Move async callbacks infra to boring itself

This helps drive async callbacks from outside tokio-boring, such as in quiche.

Not a breaking change because every public item in tokio-boring is preserved as is.

boring/src/ssl/async_callbacks.rs

@@ -0,0 +1,308 @@
+use super::mut_only::MutOnly;
+use super::{
+    ClientHello, GetSessionPendingError, PrivateKeyMethod, PrivateKeyMethodError, SelectCertError,
+    Ssl, SslContextBuilder, SslRef, SslSession, SslSignatureAlgorithm,
+};
+use crate::ex_data::Index;
+use once_cell::sync::Lazy;
+use std::convert::identity;
+use std::future::Future;
+use std::pin::Pin;
+use std::task::{ready, Context, Poll, Waker};
+
+/// The type of futures to pass to [`SslContextBuilderExt::set_async_select_certificate_callback`].
+pub type BoxSelectCertFuture = ExDataFuture<Result<BoxSelectCertFinish, AsyncSelectCertError>>;
+
+/// The type of callbacks returned by [`BoxSelectCertFuture`] methods.
+pub type BoxSelectCertFinish = Box<dyn FnOnce(ClientHello<'_>) -> Result<(), AsyncSelectCertError>>;
+
+/// The type of futures returned by [`AsyncPrivateKeyMethod`] methods.
+pub type BoxPrivateKeyMethodFuture =
+    ExDataFuture<Result<BoxPrivateKeyMethodFinish, AsyncPrivateKeyMethodError>>;
+
+/// The type of callbacks returned by [`BoxPrivateKeyMethodFuture`].
+pub type BoxPrivateKeyMethodFinish =
+    Box<dyn FnOnce(&mut SslRef, &mut [u8]) -> Result<usize, AsyncPrivateKeyMethodError>>;
+
+/// The type of futures to pass to [`SslContextBuilderExt::set_async_get_session_callback`].
+pub type BoxGetSessionFuture = ExDataFuture<Option<BoxGetSessionFinish>>;
+
+/// The type of callbacks returned by [`BoxSelectCertFuture`] methods.
+pub type BoxGetSessionFinish = Box<dyn FnOnce(&mut SslRef, &[u8]) -> Option<SslSession>>;
+
+/// Convenience alias for futures stored in [`Ssl`] ex data by [`SslContextBuilderExt`] methods.
+///
+/// Public for documentation purposes.
+pub type ExDataFuture<T> = Pin<Box<dyn Future<Output = T> + Send>>;
+
+pub(crate) static TASK_WAKER_INDEX: Lazy<Index<Ssl, Option<Waker>>> =
+    Lazy::new(|| Ssl::new_ex_index().unwrap());
+pub(crate) static SELECT_CERT_FUTURE_INDEX: Lazy<Index<Ssl, MutOnly<Option<BoxSelectCertFuture>>>> =
+    Lazy::new(|| Ssl::new_ex_index().unwrap());
+pub(crate) static SELECT_PRIVATE_KEY_METHOD_FUTURE_INDEX: Lazy<
+    Index<Ssl, MutOnly<Option<BoxPrivateKeyMethodFuture>>>,
+> = Lazy::new(|| Ssl::new_ex_index().unwrap());
+pub(crate) static SELECT_GET_SESSION_FUTURE_INDEX: Lazy<
+    Index<Ssl, MutOnly<Option<BoxGetSessionFuture>>>,
+> = Lazy::new(|| Ssl::new_ex_index().unwrap());
+
+impl SslContextBuilder {
+    /// Sets a callback that is called before most [`ClientHello`] processing
+    /// and before the decision whether to resume a session is made. The
+    /// callback may inspect the [`ClientHello`] and configure the connection.
+    ///
+    /// This method uses a function that returns a future whose output is
+    /// itself a closure that will be passed [`ClientHello`] to configure
+    /// the connection based on the computations done in the future.
+    ///
+    /// A task waker must be set on `Ssl` values associated with the resulting
+    /// `SslContext` with [`SslRef::set_task_waker`].
+    ///
+    /// See [`SslContextBuilder::set_select_certificate_callback`] for the sync
+    /// setter of this callback.
+    pub fn set_async_select_certificate_callback<F>(&mut self, callback: F)
+    where
+        F: Fn(&mut ClientHello<'_>) -> Result<BoxSelectCertFuture, AsyncSelectCertError>
+            + Send
+            + Sync
+            + 'static,
+    {
+        self.set_select_certificate_callback(move |mut client_hello| {
+            let fut_poll_result = with_ex_data_future(
+                &mut client_hello,
+                *SELECT_CERT_FUTURE_INDEX,
+                ClientHello::ssl_mut,
+                &callback,
+                identity,
+            );
+
+            let fut_result = match fut_poll_result {
+                Poll::Ready(fut_result) => fut_result,
+                Poll::Pending => return Err(SelectCertError::RETRY),
+            };
+
+            let finish = fut_result.or(Err(SelectCertError::ERROR))?;
+
+            finish(client_hello).or(Err(SelectCertError::ERROR))
+        })
+    }
+
+    /// Configures a custom private key method on the context.
+    ///
+    /// A task waker must be set on `Ssl` values associated with the resulting
