1 files changed, 103 insertions, 0 deletions
diff --git a/third_party/rust/tokio-util/src/io/sync_bridge.rs b/third_party/rust/tokio-util/src/io/sync_bridge.rs
new file mode 100644
index 0000000000..9be9446a7d
--- /dev/null
+++ b/third_party/rust/tokio-util/src/io/sync_bridge.rs
@@ -0,0 +1,103 @@
+use std::io::{Read, Write};
+use tokio::io::{AsyncRead, AsyncReadExt, AsyncWrite, AsyncWriteExt};
+
+/// Use a [`tokio::io::AsyncRead`] synchronously as a [`std::io::Read`] or
+/// a [`tokio::io::AsyncWrite`] as a [`std::io::Write`].
+#[derive(Debug)]
+pub struct SyncIoBridge<T> {
+    src: T,
+    rt: tokio::runtime::Handle,
+}
+
+impl<T: AsyncRead + Unpin> Read for SyncIoBridge<T> {
+    fn read(&mut self, buf: &mut [u8]) -> std::io::Result<usize> {
+        let src = &mut self.src;
+        self.rt.block_on(AsyncReadExt::read(src, buf))
+    }
+
+    fn read_to_end(&mut self, buf: &mut Vec<u8>) -> std::io::Result<usize> {
+        let src = &mut self.src;
+        self.rt.block_on(src.read_to_end(buf))
+    }
+
+    fn read_to_string(&mut self, buf: &mut String) -> std::io::Result<usize> {
+        let src = &mut self.src;
+        self.rt.block_on(src.read_to_string(buf))
+    }
+
+    fn read_exact(&mut self, buf: &mut [u8]) -> std::io::Result<()> {
+        let src = &mut self.src;
+        // The AsyncRead trait returns the count, synchronous doesn't.
+        let _n = self.rt.block_on(src.read_exact(buf))?;
+        Ok(())
+    }
+}
+
+impl<T: AsyncWrite + Unpin> Write for SyncIoBridge<T> {
+    fn write(&mut self, buf: &[u8]) -> std::io::Result<usize> {
+        let src = &mut self.src;
+        self.rt.block_on(src.write(buf))
+    }
+
+    fn flush(&mut self) -> std::io::Result<()> {
+        let src = &mut self.src;
+        self.rt.block_on(src.flush())
+    }
+
+    fn write_all(&mut self, buf: &[u8]) -> std::io::Result<()> {
+        let src = &mut self.src;
+        self.rt.block_on(src.write_all(buf))
+    }
+
+    fn write_vectored(&mut self, bufs: &[std::io::IoSlice<'_>]) -> std::io::Result<usize> {
+        let src = &mut self.src;
+        self.rt.block_on(src.write_vectored(bufs))
+    }
+}
+
+// Because https://doc.rust-lang.org/std/io/trait.Write.html#method.is_write_vectored is at the time
+// of this writing still unstable, we expose this as part of a standalone method.
+impl<T: AsyncWrite> SyncIoBridge<T> {
+    /// Determines if the underlying [`tokio::io::AsyncWrite`] target supports efficient vectored writes.
+    ///
+    /// See [`tokio::io::AsyncWrite::is_write_vectored`].
+    pub fn is_write_vectored(&self) -> bool {
+        self.src.is_write_vectored()
+    }
+}
+
+impl<T: Unpin> SyncIoBridge<T> {
+    /// Use a [`tokio::io::AsyncRead`] synchronously as a [`std::io::Read`] or
+    /// a [`tokio::io::AsyncWrite`] as a [`std::io::Write`].
+    ///
+    /// When this struct is created, it captures a handle to the current thread's runtime with [`tokio::runtime::Handle::current`].
+    /// It is hence OK to move this struct into a separate thread outside the runtime, as created
+    /// by e.g. [`tokio::task::spawn_blocking`].
+    ///
+    /// Stated even more strongly: to make use of this bridge, you *must* move
+    /// it into a separate thread outside the runtime.  The synchronous I/O will use the
+    /// underlying handle to block on the backing asynchronous source, via
+    /// [`tokio::runtime::Handle::block_on`].  As noted in the documentation for that
+    /// function, an attempt to `block_on` from an asynchronous execution context
+    /// will panic.
+    ///
+    /// # Wrapping `!Unpin` types
+    ///
+    /// Use e.g. `SyncIoBridge::new(Box::pin(src))`.
+    ///
+    /// # Panic
+    ///
+    /// This will panic if called outside the context of a Tokio runtime.
+    pub fn new(src: T) -> Self {
+        Self::new_with_handle(src, tokio::runtime::Handle::current())
+    }
+
+    /// Use a [`tokio::io::AsyncRead`] synchronously as a [`std::io::Read`] or
+    /// a [`tokio::io::AsyncWrite`] as a [`std::io::Write`].
+    ///
+    /// This is the same as [`SyncIoBridge::new`], but allows passing an arbitrary handle and hence may
+    /// be initially invoked outside of an asynchronous context.
+    pub fn new_with_handle(src: T, rt: tokio::runtime::Handle) -> Self {
+        Self { src, rt }
+    }
+}