fs: future proof `File` (#2930)

Changes inherent methods to take `&self` instead of `&mut self`. This
brings the API in line with `std`.

This patch is implemented by using a `tokio::sync::Mutex` to guard the
internal `File` state. This is not an ideal implementation strategy
doesn't make a big impact compared to having to dispatch operations to a
background thread followed by a blocking syscall.

In the future, the implementation can be improved as we explore async
file-system APIs provided by the operating-system (iocp / io_uring).

Closes #2927

1 files changed, 5 insertions, 2 deletions

diff --git a/tokio/tests/fs_file_mocked.rs b/tokio/tests/fs_file_mocked.rs
index 2e7e8b7c..edb74a73 100644
--- a/tokio/tests/fs_file_mocked.rs
+++ b/tokio/tests/fs_file_mocked.rs
@@ -57,6 +57,9 @@ pub(crate) mod fs {
 
     pub(crate) use crate::support::mock_pool::asyncify;
 }
+pub(crate) mod sync {
+    pub(crate) use tokio::sync::Mutex;
+}
 use fs::sys;
 
 use tokio::prelude::*;
@@ -710,7 +713,7 @@ fn open_set_len_ok() {
     let (mock, file) = sys::File::mock();
     mock.set_len(123);
 
-    let mut file = File::from_std(file);
+    let file = File::from_std(file);
     let mut t = task::spawn(file.set_len(123));
 
     assert_pending!(t.poll());
@@ -728,7 +731,7 @@ fn open_set_len_err() {
     let (mock, file) = sys::File::mock();
     mock.set_len_err(123);
 
-    let mut file = File::from_std(file);
+    let file = File::from_std(file);
     let mut t = task::spawn(file.set_len(123));
 
     assert_pending!(t.poll());