refactor(tvix/store/blobsvc): make BlobStore async

We previously kept the trait of a BlobService sync.

This however had some annoying consequences:

 - It became more and more complicated to track when we're in a context
   with an async runtime in the context or not, producing bugs like
   https://b.tvl.fyi/issues/304
 - The sync trait shielded away async clients from async worloads,
   requiring manual block_on code inside the gRPC client code, and
   spawn_blocking calls in consumers of the trait, even if they were
   async (like the gRPC server)
 - We had to write our own custom glue code (SyncReadIntoAsyncRead)
   to convert a sync io::Read into a tokio::io::AsyncRead, which already
   existed in tokio internally, but upstream ia hesitant to expose.

This now makes the BlobService trait async (via the async_trait macro,
like we already do in various gRPC parts), and replaces the sync readers
and writers with their async counterparts.

Tests interacting with a BlobService now need to have an async runtime
available, the easiest way for this is to mark the test functions
with the tokio::test macro, allowing us to directly .await in the test
function.

In places where we don't have an async runtime available from context
(like tvix-cli), we can pass one down explicitly.

Now that we don't provide a sync interface anymore, the (sync) FUSE
library now holds a pointer to a tokio runtime handle, and needs to at
least have 2 threads available when talking to a blob service (which is
why some of the tests now use the multi_thread flavor).

The FUSE tests got a bit more verbose, as we couldn't use the
setup_and_mount function accepting a callback anymore. We can hopefully
move some of the test fixture setup to rstest in the future to make this
less repetitive.

Co-Authored-By: Connor Brewster <cbrewster@hey.com>
Change-Id: Ia0501b606e32c852d0108de9c9016b21c94a3c05
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9329
Reviewed-by: Connor Brewster <cbrewster@hey.com>
Tested-by: BuildkiteCI
Reviewed-by: raitobezarius <tvl@lahfa.xyz>

tvix/store/src/blobservice/memory.rs

@@ -1,9 +1,11 @@
-use std::io::{self, Cursor};
+use std::io::{self, Cursor, Write};
+use std::task::Poll;
 use std::{
     collections::HashMap,
     sync::{Arc, RwLock},
 };
-use tracing::{instrument, warn};
+use tonic::async_trait;
+use tracing::instrument;
 
 use super::{BlobReader, BlobService, BlobWriter};
 use crate::{B3Digest, Error};
@@ -13,6 +15,7 @@ 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://`
@@ -31,12 +34,12 @@ impl BlobService for MemoryBlobService {
     }
 
     #[instrument(skip(self, digest), fields(blob.digest=%digest))]
-    fn has(&self, digest: &B3Digest) -> Result<bool, Error> {
+    async fn has(&self, digest: &B3Digest) -> Result<bool, Error> {
         let db = self.db.read().unwrap();
         Ok(db.contains_key(digest))
     }
 
-    fn open_read(&self, digest: &B3Digest) -> Result<Option<Box<dyn BlobReader>>, Error> {
+    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())) {
@@ -46,7 +49,7 @@ impl BlobService for MemoryBlobService {
     }
 
     #[instrument(skip(self))]
-    fn open_write(&self) -> Box<dyn BlobWriter> {
+    async fn open_write(&self) -> Box<dyn BlobWriter> {
         Box::new(MemoryBlobWriter::new(self.db.clone()))
     }
 }
@@ -70,9 +73,13 @@ impl MemoryBlobWriter {
         }
     }
 }
-impl std::io::Write for MemoryBlobWriter {
-    fn write(&mut self, b: &[u8]) -> std::io::Result<usize> {
-        match &mut self.writers {
+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",
@@ -81,22 +88,34 @@ impl std::io::Write for MemoryBlobWriter {
                 let bytes_written = buf.write(b)?;
                 hasher.write(&b[..bytes_written])
             }
-        }
+        })
     }
 
-    fn flush(&mut self) -> std::io::Result<()> {
-        match &mut self.writers {
+    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 {
-    fn close(&mut self) -> Result<B3Digest, Error> {
+    async fn close(&mut self) -> Result<B3Digest, Error> {
         if self.writers.is_none() {
             match &self.digest {
                 Some(digest) => Ok(digest.clone()),