Ignore ENOENT errors when checking if paths are mounted

It's safe to say that paths that do not exist are not mounted for all of
the purposes in the spfs setup logic

Signed-off-by: Ryan Bottriell <ryan@bottriell.ca>

crates/spfs/src/env.rs

@@ -283,6 +283,8 @@ where
     }
 
     /// Check if the identified directory is an active mount point.
+    ///
+    /// Returns false in the case where the path or its parent do not exist.
     async fn is_mounted<P: Into<PathBuf>>(&self, target: P) -> Result<bool> {
         let target = target.into();
         let parent = match target.parent() {
@@ -292,11 +294,9 @@ where
 
         // A new thread created while holding _guard will be inside the same
         // mount namespace...
-        let stat_parent_thread =
-            std::thread::spawn(move || nix::sys::stat::stat(&parent).map_err(Error::from));
+        let stat_parent_thread = std::thread::spawn(move || nix::sys::stat::stat(&parent));
 
-        let stat_target_thread =
-            std::thread::spawn(move || nix::sys::stat::stat(&target).map_err(Error::from));
+        let stat_target_thread = std::thread::spawn(move || nix::sys::stat::stat(&target));
 
         let (st_parent, st_target) = tokio::task::spawn_blocking(move || {
             let st_parent = stat_parent_thread.join();
@@ -306,9 +306,22 @@ where
         .await?;
 
         let st_parent =
-            st_parent.map_err(|_| Error::String("Failed to stat parent".to_owned()))??;
+            match st_parent.map_err(|_| Error::String("Failed to stat parent".to_owned()))? {
+                // the parent not existing means the child also doesn't exist
+                // and so cannot be considered as mounted
+                Err(nix::errno::Errno::ENOENT) => {
+                    return Ok(false);
+                }
+                r => r?,
+            };
         let st_target =
-            st_target.map_err(|_| Error::String("Failed to stat target".to_owned()))??;
+            match st_target.map_err(|_| Error::String("Failed to stat target".to_owned()))? {
+                // a non-existent directory is considered not mounted
+                Err(nix::errno::Errno::ENOENT) => {
+                    return Ok(false);
+                }
+                r => r?,
+            };
 
         Ok(st_target.st_dev != st_parent.st_dev)
     }
@@ -346,8 +359,8 @@ where
             ));
         }
 
-        if self.is_mounted("/spfs").await? {
-            res = mount(NONE, "/spfs", NONE, MsFlags::MS_PRIVATE, NONE);
+        if self.is_mounted(SPFS_DIR).await? {
+            res = mount(NONE, SPFS_DIR, NONE, MsFlags::MS_PRIVATE, NONE);
             if let Err(err) = res {
                 return Err(Error::wrap_nix(
                     err,