+    /// `SslContext` with [`SslRef::set_task_waker`].
+    ///
+    /// See [`AsyncPrivateKeyMethod`] for more details.
+    pub fn set_async_private_key_method(&mut self, method: impl AsyncPrivateKeyMethod) {
+        self.set_private_key_method(AsyncPrivateKeyMethodBridge(Box::new(method)));
+    }
+
+    /// Sets a callback that is called when a client proposed to resume a session
+    /// but it was not found in the internal cache.
+    ///
+    /// The callback is passed a reference to the session ID provided by the client.
+    /// It should return the session corresponding to that ID if available. This is
+    /// only used for servers, not clients.
+    ///
+    /// A task waker must be set on `Ssl` values associated with the resulting
+    /// `SslContext` with [`SslRef::set_task_waker`].
+    ///
+    /// See [`SslContextBuilder::set_get_session_callback`] for the sync setter
+    /// of this callback.
+    ///
+    /// # Safety
+    ///
+    /// The returned [`SslSession`] must not be associated with a different [`SslContext`].
+    pub unsafe fn set_async_get_session_callback<F>(&mut self, callback: F)
+    where
+        F: Fn(&mut SslRef, &[u8]) -> Option<BoxGetSessionFuture> + Send + Sync + 'static,
+    {
+        let async_callback = move |ssl: &mut SslRef, id: &[u8]| {
+            let fut_poll_result = with_ex_data_future(
+                &mut *ssl,
+                *SELECT_GET_SESSION_FUTURE_INDEX,
+                |ssl| ssl,
+                |ssl| callback(ssl, id).ok_or(()),
+                |option| option.ok_or(()),
+            );
+
+            match fut_poll_result {
+                Poll::Ready(Err(())) => Ok(None),
+                Poll::Ready(Ok(finish)) => Ok(finish(ssl, id)),
+                Poll::Pending => Err(GetSessionPendingError),
+            }
+        };
+
+        self.set_get_session_callback(async_callback)
+    }
+}
+
+impl SslRef {
+    /// Sets the task waker to be used in async callbacks installed on this `Ssl`.
+    pub fn set_task_waker(&mut self, waker: Option<Waker>) {
+        self.replace_ex_data(*TASK_WAKER_INDEX, waker);
+    }
+}
+
+/// A fatal error to be returned from async select certificate callbacks.
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+pub struct AsyncSelectCertError;
+
+/// Describes async private key hooks. This is used to off-load signing
+/// operations to a custom, potentially asynchronous, backend. Metadata about the
+/// key such as the type and size are parsed out of the certificate.
+///
+/// See [`PrivateKeyMethod`] for the sync version of those hooks.
+///
+/// [`ssl_private_key_method_st`]: https://commondatastorage.googleapis.com/chromium-boringssl-docs/ssl.h.html#ssl_private_key_method_st
+pub trait AsyncPrivateKeyMethod: Send + Sync + 'static {
+    /// Signs the message `input` using the specified signature algorithm.
+    ///
+    /// This method uses a function that returns a future whose output is
+    /// itself a closure that will be passed `ssl` and `output`
+    /// to finish writing the signature.
+    ///
+    /// See [`PrivateKeyMethod::sign`] for the sync version of this method.
+    fn sign(
+        &self,
+        ssl: &mut SslRef,
+        input: &[u8],
+        signature_algorithm: SslSignatureAlgorithm,
+        output: &mut [u8],
+    ) -> Result<BoxPrivateKeyMethodFuture, AsyncPrivateKeyMethodError>;
+
+    /// Decrypts `input`.
+    ///
+    /// This method uses a function that returns a future whose output is
+    /// itself a closure that will be passed `ssl` and `output`
+    /// to finish decrypting the input.
+    ///
+    /// See [`PrivateKeyMethod::decrypt`] for the sync version of this method.
+    fn decrypt(
+        &self,
+        ssl: &mut SslRef,
+        input: &[u8],
+        output: &mut [u8],
+    ) -> Result<BoxPrivateKeyMethodFuture, AsyncPrivateKeyMethodError>;
+}
+
+/// A fatal error to be returned from async private key methods.
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+pub struct AsyncPrivateKeyMethodError;
+
+struct AsyncPrivateKeyMethodBridge(Box<dyn AsyncPrivateKeyMethod>);
+
+impl PrivateKeyMethod for AsyncPrivateKeyMethodBridge {
+    fn sign(
+        &self,
+        ssl: &mut SslRef,
+        input: &[u8],
+        signature_algorithm: SslSignatureAlgorithm,
+        output: &mut [u8],
+    ) -> Result<usize, PrivateKeyMethodError> {
+        with_private_key_method(ssl, output, |ssl, output| {
