signal: expose CtrlC stream on windows (#3186)

* Make tokio::signal::windows::ctrl_c() public.
* Stop referring to private tokio::signal::windows::Event in module
  documentation.

Closes #3178

tokio/src/signal/windows.rs

@@ -1,9 +1,9 @@
 //! Windows-specific types for signal handling.
 //!
-//! This module is only defined on Windows and contains the primary `Event` type
-//! for receiving notifications of events. These events are listened for via the
+//! This module is only defined on Windows and allows receiving "ctrl-c"
+//! and "ctrl-break" notifications. These events are listened for via the
 //! `SetConsoleCtrlHandler` function which receives events of the type
-//! `CTRL_C_EVENT` and `CTRL_BREAK_EVENT`
+//! `CTRL_C_EVENT` and `CTRL_BREAK_EVENT`.
 
 #![cfg(windows)]
 
@@ -79,10 +79,6 @@ pub(crate) struct Event {
     rx: Receiver<()>,
 }
 
-pub(crate) fn ctrl_c() -> io::Result<Event> {
-    Event::new(CTRL_C_EVENT)
-}
-
 impl Event {
     fn new(signum: DWORD) -> io::Result<Self> {
         global_init()?;
@@ -135,6 +131,116 @@ unsafe extern "system" fn handler(ty: DWORD) -> BOOL {
     }
 }
 
+/// Creates a new stream which receives "ctrl-c" notifications sent to the
+/// process.
+///
+/// # Examples
+///
+/// ```rust,no_run
+/// use tokio::signal::windows::ctrl_c;
+///
+/// #[tokio::main]
+/// async fn main() -> Result<(), Box<dyn std::error::Error>> {
+///     // An infinite stream of CTRL-C events.
+///     let mut stream = ctrl_c()?;
+///
+///     // Print whenever a CTRL-C event is received.
+///     for countdown in (0..3).rev() {
+///         stream.recv().await;
+///         println!("got CTRL-C. {} more to exit", countdown);
+///     }
+///
+///     Ok(())
+/// }
+/// ```
+pub fn ctrl_c() -> io::Result<CtrlC> {
+    Event::new(CTRL_C_EVENT).map(|inner| CtrlC { inner })
+}
+
+/// Represents a stream which receives "ctrl-c" notifications sent to the process
+/// via `SetConsoleCtrlHandler`.
+///
+/// A notification to this process notifies *all* streams listening for
+/// this event. Moreover, the notifications **are coalesced** if they aren't processed
+/// quickly enough. This means that if two notifications are received back-to-back,
+/// then the stream may only receive one item about the two notifications.
+#[must_use = "streams do nothing unless polled"]
+#[derive(Debug)]
+pub struct CtrlC {
+    inner: Event,
+}
+
+impl CtrlC {
+    /// Receives the next signal notification event.
+    ///
+    /// `None` is returned if no more events can be received by this stream.
+    ///
+    /// # Examples
+    ///
+    /// ```rust,no_run
+    /// use tokio::signal::windows::ctrl_c;
+    ///
+    /// #[tokio::main]
+    /// async fn main() -> Result<(), Box<dyn std::error::Error>> {
+    ///     // An infinite stream of CTRL-C events.
+    ///     let mut stream = ctrl_c()?;
+    ///
+    ///     // Print whenever a CTRL-C event is received.
+    ///     for countdown in (0..3).rev() {
+    ///         stream.recv().await;
+    ///         println!("got CTRL-C. {} more to exit", countdown);
+    ///     }
+    ///
+    ///     Ok(())
+    /// }
+    /// ```
+    pub async fn recv(&mut self) -> Option<()> {
+        self.inner.recv().await
+    }
+
+    /// Polls to receive the next signal notification event, outside of an
+    /// `async` context.
+    ///
+    /// `None` is returned if no more events can be received by this stream.
+    ///
+    /// # Examples
+    ///
+    /// Polling from a manually implemented future
+    ///
+    /// ```rust,no_run
+    /// use std::pin::Pin;
+    /// use std::future::Future;
+    /// use std::task::{Context, Poll};
+    /// use tokio::signal::windows::CtrlC;
+    ///
+    /// struct MyFuture {
+    ///     ctrl_c: CtrlC,
+    /// }
+    ///
+    /// impl Future for MyFuture {
+    ///     type Output = Option<()>;
+    ///
+    ///     fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+    ///         println!("polling MyFuture");
+    ///         self.ctrl_c.poll_recv(cx)
+    ///     }
+    /// }
+    /// ```
+    pub fn poll_recv(&mut self, cx: &mut Context<'_>) -> Poll<Option<()>> {
+        self.inner.rx.poll_recv(cx)
+    }
+}
+
+cfg_stream! {
+    impl crate::stream::Stream for CtrlC {
+        type Item = ();
+
+        fn poll_next(mut self: std::pin::Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<()>> {
+            self.poll_recv(cx)
+        }
+    }
+}
+
 /// Represents a stream which receives "ctrl-break" notifications sent to the process
 /// via `SetConsoleCtrlHandler`.
 ///
@@ -163,16 +269,15 @@ impl CtrlBreak {
     ///     // An infinite stream of CTRL-BREAK events.
     ///     let mut stream = ctrl_break()?;
     ///
-    ///     // Print whenever a CTRL-BREAK event is received
+    ///     // Print whenever a CTRL-BREAK event is received.
     ///     loop {
     ///         stream.recv().await;
     ///         println!("got signal CTRL-BREAK");
     ///     }
     /// }
     /// ```
     pub async fn recv(&mut self) -> Option<()> {
-        use crate::future::poll_fn;
-        poll_fn(|cx| self.poll_recv(cx)).await
+        self.inner.recv().await
     }
 
     /// Polls to receive the next signal notification event, outside of an
@@ -231,7 +336,7 @@ cfg_stream! {
 ///     // An infinite stream of CTRL-BREAK events.
 ///     let mut stream = ctrl_break()?;
 ///
-///     // Print whenever a CTRL-BREAK event is received
+///     // Print whenever a CTRL-BREAK event is received.
 ///     loop {
 ///         stream.recv().await;
 ///         println!("got signal CTRL-BREAK");