Fix unnecessary zeroing for AsyncRead implementors

tokio/src/fs/file.rs

@@ -537,6 +537,12 @@ impl File {
 }
 
 impl AsyncRead for File {
+    unsafe fn prepare_uninitialized_buffer(&self, _buf: &mut [std::mem::MaybeUninit<u8>]) -> bool {
+        // std::fs::File does not need zeroing
+        // https://github.com/rust-lang/rust/blob/09c817eeb29e764cfc12d0a8d94841e3ffe34023/src/libstd/fs.rs#L668
+        false
+    }
+
     fn poll_read(
         mut self: Pin<&mut Self>,
         cx: &mut Context<'_>,

tokio/src/io/async_read.rs

@@ -5,6 +5,28 @@ use std::ops::DerefMut;
 use std::pin::Pin;
 use std::task::{Context, Poll};
 
+/// This utility function shoyld be used to override `AsyncRead::prepare_uninitialized_buffer`
+/// for any AsyncRead impl which wraps an `std::io::Read` instance.
+pub(crate) fn prepare_uninitialized_buffer_std_read<R: std::io::Read>(
+    buf: &mut [MaybeUninit<u8>],
+) -> bool {
+    // TODO: when std::io::Initializer is stable, it should be used
+    // to override `prepare_uninitialized_buffer`
+    /*use std::io::Read;
+    if !T::initializer::should_initialize() {
+        return false;
+    }
+    */
+    for x in buf {
+        // we could use safe method x.write(0) here, but it is unstable.
+        // Safety: as_mut_ptr() returns valid writeable pointer.
+        unsafe {
+            x.as_mut_ptr().write(0);
+        }
+    }
+    true
+}
+
 /// Reads bytes from a source.
 ///
 /// This trait is analogous to the [`std::io::Read`] trait, but integrates with

tokio/src/io/blocking.rs

@@ -50,6 +50,10 @@ impl<T> AsyncRead for Blocking<T>
 where
     T: Read + Unpin + Send + 'static,
 {
+    unsafe fn prepare_uninitialized_buffer(&self, buf: &mut [std::mem::MaybeUninit<u8>]) -> bool {
+        crate::io::prepare_uninitialized_buffer_std_read::<T>(buf)
+    }
+
     fn poll_read(
         mut self: Pin<&mut Self>,
         cx: &mut Context<'_>,

tokio/src/io/mod.rs

@@ -163,6 +163,7 @@ pub use self::async_buf_read::AsyncBufRead;
 
 mod async_read;
 pub use self::async_read::AsyncRead;
+pub(crate) use self::async_read::prepare_uninitialized_buffer_std_read;
 
 mod async_seek;
 pub use self::async_seek::AsyncSeek;

tokio/src/io/poll_evented.rs

@@ -381,6 +381,10 @@ impl<E> AsyncRead for PollEvented<E>
 where
     E: Evented + Read + Unpin,
 {
+    unsafe fn prepare_uninitialized_buffer(&self, buf: &mut [std::mem::MaybeUninit<u8>]) -> bool {
+        crate::io::prepare_uninitialized_buffer_std_read::<E>(buf)
+    }
+
     fn poll_read(
         mut self: Pin<&mut Self>,
         cx: &mut Context<'_>,

tokio/src/io/stdin.rs

@@ -63,6 +63,11 @@ impl std::os::windows::io::AsRawHandle for Stdin {
 }
 
 impl AsyncRead for Stdin {
+    unsafe fn prepare_uninitialized_buffer(&self, _buf: &mut [std::mem::MaybeUninit<u8>]) -> bool {
+        // https://github.com/rust-lang/rust/blob/09c817eeb29e764cfc12d0a8d94841e3ffe34023/src/libstd/io/stdio.rs#L97
+        false
+    }
+
     fn poll_read(
         mut self: Pin<&mut Self>,
         cx: &mut Context<'_>,

tokio/src/io/util/chain.rs

@@ -84,6 +84,15 @@ where
     T: AsyncRead,
     U: AsyncRead,
 {
+    unsafe fn prepare_uninitialized_buffer(&self, buf: &mut [std::mem::MaybeUninit<u8>]) -> bool {
+        if self.first.prepare_uninitialized_buffer(buf) {
+            return true;
+        }
+        if self.second.prepare_uninitialized_buffer(buf) {
+            return true;
+        }
+        false
+    }
     fn poll_read(
         self: Pin<&mut Self>,
         cx: &mut Context<'_>,

tokio/src/io/util/empty.rs

@@ -47,6 +47,9 @@ cfg_io_util! {
 }
 
 impl AsyncRead for Empty {
+    unsafe fn prepare_uninitialized_buffer(&self, _buf: &mut [std::mem::MaybeUninit<u8>]) -> bool {
+        false
+    }
     #[inline]
     fn poll_read(
         self: Pin<&mut Self>,

tokio/src/io/util/repeat.rs

@@ -47,6 +47,9 @@ cfg_io_util! {
 }
 
 impl AsyncRead for Repeat {
+    unsafe fn prepare_uninitialized_buffer(&self, _buf: &mut [std::mem::MaybeUninit<u8>]) -> bool {
+        false
+    }
     #[inline]
     fn poll_read(
         self: Pin<&mut Self>,

tokio/src/process/mod.rs

@@ -872,6 +872,11 @@ impl AsyncWrite for ChildStdin {
 }
 
 impl AsyncRead for ChildStdout {
+    unsafe fn prepare_uninitialized_buffer(&self, _buf: &mut [std::mem::MaybeUninit<u8>]) -> bool {
+        // https://github.com/rust-lang/rust/blob/09c817eeb29e764cfc12d0a8d94841e3ffe34023/src/libstd/process.rs#L314
+        false
+    }
+
     fn poll_read(
         mut self: Pin<&mut Self>,
         cx: &mut Context<'_>,
@@ -882,6 +887,11 @@ impl AsyncRead for ChildStdout {
 }
 
 impl AsyncRead for ChildStderr {
+    unsafe fn prepare_uninitialized_buffer(&self, _buf: &mut [std::mem::MaybeUninit<u8>]) -> bool {
+        // https://github.com/rust-lang/rust/blob/09c817eeb29e764cfc12d0a8d94841e3ffe34023/src/libstd/process.rs#L375
+        false
+    }
+
     fn poll_read(
         mut self: Pin<&mut Self>,
         cx: &mut Context<'_>,