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

3 files changed, 11 insertions, 1 deletions

diff --git a/tokio/src/sync/batch_semaphore.rs b/tokio/src/sync/batch_semaphore.rs
index d09528be..0b50e4f7 100644
--- a/tokio/src/sync/batch_semaphore.rs
+++ b/tokio/src/sync/batch_semaphore.rs
@@ -1,3 +1,4 @@
+#![cfg_attr(not(feature = "sync"), allow(unreachable_pub, dead_code))]
 //! # Implementation Details
 //!
 //! The semaphore is implemented using an intrusive linked list of waiters. An
diff --git a/tokio/src/sync/mod.rs b/tokio/src/sync/mod.rs
index 4919ad8e..ed9f07a0 100644
--- a/tokio/src/sync/mod.rs
+++ b/tokio/src/sync/mod.rs
@@ -456,6 +456,14 @@ cfg_sync! {
 }
 
 cfg_not_sync! {
+    #[cfg(any(feature = "fs", feature = "signal", all(unix, feature = "process")))]
+    pub(crate) mod batch_semaphore;
+
+    cfg_fs! {
+        mod mutex;
+        pub(crate) use mutex::Mutex;
+    }
+
     mod notify;
     pub(crate) use notify::Notify;
 
@@ -472,7 +480,6 @@ cfg_not_sync! {
 
     cfg_signal_internal! {
         pub(crate) mod mpsc;
-        pub(crate) mod batch_semaphore;
     }
 }
 
diff --git a/tokio/src/sync/mutex.rs b/tokio/src/sync/mutex.rs
index b2cf64d3..21e44ca9 100644
--- a/tokio/src/sync/mutex.rs
+++ b/tokio/src/sync/mutex.rs
@@ -1,3 +1,5 @@
+#![cfg_attr(not(feature = "sync"), allow(unreachable_pub, dead_code))]
+
 use crate::sync::batch_semaphore as semaphore;
 
 use std::cell::UnsafeCell;