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/tests/import.rs

@@ -9,8 +9,8 @@ use tempfile::TempDir;
 use std::os::unix::ffi::OsStrExt;
 
 #[cfg(target_family = "unix")]
-#[test]
-fn symlink() {
+#[tokio::test]
+async fn symlink() {
     let tmpdir = TempDir::new().unwrap();
 
     std::fs::create_dir_all(&tmpdir).unwrap();
@@ -25,6 +25,7 @@ fn symlink() {
         gen_directory_service(),
         tmpdir.path().join("doesntmatter"),
     )
+    .await
     .expect("must succeed");
 
     assert_eq!(
@@ -36,8 +37,8 @@ fn symlink() {
     )
 }
 
-#[test]
-fn single_file() {
+#[tokio::test]
+async fn single_file() {
     let tmpdir = TempDir::new().unwrap();
 
     std::fs::write(tmpdir.path().join("root"), HELLOWORLD_BLOB_CONTENTS).unwrap();
@@ -49,6 +50,7 @@ fn single_file() {
         gen_directory_service(),
         tmpdir.path().join("root"),
     )
+    .await
     .expect("must succeed");
 
     assert_eq!(
@@ -62,12 +64,12 @@ fn single_file() {
     );
 
     // ensure the blob has been uploaded
-    assert!(blob_service.has(&HELLOWORLD_BLOB_DIGEST).unwrap());
+    assert!(blob_service.has(&HELLOWORLD_BLOB_DIGEST).await.unwrap());
 }
 
 #[cfg(target_family = "unix")]
-#[test]
-fn complicated() {
+#[tokio::test]
+async fn complicated() {
     let tmpdir = TempDir::new().unwrap();
 
     // File ``.keep`
@@ -87,6 +89,7 @@ fn complicated() {
         directory_service.clone(),
         tmpdir.path(),
     )
+    .await
     .expect("must succeed");
 
     // ensure root_node matched expectations
@@ -116,5 +119,5 @@ fn complicated() {
         .is_some());
 
     // ensure EMPTY_BLOB_CONTENTS has been uploaded
-    assert!(blob_service.has(&EMPTY_BLOB_DIGEST).unwrap());
+    assert!(blob_service.has(&EMPTY_BLOB_DIGEST).await.unwrap());
 }