fix(tvix/store): Fix FUSE support on MacOS

This partially fixes b/312 and gets FUSE to work again on MacOS.

It is mostly small type changes and an update to fuse-backend-rs because
upstream currently doesn't work with MacFuse. It also sets the default
FUSE thread count on MacOS to 1 because otherwise the mount command will
hang when shutting down as only one thread gets ENODEV and all the others
just keep blocking.

Change-Id: Ifb3c4268caf296c487049c1dc4618acb32497f44
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9490
Tested-by: BuildkiteCI
Reviewed-by: Connor Brewster <cbrewster@hey.com>
Reviewed-by: flokli <flokli@flokli.de>

tvix/store/src/bin/tvix-store.rs

@@ -139,12 +139,19 @@ enum Commands {
     },
 }
 
-#[cfg(feature = "fuse")]
+#[cfg(all(feature = "fuse", not(target_os = "macos")))]
 fn default_threads() -> usize {
     std::thread::available_parallelism()
         .map(|threads| threads.into())
         .unwrap_or(4)
 }
+// On MacFUSE only a single channel will receive ENODEV when the file system is
+// unmounted and so all the other channels will block forever.
+// See https://github.com/osxfuse/osxfuse/issues/974
+#[cfg(all(feature = "fuse", target_os = "macos"))]
+fn default_threads() -> usize {
+    1
+}
 
 #[tokio::main]
 async fn main() -> Result<(), Box<dyn std::error::Error>> {