filehandle: trim the filehandle slice before returning it to user

This way, once the user gets it, the filehandle slice is only as large
as it needs to be. Then if the user copies it, excess unused bytes are
not copied.

Author: bertschingert

src/fs/filehandle.rs

@@ -67,6 +67,16 @@ impl FileHandle {
         &self.raw[HANDLE_STRUCT_SIZE..]
     }
 
+    /// We allocate the "maximum" size for a file handle straight away in order to avoid needing
+    /// multiple syscalls / reallocations whenever possible. However, that leaves raw.len()
+    /// excessively high when the filehandle will usually be much smaller than MAX_HANDLE_SIZE.
+    /// This function "trims" the filehandle so that the slice is only as large as it needs to be.
+    fn trim(&mut self) {
+        let len = self.get_handle_len() + HANDLE_STRUCT_SIZE;
+
+        self.raw = Box::from(&self.raw[0..len]);
+    }
+
     /// Set the `handle_bytes` field (first 4 bytes of the struct) to the given length.
     fn set_handle_len(&mut self, size: usize) {
         self.raw[0..size_of::<ffi::c_uint>()].copy_from_slice(&(size as ffi::c_uint).to_ne_bytes());
@@ -144,6 +154,9 @@ pub fn name_to_handle_at<Fd: AsFd, P: path::Arg>(
             mount_id_int as u64
         };
 
+        // Ensure the slice is only as large as it needs to be before returning it to the user.
+        file_handle.trim();
+
         return ret.map(|_| (file_handle, mount_id));
     })
 }