feat(fusedev): add try_with_writer on FuseSession; update

`with_writer` docs

- Introduce `try_with_writer<F, R, E>` that builds a `FuseDevWriter` and
returns
  the closure’s `Result`, converting `super::Error` via `From`.
  - Return `SessionFailure("invalid fuse session")` when `self.file` is
  absent.
  - Tweak `with_writer` doc to clarify it passes a writer to the given
  closure.

example(passthrough): update example to show how to use try_with_writer

Author: Xu Jihui

src/transport/fusedev/linux_session.rs

@@ -238,7 +238,7 @@ impl FuseSession {
         }
     }
 
-    /// Create a new fuse message channel with a specific buffer size.
+    /// Create a new fuse message writer and pass it to the given closure.
     pub fn with_writer<F>(&mut self, f: F)
     where
         F: FnOnce(FuseDevWriter),
@@ -251,6 +251,22 @@ impl FuseSession {
         }
     }
 
+    /// Create a new fuse message writer and pass it to the given closure. and return the result from the closure.
+    pub fn try_with_writer<F, R, E>(&mut self, f: F) -> std::result::Result<R, E>
+    where
+        F: FnOnce(FuseDevWriter) -> std::result::Result<R, E>,
+        E: From<super::Error>,
+    {
+        if let Some(file) = &self.file {
+            let fd = file.as_raw_fd();
+            let mut buf = vec![0x0u8; self.bufsize];
+            let writer = FuseDevWriter::new(fd, &mut buf)?;
+            f(writer)
+        } else {
+            Err(SessionFailure("invalid fuse session".into()).into())
+        }
+    }
+
     /// Wake channel loop and exit
     pub fn wake(&self) -> Result<()> {
         let wakers = self

tests/passthrough/src/main.rs

@@ -23,6 +23,17 @@ pub struct Daemon {
     session: Option<FuseSession>,
 }
 
+pub enum PassthroughFsError {
+    FuseError(fuse_backend_rs::Error),
+    TransportError(fuse_backend_rs::transport::Error),
+}
+
+impl From<fuse_backend_rs::transport::Error> for PassthroughFsError {
+    fn from(e: fuse_backend_rs::transport::Error) -> Self {
+        PassthroughFsError::TransportError(e)
+    }
+}
+
 #[allow(dead_code)]
 impl Daemon {
     /// Creates a fusedev daemon instance
@@ -59,11 +70,10 @@ impl Daemon {
             FuseSession::new(Path::new(&self.mountpoint), "testpassthrough", "", false).unwrap();
         se.mount().unwrap();
 
-        se.with_writer(|writer| {
+        se.try_with_writer(|writer| {
             self.server
-                .notify_resend(writer)
-                .unwrap_or_else(|e| println!("failed to send resend notification {}", e));
-        });
+                .notify_resend(writer).map_err(PassthroughFsError::FuseError)
+        }).map_err(|_| Error::from_raw_os_error(libc::EINVAL))?;
 
         for _ in 0..self.thread_cnt {
             let mut server = FuseServer {