+            <dyn AsyncPrivateKeyMethod>::sign(&*self.0, ssl, input, signature_algorithm, output)
+        })
+    }
+
+    fn decrypt(
+        &self,
+        ssl: &mut SslRef,
+        input: &[u8],
+        output: &mut [u8],
+    ) -> Result<usize, PrivateKeyMethodError> {
+        with_private_key_method(ssl, output, |ssl, output| {
+            <dyn AsyncPrivateKeyMethod>::decrypt(&*self.0, ssl, input, output)
+        })
+    }
+
+    fn complete(
+        &self,
+        ssl: &mut SslRef,
+        output: &mut [u8],
+    ) -> Result<usize, PrivateKeyMethodError> {
+        with_private_key_method(ssl, output, |_, _| {
+            // This should never be reached, if it does, that's a bug on boring's side,
+            // which called `complete` without having been returned to with a pending
+            // future from `sign` or `decrypt`.
+
+            if cfg!(debug_assertions) {
+                panic!("BUG: boring called complete without a pending operation");
+            }
+
+            Err(AsyncPrivateKeyMethodError)
+        })
+    }
+}
+
+/// Creates and drives a private key method future.
+///
+/// This is a convenience function for the three methods of impl `PrivateKeyMethod``
+/// for `dyn AsyncPrivateKeyMethod`. It relies on [`with_ex_data_future`] to
+/// drive the future and then immediately calls the final [`BoxPrivateKeyMethodFinish`]
+/// when the future is ready.
+fn with_private_key_method(
+    ssl: &mut SslRef,
+    output: &mut [u8],
+    create_fut: impl FnOnce(
+        &mut SslRef,
+        &mut [u8],
+    ) -> Result<BoxPrivateKeyMethodFuture, AsyncPrivateKeyMethodError>,
+) -> Result<usize, PrivateKeyMethodError> {
+    let fut_poll_result = with_ex_data_future(
+        ssl,
+        *SELECT_PRIVATE_KEY_METHOD_FUTURE_INDEX,
+        |ssl| ssl,
+        |ssl| create_fut(ssl, output),
+        identity,
+    );
+
+    let fut_result = match fut_poll_result {
+        Poll::Ready(fut_result) => fut_result,
+        Poll::Pending => return Err(PrivateKeyMethodError::RETRY),
+    };
+
+    let finish = fut_result.or(Err(PrivateKeyMethodError::FAILURE))?;
+
+    finish(ssl, output).or(Err(PrivateKeyMethodError::FAILURE))
+}
+
+/// Creates and drives a future stored in `ssl_handle`'s `Ssl` at ex data index `index`.
+///
+/// This function won't even bother storing the future in `index` if the future
+/// created by `create_fut` returns `Poll::Ready(_)` on the first poll call.
+fn with_ex_data_future<H, R, T, E>(
+    ssl_handle: &mut H,
+    index: Index<Ssl, MutOnly<Option<ExDataFuture<R>>>>,
+    get_ssl_mut: impl Fn(&mut H) -> &mut SslRef,
+    create_fut: impl FnOnce(&mut H) -> Result<ExDataFuture<R>, E>,
+    into_result: impl Fn(R) -> Result<T, E>,
+) -> Poll<Result<T, E>> {
+    let ssl = get_ssl_mut(ssl_handle);
+    let waker = ssl
+        .ex_data(*TASK_WAKER_INDEX)
+        .cloned()
+        .flatten()
+        .expect("task waker should be set");
+
+    let mut ctx = Context::from_waker(&waker);
+
+    if let Some(data @ Some(_)) = ssl.ex_data_mut(index).map(MutOnly::get_mut) {
+        let fut_result = into_result(ready!(data.as_mut().unwrap().as_mut().poll(&mut ctx)));
+
+        *data = None;
+
+        Poll::Ready(fut_result)
+    } else {
+        let mut fut = create_fut(ssl_handle)?;
+
+        match fut.as_mut().poll(&mut ctx) {
+            Poll::Ready(fut_result) => Poll::Ready(into_result(fut_result)),
+            Poll::Pending => {
+                get_ssl_mut(ssl_handle).replace_ex_data(index, MutOnly::new(Some(fut)));
+
+                Poll::Pending
+            }
+        }
+    }
+}

boring/src/ssl/mod.rs

@@ -97,15 +97,22 @@ use crate::x509::{
 };
 use crate::{cvt, cvt_0i, cvt_n, cvt_p, init};
 
-pub use crate::ssl::connector::{
+pub use self::async_callbacks::{
+    AsyncPrivateKeyMethod, AsyncPrivateKeyMethodError, AsyncSelectCertError, BoxGetSessionFinish,
+    BoxGetSessionFuture, BoxPrivateKeyMethodFinish, BoxPrivateKeyMethodFuture, BoxSelectCertFinish,
+    BoxSelectCertFuture, ExDataFuture,
+};
+pub use self::connector::{
     ConnectConfiguration, SslAcceptor, SslAcceptorBuilder, SslConnector, SslConnectorBuilder,
 };
-pub use crate::ssl::error::{Error, ErrorCode, HandshakeError};
+pub use self::error::{Error, ErrorCode, HandshakeError};
 
+mod async_callbacks;
 mod bio;
 mod callbacks;
 mod connector;
 mod error;
+mod mut_only;
 #[cfg(test)]
 mod test;