() != "sled" {
+            return Err(crate::Error::StorageError("invalid scheme".to_string()));
+        }
+
+        if url.has_host() {
+            return Err(crate::Error::StorageError(format!(
+                "invalid host: {}",
+                url.host().unwrap()
+            )));
+        }
+
+        // TODO: expose compression and other parameters as URL parameters, drop new and new_temporary?
+        if url.path().is_empty() {
+            Self::new_temporary().map_err(|e| Error::StorageError(e.to_string()))
+        } else if url.path() == "/" {
+            Err(crate::Error::StorageError(
+                "cowardly refusing to open / with sled".to_string(),
+            ))
+        } else {
+            Self::new(url.path().into()).map_err(|e| Error::StorageError(e.to_string()))
+        }
+    }
+
+    #[instrument(skip(self), fields(blob.digest=%digest))]
+    async fn has(&self, digest: &B3Digest) -> Result<bool, Error> {
+        match self.db.contains_key(digest.to_vec()) {
+            Ok(has) => Ok(has),
+            Err(e) => Err(Error::StorageError(e.to_string())),
+        }
+    }
+
+    #[instrument(skip(self), fields(blob.digest=%digest))]
+    async fn open_read(&self, digest: &B3Digest) -> Result<Option<Box<dyn BlobReader>>, Error> {
+        match self.db.get(digest.to_vec()) {
+            Ok(None) => Ok(None),
+            Ok(Some(data)) => Ok(Some(Box::new(Cursor::new(data[..].to_vec())))),
+            Err(e) => Err(Error::StorageError(e.to_string())),
+        }
+    }
+
+    #[instrument(skip(self))]
+    async fn open_write(&self) -> Box<dyn BlobWriter> {
+        Box::new(SledBlobWriter::new(self.db.clone()))
+    }
+}
+
+pub struct SledBlobWriter {
+    db: sled::Db,
+
+    /// Contains the buffer Vec and hasher, or None if already closed
+    writers: Option<(Vec<u8>, blake3::Hasher)>,
+
+    /// The digest that has been returned, if we successfully closed.
+    digest: Option<B3Digest>,
+}
+
+impl SledBlobWriter {
+    pub fn new(db: sled::Db) -> Self {
+        Self {
+            db,
+            writers: Some((Vec::new(), blake3::Hasher::new())),
+            digest: None,
+        }
+    }
+}
+
+impl tokio::io::AsyncWrite for SledBlobWriter {
+    fn poll_write(
+        mut self: std::pin::Pin<&mut Self>,
+        _cx: &mut std::task::Context<'_>,
+        b: &[u8],
+    ) -> std::task::Poll<Result<usize, io::Error>> {
+        Poll::Ready(match &mut self.writers {
+            None => Err(io::Error::new(
+                io::ErrorKind::NotConnected,
+                "already closed",
+            )),
+            Some((ref mut buf, ref mut hasher)) => {
+                let bytes_written = buf.write(b)?;
+                hasher.write(&b[..bytes_written])
+            }
+        })
+    }
+
+    fn poll_flush(
+        mut self: std::pin::Pin<&mut Self>,
+        _cx: &mut std::task::Context<'_>,
+    ) -> std::task::Poll<Result<(), io::Error>> {
+        Poll::Ready(match &mut self.writers {
+            None => Err(io::Error::new(
+                io::ErrorKind::NotConnected,
+                "already closed",
+            )),
+            Some(_) => Ok(()),
+        })
+    }
+
+    fn poll_shutdown(
+        self: std::pin::Pin<&mut Self>,
+        _cx: &mut std::task::Context<'_>,
+    ) -> std::task::Poll<Result<(), io::Error>> {
+        // shutdown is "instantaneous", we only write to a Vec<u8> as buffer.
+        Poll::Ready(Ok(()))
+    }
+}
+
+#[async_trait]
+impl BlobWriter for SledBlobWriter {
+    async fn close(&mut self) -> Result<B3Digest, Error> {
+        if self.writers.is_none() {
+            match &self.digest {
+                Some(digest) => Ok(digest.clone()),
+                None => Err(crate::Error::StorageError(
+                    "previously closed with error".to_string(),
+                )),
+            }
+        } else {
+            let (buf, hasher) = self.writers.take().unwrap();
+
+            let digest: B3Digest = hasher.finalize().as_bytes().into();
+
+            // Only insert if the blob doesn't already exist.
