Add ability to adjust INITIAL_WINDOW_SIZE setting on an existing connection

src/client.rs

@@ -1201,6 +1201,25 @@ where
         self.inner.set_target_window_size(size);
     }
 
+    /// Set a new `INITIAL_WINDOW_SIZE` setting (in octets) for stream-level
+    /// flow control for received data.
+    ///
+    /// The `SETTINGS` will be sent to the remote, and only applied once the
+    /// remote acknowledges the change.
+    ///
+    /// This can be used to increase or decrease the window size for existing
+    /// streams.
+    ///
+    /// # Errors
+    ///
+    /// Returns an error if a previous call is still pending acknowledgement
+    /// from the remote endpoint.
+    pub fn set_initial_window_size(&mut self, size: u32) -> Result<(), crate::Error> {
+        assert!(size <= proto::MAX_WINDOW_SIZE);
+        self.inner.set_initial_window_size(size)?;
+        Ok(())
+    }
+
     /// Takes a `PingPong` instance from the connection.
     ///
     /// # Note

src/codec/error.rs

@@ -60,6 +60,9 @@ pub enum UserError {
 
     /// Calls `PingPong::send_ping` before receiving a pong.
     SendPingWhilePending,
+
+    /// Tries to update local SETTINGS while ACK has not been received.
+    SendSettingsWhilePending,
 }
 
 // ===== impl RecvError =====
@@ -140,6 +143,7 @@ impl error::Error for UserError {
             MissingUriSchemeAndAuthority => "request URI missing scheme and authority",
             PollResetAfterSendResponse => "poll_reset after send_response is illegal",
             SendPingWhilePending => "send_ping before received previous pong",
+            SendSettingsWhilePending => "sending SETTINGS before received previous ACK",
         }
     }
 }

src/proto/connection.rs

@@ -1,4 +1,4 @@
-use crate::codec::RecvError;
+use crate::codec::{RecvError, UserError};
 use crate::frame::{Reason, StreamId};
 use crate::{client, frame, proto, server};
 
@@ -99,16 +99,24 @@ where
             codec,
             go_away: GoAway::new(),
             ping_pong: PingPong::new(),
-            settings: Settings::new(),
+            settings: Settings::new(config.settings),
             streams,
             _phantom: PhantomData,
         }
     }
 
-    pub fn set_target_window_size(&mut self, size: WindowSize) {
+    /// connection flow control
+    pub(crate) fn set_target_window_size(&mut self, size: WindowSize) {
         self.streams.set_target_connection_window_size(size);
     }
 
+    /// Send a new SETTINGS frame with an updated initial window size.
+    pub(crate) fn set_initial_window_size(&mut self, size: WindowSize) -> Result<(), UserError> {
+        let mut settings = frame::Settings::default();
+        settings.set_initial_window_size(Some(size));
+        self.settings.send_settings(settings)
+    }
+
     /// Returns `Ready` when the connection is ready to receive a frame.
     ///
     /// Returns `RecvError` as this may raise errors that are caused by delayed
@@ -119,7 +127,7 @@ where
         ready!(self.ping_pong.send_pending_ping(cx, &mut self.codec))?;
         ready!(self
             .settings
-            .send_pending_ack(cx, &mut self.codec, &mut self.streams))?;
+            .poll_send(cx, &mut self.codec, &mut self.streams))?;
         ready!(self.streams.send_pending_refusal(cx, &mut self.codec))?;
 
         Poll::Ready(Ok(()))
@@ -327,7 +335,8 @@ where
                 }
                 Some(Settings(frame)) => {
                     log::trace!("recv SETTINGS; frame={:?}", frame);
-                    self.settings.recv_settings(frame);
+                    self.settings
+                        .recv_settings(frame, &mut self.codec, &mut self.streams)?;
                 }
                 Some(GoAway(frame)) => {
                     log::trace!("recv GOAWAY; frame={:?}", frame);

src/proto/settings.rs

@@ -1,32 +1,98 @@
-use crate::codec::RecvError;
+use crate::codec::{RecvError, UserError};
+use crate::error::Reason;
 use crate::frame;
 use crate::proto::*;
 use std::task::{Context, Poll};
 
 #[derive(Debug)]
 pub(crate) struct Settings {
+    /// Our local SETTINGS sync state with the remote.
+    local: Local,
     /// Received SETTINGS frame pending processing. The ACK must be written to
     /// the socket first then the settings applied **before** receiving any
     /// further frames.
-    pending: Option<frame::Settings>,
+    remote: Option<frame::Settings>,
+}
+
+#[derive(Debug)]
+enum Local {
+    /// We want to send these SETTINGS to the remote when the socket is ready.
+    ToSend(frame::Settings),
+    /// We have sent these SETTINGS and are waiting for the remote to ACK
+    /// before we apply them.
+    WaitingAck(frame::Settings),
+    /// Our local settings are in sync with the remote.
+    Synced,
 }
 
 impl Settings {
-    pub fn new() -> Self {
-        Settings { pending: None }
+    pub(crate) fn new(local: frame::Settings) -> Self {
+        Settings {
+            // We assume the initial local SETTINGS were flushed during
+            // the handshake process.
+            local: Local::WaitingAck(local),
+            remote: None,
+        }
     }
 
