feat: add max_pending_accept_reset_streams(n) options

The new option is available to both client and server `Builder`s.

src/client.rs

@@ -326,6 +326,10 @@ pub struct Builder {
     /// Maximum number of locally reset streams to keep at a time.
     reset_stream_max: usize,
 
+    /// Maximum number of remotely reset streams to allow in the pending
+    /// accept queue.
+    pending_accept_reset_stream_max: usize,
+
     /// Initial `Settings` frame to send as part of the handshake.
     settings: Settings,
 
@@ -634,6 +638,7 @@ impl Builder {
             max_send_buffer_size: proto::DEFAULT_MAX_SEND_BUFFER_SIZE,
             reset_stream_duration: Duration::from_secs(proto::DEFAULT_RESET_STREAM_SECS),
             reset_stream_max: proto::DEFAULT_RESET_STREAM_MAX,
+            pending_accept_reset_stream_max: proto::DEFAULT_REMOTE_RESET_STREAM_MAX,
             initial_target_connection_window_size: None,
             initial_max_send_streams: usize::MAX,
             settings: Default::default(),
@@ -966,6 +971,37 @@ impl Builder {
         self
     }
 
+    /// Sets the maximum number of pending-accept remotely-reset streams.
+    ///
+    /// When a
+    ///
+    /// The default value is 20.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use tokio::io::{AsyncRead, AsyncWrite};
+    /// # use h2::client::*;
+    /// # use bytes::Bytes;
+    /// #
+    /// # async fn doc<T: AsyncRead + AsyncWrite + Unpin>(my_io: T)
+    /// # -> Result<((SendRequest<Bytes>, Connection<T, Bytes>)), h2::Error>
+    /// # {
+    /// // `client_fut` is a future representing the completion of the HTTP/2
+    /// // handshake.
+    /// let client_fut = Builder::new()
+    ///     .max_pending_accept_reset_streams(100)
+    ///     .handshake(my_io);
+    /// # client_fut.await
+    /// # }
+    /// #
+    /// # pub fn main() {}
+    /// ```
+    pub fn max_pending_accept_reset_streams(&mut self, max: usize) -> &mut Self {
+        self.pending_accept_reset_stream_max = max;
+        self
+    }
+
     /// Sets the maximum send buffer size per stream.
     ///
     /// Once a stream has buffered up to (or over) the maximum, the stream's
@@ -1209,6 +1245,7 @@ where
                 max_send_buffer_size: builder.max_send_buffer_size,
                 reset_stream_duration: builder.reset_stream_duration,
                 reset_stream_max: builder.reset_stream_max,
+                remote_reset_stream_max: builder.pending_accept_reset_stream_max,
                 settings: builder.settings.clone(),
             },
         );

src/proto/connection.rs

@@ -14,8 +14,6 @@ use std::task::{Context, Poll};
 use std::time::Duration;
 use tokio::io::{AsyncRead, AsyncWrite};
 
-const DEFAULT_MAX_REMOTE_RESET_STREAMS: usize = 20;
-
 /// An H2 connection
 #[derive(Debug)]
 pub(crate) struct Connection<T, P, B: Buf = Bytes>
@@ -82,6 +80,7 @@ pub(crate) struct Config {
     pub max_send_buffer_size: usize,
     pub reset_stream_duration: Duration,
     pub reset_stream_max: usize,
+    pub remote_reset_stream_max: usize,
     pub settings: frame::Settings,
 }
 
@@ -120,7 +119,7 @@ where
                     .unwrap_or(false),
                 local_reset_duration: config.reset_stream_duration,
                 local_reset_max: config.reset_stream_max,
-                remote_reset_max: DEFAULT_MAX_REMOTE_RESET_STREAMS,
+                remote_reset_max: config.remote_reset_stream_max,
                 remote_init_window_sz: DEFAULT_INITIAL_WINDOW_SIZE,
                 remote_max_initiated: config
                     .settings

src/proto/mod.rs

@@ -31,6 +31,7 @@ pub type WindowSize = u32;
 
