Remove clean_for_close/flush in FileLike

kernel/aster-nix/src/fs/file_handle.rs

@@ -54,10 +54,6 @@ pub trait FileLike: Pollable + Send + Sync + Any {
         return_errno_with_message!(Errno::EINVAL, "resize is not supported");
     }
 
-    fn flush(&self) -> Result<()> {
-        Ok(())
-    }
-
     fn metadata(&self) -> Metadata {
         panic!("metadata unsupported");
     }
@@ -102,11 +98,6 @@ pub trait FileLike: Pollable + Send + Sync + Any {
         return_errno_with_message!(Errno::ESPIPE, "seek is not supported");
     }
 
-    fn clean_for_close(&self) -> Result<()> {
-        self.flush()?;
-        Ok(())
-    }
-
     fn register_observer(
         &self,
         observer: Weak<dyn Observer<IoEvents>>,

kernel/aster-nix/src/fs/inode_handle/dyn_cap.rs

@@ -138,11 +138,6 @@ impl FileLike for InodeHandle<Rights> {
         Ok(())
     }
 
-    fn clean_for_close(&self) -> Result<()> {
-        // Close does not guarantee that the data has been successfully saved to disk.
-        Ok(())
-    }
-
     fn as_device(&self) -> Option<Arc<dyn Device>> {
         self.dentry().inode().as_device()
     }

kernel/aster-nix/src/process/exit.rs

@@ -40,9 +40,7 @@ pub fn do_exit_group(term_status: TermStatus) {
 
     // Close all files then exit the process
     let files = current.file_table().lock().close_all();
-    for file in files {
-        let _ = file.clean_for_close();
-    }
+    drop(files);
 
     // Move children to the init process
     if !is_init_process(&current) {

kernel/aster-nix/src/syscall/close.rs

@@ -5,10 +5,27 @@ use crate::{fs::file_table::FileDesc, prelude::*};
 
 pub fn sys_close(fd: FileDesc) -> Result<SyscallReturn> {
     debug!("fd = {}", fd);
-    let current = current!();
-    let mut file_table = current.file_table().lock();
-    let _ = file_table.get_file(fd)?;
-    let file = file_table.close_file(fd).unwrap();
-    file.clean_for_close()?;
+
+    let file = {
+        let current = current!();
+        let mut file_table = current.file_table().lock();
+        let _ = file_table.get_file(fd)?;
+        file_table.close_file(fd).unwrap()
+    };
+
+    // Cleanup work needs to be done in the `Drop` impl.
+    //
+    // We don't mind the races between closing the file descriptor and using the file descriptor,
+    // because such races are explicitly allowed in the man pages. See the "Multithreaded processes
+    // and close()" section in <https://man7.org/linux/man-pages/man2/close.2.html>.
+    drop(file);
+
+    // Linux has error codes for the close() system call for diagnostic and remedial purposes, but
+    // only for a small subset of file systems such as NFS. We currently have no support for such
+    // file systems, so it's fine to just return zero.
+    //
+    // For details, see the discussion at <https://github.com/asterinas/asterinas/issues/506> and
+    // the "Dealing with error returns from close()" section at
+    // <https://man7.org/linux/man-pages/man2/close.2.html>.
     Ok(SyscallReturn::Return(0))
 }

kernel/aster-nix/src/syscall/execve.rs

@@ -105,9 +105,7 @@ fn do_execve(
 
     // Ensure that the file descriptors with the close-on-exec flag are closed.
     let closed_files = current.file_table().lock().close_files_on_exec();
-    for file in closed_files {
-        file.clean_for_close()?;
-    }
+    drop(closed_files);
 
     debug!("load program to root vmar");
     let (new_executable_path, elf_load_info) = {