+            if !self.db.contains_key(digest.to_vec()).map_err(|e| {
+                Error::StorageError(format!("Unable to check if we have blob {}: {}", digest, e))
+            })? {
+                // put buf in there. This will move buf out.
+                self.db
+                    .insert(digest.to_vec(), buf)
+                    .map_err(|e| Error::StorageError(format!("unable to insert blob: {}", e)))?;
+            }
+
+            self.digest = Some(digest.clone());
+
+            Ok(digest)
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use tempfile::TempDir;
+
+    use super::BlobService;
+    use super::SledBlobService;
+
+    /// This uses a wrong scheme.
+    #[test]
+    fn test_invalid_scheme() {
+        let url = url::Url::parse("http://foo.example/test").expect("must parse");
+
+        assert!(SledBlobService::from_url(&url).is_err());
+    }
+
+    /// This uses the correct scheme, and doesn't specify a path (temporary sled).
+    #[test]
+    fn test_valid_scheme_temporary() {
+        let url = url::Url::parse("sled://").expect("must parse");
+
+        assert!(SledBlobService::from_url(&url).is_ok());
+    }
+
+    /// This sets the path to a location that doesn't exist, which should fail (as sled doesn't mkdir -p)
+    #[test]
+    fn test_nonexistent_path() {
+        let tmpdir = TempDir::new().unwrap();
+
+        let mut url = url::Url::parse("sled://foo.example").expect("must parse");
+        url.set_path(tmpdir.path().join("foo").join("bar").to_str().unwrap());
+
+        assert!(SledBlobService::from_url(&url).is_err());
+    }
+
+    /// This uses the correct scheme, and specifies / as path (which should fail
+    // for obvious reasons)
+    #[test]
+    fn test_invalid_path_root() {
+        let url = url::Url::parse("sled:///").expect("must parse");
+
+        assert!(SledBlobService::from_url(&url).is_err());
+    }
+
+    /// This uses the correct scheme, and sets a tempdir as location.
+    #[test]
+    fn test_valid_scheme_path() {
+        let tmpdir = TempDir::new().unwrap();
+
+        let mut url = url::Url::parse("sled://").expect("must parse");
+        url.set_path(tmpdir.path().to_str().unwrap());
+
+        assert!(SledBlobService::from_url(&url).is_ok());
+    }
+
+    /// This sets a host, rather than a path, which should fail.
+    #[test]
+    fn test_invalid_host() {
+        let url = url::Url::parse("sled://foo.example").expect("must parse");
+
+        assert!(SledBlobService::from_url(&url).is_err());
+    }
+
+    /// This sets a host AND a valid path, which should fail
+    #[test]
+    fn test_invalid_host_and_path() {
+        let tmpdir = TempDir::new().unwrap();
+
+        let mut url = url::Url::parse("sled://foo.example").expect("must parse");
+        url.set_path(tmpdir.path().to_str().unwrap());
+
+        assert!(SledBlobService::from_url(&url).is_err());
+    }
+}
diff --git a/tvix/castore/src/blobservice/tests.rs b/tvix/castore/src/blobservice/tests.rs
new file mode 100644
index 0000000000..fe390b537e
--- /dev/null
+++ b/tvix/castore/src/blobservice/tests.rs
@@ -0,0 +1,246 @@
+use std::io;
+use std::pin::pin;
+
+use test_case::test_case;
+use tokio::io::AsyncReadExt;
+use tokio::io::AsyncSeekExt;
+
+use super::B3Digest;
+use super::BlobService;
+use super::MemoryBlobService;
+use super::SledBlobService;
+use crate::fixtures;
+
+// TODO: avoid having to define all different services we test against for all functions.
+// maybe something like rstest can be used?