 // Constants
 pub const MAX_WINDOW_SIZE: WindowSize = (1 << 31) - 1;
+pub const DEFAULT_REMOTE_RESET_STREAM_MAX: usize = 20;
 pub const DEFAULT_RESET_STREAM_MAX: usize = 10;
 pub const DEFAULT_RESET_STREAM_SECS: u64 = 30;
 pub const DEFAULT_MAX_SEND_BUFFER_SIZE: usize = 1024 * 400;

src/server.rs

@@ -240,6 +240,10 @@ pub struct Builder {
     /// Maximum number of locally reset streams to keep at a time.
     reset_stream_max: usize,
 
+    /// Maximum number of remotely reset streams to allow in the pending
+    /// accept queue.
+    pending_accept_reset_stream_max: usize,
+
     /// Initial `Settings` frame to send as part of the handshake.
     settings: Settings,
 
@@ -642,6 +646,7 @@ impl Builder {
         Builder {
             reset_stream_duration: Duration::from_secs(proto::DEFAULT_RESET_STREAM_SECS),
             reset_stream_max: proto::DEFAULT_RESET_STREAM_MAX,
+            pending_accept_reset_stream_max: proto::DEFAULT_REMOTE_RESET_STREAM_MAX,
             settings: Settings::default(),
             initial_target_connection_window_size: None,
             max_send_buffer_size: proto::DEFAULT_MAX_SEND_BUFFER_SIZE,
@@ -882,6 +887,49 @@ impl Builder {
         self
     }
 
+    /// Sets the maximum number of pending-accept remotely-reset streams.
+    ///
+    /// Streams that have been received by the peer, but not accepted by the
+    /// user, can also receive a RST_STREAM. This is a legitimate pattern: one
+    /// could send a request and then shortly after, realize it is not needed,
+    /// sending a CANCEL.
+    ///
+    /// However, since those streams are now "closed", they don't count towards
+    /// the max concurrent streams. So, they will sit in the accept queue,
+    /// using memory.
+    ///
+    /// When the number of remotely-reset streams sitting in the pending-accept
+    /// queue reaches this maximum value, a connection error with the code of
+    /// `ENHANCE_YOUR_CALM` will be sent to the peer, and returned by the
+    /// `Future`.
+    ///
+    /// The default value is 20.
+    ///
+    /// # Examples
+    ///
+    ///
+    /// ```
+    /// # use tokio::io::{AsyncRead, AsyncWrite};
+    /// # use h2::server::*;
+    /// #
+    /// # fn doc<T: AsyncRead + AsyncWrite + Unpin>(my_io: T)
+    /// # -> Handshake<T>
+    /// # {
+    /// // `server_fut` is a future representing the completion of the HTTP/2
+    /// // handshake.
+    /// let server_fut = Builder::new()
+    ///     .max_pending_accept_reset_streams(100)
+    ///     .handshake(my_io);
+    /// # server_fut
+    /// # }
+    /// #
+    /// # pub fn main() {}
+    /// ```
+    pub fn max_pending_accept_reset_streams(&mut self, max: usize) -> &mut Self {
+        self.pending_accept_reset_stream_max = max;
+        self
+    }
+
     /// Sets the maximum send buffer size per stream.
     ///
     /// Once a stream has buffered up to (or over) the maximum, the stream's
@@ -1312,6 +1360,7 @@ where
                             max_send_buffer_size: self.builder.max_send_buffer_size,
                             reset_stream_duration: self.builder.reset_stream_duration,
                             reset_stream_max: self.builder.reset_stream_max,
+                            remote_reset_stream_max: self.builder.pending_accept_reset_stream_max,
                             settings: self.builder.settings.clone(),
                         },
                     );

tests/h2-tests/tests/stream_states.rs

@@ -200,6 +200,7 @@ async fn reset_streams_dont_grow_memory_continuously() {
     let (io, mut client) = mock::new();
 
     const N: u32 = 50;
+    const MAX: usize = 20;
 
     let client = async move {
         let settings = client.assert_server_handshake().await;
@@ -212,14 +213,15 @@ async fn reset_streams_dont_grow_memory_continuously() {
         }
         tokio::time::timeout(
             std::time::Duration::from_secs(1),
-            client.recv_frame(frames::go_away(41).calm()),
+            client.recv_frame(frames::go_away((MAX * 2 + 1) as u32).calm()),
         )
         .await
         .expect("client goaway");
     };
 
     let srv = async move {
         let mut srv = server::Builder::new()
+            .max_pending_accept_reset_streams(MAX)
             .handshake::<_, Bytes>(io)
             .await
             .expect("handshake");