+
+fn gen_memory_blob_service() -> impl BlobService {
+    MemoryBlobService::default()
+}
+fn gen_sled_blob_service() -> impl BlobService {
+    SledBlobService::new_temporary().unwrap()
+}
+
+// TODO: add GRPC blob service here.
+
+/// Using [BlobService::has] on a non-existing blob should return false
+#[test_case(gen_memory_blob_service(); "memory")]
+#[test_case(gen_sled_blob_service(); "sled")]
+fn has_nonexistent_false(blob_service: impl BlobService) {
+    tokio::runtime::Runtime::new().unwrap().block_on(async {
+        assert!(!blob_service
+            .has(&fixtures::BLOB_A_DIGEST)
+            .await
+            .expect("must not fail"));
+    })
+}
+
+/// Trying to read a non-existing blob should return a None instead of a reader.
+#[test_case(gen_memory_blob_service(); "memory")]
+#[test_case(gen_sled_blob_service(); "sled")]
+fn not_found_read(blob_service: impl BlobService) {
+    tokio::runtime::Runtime::new().unwrap().block_on(async {
+        assert!(blob_service
+            .open_read(&fixtures::BLOB_A_DIGEST)
+            .await
+            .expect("must not fail")
+            .is_none())
+    })
+}
+
+/// Put a blob in the store, check has, get it back.
+/// We test both with small and big blobs.
+#[test_case(gen_memory_blob_service(), &fixtures::BLOB_A, &fixtures::BLOB_A_DIGEST; "memory-small")]
+#[test_case(gen_sled_blob_service(), &fixtures::BLOB_A, &fixtures::BLOB_A_DIGEST; "sled-small")]
+#[test_case(gen_memory_blob_service(), &fixtures::BLOB_B, &fixtures::BLOB_B_DIGEST; "memory-big")]
+#[test_case(gen_sled_blob_service(), &fixtures::BLOB_B, &fixtures::BLOB_B_DIGEST; "sled-big")]
+fn put_has_get(blob_service: impl BlobService, blob_contents: &[u8], blob_digest: &B3Digest) {
+    tokio::runtime::Runtime::new().unwrap().block_on(async {
+        let mut w = blob_service.open_write().await;
+
+        let l = tokio::io::copy(&mut io::Cursor::new(blob_contents), &mut w)
+            .await
+            .expect("copy must succeed");
+        assert_eq!(
+            blob_contents.len(),
+            l as usize,
+            "written bytes must match blob length"
+        );
+
+        let digest = w.close().await.expect("close must succeed");
+
+        assert_eq!(*blob_digest, digest, "returned digest must be correct");
+
+        assert!(
+            blob_service.has(blob_digest).await.expect("must not fail"),
+            "blob service should now have the blob"
+        );
+
+        let mut r = blob_service
+            .open_read(blob_digest)
+            .await
+            .expect("open_read must succeed")
+            .expect("must be some");
+
+        let mut buf: Vec<u8> = Vec::new();
+        let mut pinned_reader = pin!(r);
+        let l = tokio::io::copy(&mut pinned_reader, &mut buf)
+            .await
+            .expect("copy must succeed");
+        // let l = io::copy(&mut r, &mut buf).expect("copy must succeed");
+
+        assert_eq!(
+            blob_contents.len(),
+            l as usize,
+            "read bytes must match blob length"
+        );
+
+        assert_eq!(blob_contents, buf, "read blob contents must match");
+    })
+}
+
+/// Put a blob in the store, and seek inside it a bit.
+#[test_case(gen_memory_blob_service(); "memory")]
+#[test_case(gen_sled_blob_service(); "sled")]
+fn put_seek(blob_service: impl BlobService) {
+    tokio::runtime::Runtime::new().unwrap().block_on(async {
+        let mut w = blob_service.open_write().await;
+
+        tokio::io::copy(&mut io::Cursor::new(&fixtures::BLOB_B.to_vec()), &mut w)
+            .await
+            .expect("copy must succeed");
+        w.close().await.expect("close must succeed");
+
+        // open a blob for reading
+        let mut r = blob_service
+            .open_read(&fixtures::BLOB_B_DIGEST)
+            .await
+            .expect("open_read must succeed")
+            .expect("must be some");
+
+        let mut pos: u64 = 0;
+
+        // read the first 10 bytes, they must match the data in the fixture.
+        {
+            let mut buf = [0; 10];
+            r.read_exact(&mut buf).await.expect("must succeed");
+
+            assert_eq!(
+                &fixtures::BLOB_B[pos as usize..pos as usize + buf.len()],
+                buf,
+                "expected first 10 bytes to match"
+            );
+
+            pos += buf.len() as u64;
+        }
+        // seek by 0 bytes, using SeekFrom::Start.
+        let p = r
+            .seek(io::SeekFrom::Start(pos))
+            .await
+            .expect("must not fail");
+        assert_eq!(pos, p);
+
+        // read the next 10 bytes, they must match the data in the fixture.
+        {
+            let mut buf = [0; 10];
+            r.read_exact(&mut buf).await.expect("must succeed");
+
+            assert_eq!(
+                &fixtures::BLOB_B[pos as usize..pos as usize + buf.len()],
+                buf,
+                "expected data to match"
+            );
+
+            pos += buf.len() as u64;
+        }
+
+        // seek by 5 bytes, using SeekFrom::Start.
+        let p = r
+            .seek(io::SeekFrom::Start(pos + 5))
+            .await
+            .expect("must not fail");
+        pos += 5;
+        assert_eq!(pos, p);
+
+        // read the next 10 bytes, they must match the data in the fixture.
+        {
+            let mut buf = [0; 10];
+            r.read_exact(&mut buf).await.expect("must succeed");
+
+            assert_eq!(
+                &fixtures::BLOB_B[pos as usize..pos as usize + buf.len()],
+                buf,
+                "expected data to match"
+            );
+
+            pos += buf.len() as u64;
+        }
+
+        // seek by 12345 bytes, using SeekFrom::
+        let p = r
+            .seek(io::SeekFrom::Current(12345))
+            .await
+            .expect("must not fail");
+        pos += 12345;
+        assert_eq!(pos, p);
+
+        // read the next 10 bytes, they must match the data in the fixture.
+        {
+            let mut buf = [0; 10];
+            r.read_exact(&mut buf).await.expect("must succeed");
+
+            assert_eq!(
+                &fixtures::BLOB_B[pos as usize..pos as usize + buf.len()],
+                buf,
+                "expected data to match"
+            );
+
+            #[allow(unused_assignments)]
+            {
+                pos += buf.len() as u64;
+            }
+        }
+
+        // seeking to the end is okay…
+        let p = r
+            .seek(io::SeekFrom::Start(fixtures::BLOB_B.len() as u64))
+            .await
+            .expect("must not fail");
+        pos = fixtures::BLOB_B.len() as u64;
+        assert_eq!(pos, p);
+
+        {
+            // but it returns no more data.
+            let mut buf: Vec<u8> = Vec::new();
+            r.read_to_end(&mut buf).await.expect("must not fail");
+            assert!(buf.is_empty(), "expected no more data to be read");
+        }
+
+        // seeking past the end…
+        match r
+            .seek(io::SeekFrom::Start(fixtures::BLOB_B.len() as u64 + 1))
+            .await
+        {
+            // should either be ok, but then return 0 bytes.
+            // this matches the behaviour or a Cursor<Vec<u8>>.
+            Ok(_pos) => {
+                let mut buf: Vec<u8> = Vec::new();
+                r.read_to_end(&mut buf).await.expect("must not fail");
+                assert!(buf.is_empty(), "expected no more data to be read");
+            }
+            // or not be okay.
+            Err(_) => {}
+        }
+
+        // TODO: this is only broken for the gRPC version
+        // We expect seeking backwards or relative to the end to fail.
+        // r.seek(io::SeekFrom::Current(-1))
+        //     .expect_err("SeekFrom::Current(-1) expected to fail");
+
+        // r.seek(io::SeekFrom::Start(pos - 1))
+        //     .expect_err("SeekFrom::Start(pos-1) expected to fail");
+
+        // r.seek(io::SeekFrom::End(0))
+        //     .expect_err("SeekFrom::End(_) expected to fail");
+    })
+}