-    pub fn recv_settings(&mut self, frame: frame::Settings) {
+    pub(crate) fn recv_settings<T, B, C, P>(
+        &mut self,
+        frame: frame::Settings,
+        codec: &mut Codec<T, B>,
+        streams: &mut Streams<C, P>,
+    ) -> Result<(), RecvError>
+    where
+        T: AsyncWrite + Unpin,
+        B: Buf + Unpin,
+        C: Buf + Unpin,
+        P: Peer,
+    {
         if frame.is_ack() {
-            log::debug!("received remote settings ack");
-        // TODO: handle acks
+            match &self.local {
+                Local::WaitingAck(local) => {
+                    log::debug!("received settings ACK; applying {:?}", local);
+
+                    if let Some(max) = local.max_frame_size() {
+                        codec.set_max_recv_frame_size(max as usize);
+                    }
+
+                    if let Some(max) = local.max_header_list_size() {
+                        codec.set_max_recv_header_list_size(max as usize);
+                    }
+
+                    streams.apply_local_settings(local)?;
+                    self.local = Local::Synced;
+                    Ok(())
+                }
+                Local::ToSend(..) | Local::Synced => {
+                    // We haven't sent any SETTINGS frames to be ACKed, so
+                    // this is very bizarre! Remote is either buggy or malicious.
+                    proto_err!(conn: "received unexpected settings ack");
+                    Err(RecvError::Connection(Reason::PROTOCOL_ERROR))
+                }
+            }
         } else {
-            assert!(self.pending.is_none());
-            self.pending = Some(frame);
+            // We always ACK before reading more frames, so `remote` should
+            // always be none!
+            assert!(self.remote.is_none());
+            self.remote = Some(frame);
+            Ok(())
+        }
+    }
+
+    pub(crate) fn send_settings(&mut self, frame: frame::Settings) -> Result<(), UserError> {
+        assert!(!frame.is_ack());
+        match &self.local {
+            Local::ToSend(..) | Local::WaitingAck(..) => Err(UserError::SendSettingsWhilePending),
+            Local::Synced => {
+                log::trace!("queue to send local settings: {:?}", frame);
+                self.local = Local::ToSend(frame);
+                Ok(())
+            }
         }
     }
 
-    pub fn send_pending_ack<T, B, C, P>(
+    pub(crate) fn poll_send<T, B, C, P>(
         &mut self,
         cx: &mut Context,
         dst: &mut Codec<T, B>,
@@ -38,11 +104,8 @@ impl Settings {
         C: Buf + Unpin,
         P: Peer,
     {
-        log::trace!("send_pending_ack; pending={:?}", self.pending);
-
-        if let Some(settings) = &self.pending {
+        if let Some(settings) = &self.remote {
             if !dst.poll_ready(cx)?.is_ready() {
-                log::trace!("failed to send ACK");
                 return Poll::Pending;
             }
 
@@ -61,7 +124,23 @@ impl Settings {
             streams.apply_remote_settings(settings)?;
         }
 
-        self.pending = None;
+        self.remote = None;
+
+        match &self.local {
+            Local::ToSend(settings) => {
+                if !dst.poll_ready(cx)?.is_ready() {
+                    return Poll::Pending;
+                }
+
+                // Buffer the settings frame
+                dst.buffer(settings.clone().into())
+                    .expect("invalid settings frame");
+                log::trace!("local settings sent; waiting for ack: {:?}", settings);
+
+                self.local = Local::WaitingAck(settings.clone());
+            }
+            Local::WaitingAck(..) | Local::Synced => {}
+        }
 
         Poll::Ready(Ok(()))
     }

src/proto/streams/flow_control.rs

@@ -131,11 +131,11 @@ impl FlowControl {
         Ok(())
     }
 
-    /// Decrement the window size.
+    /// Decrement the send-side window size.
     ///
     /// This is called after receiving a SETTINGS frame with a lower
     /// INITIAL_WINDOW_SIZE value.
-    pub fn dec_window(&mut self, sz: WindowSize) {
+    pub fn dec_send_window(&mut self, sz: WindowSize) {
         log::trace!(
             "dec_window; sz={}; window={}, available={}",
             sz,
@@ -146,6 +146,22 @@ impl FlowControl {
         self.window_size -= sz;
     }
 
+    /// Decrement the recv-side window size.
+    ///
+    /// This is called after receiving a SETTINGS ACK frame with a lower
+    /// INITIAL_WINDOW_SIZE value.
+    pub fn dec_recv_window(&mut self, sz: WindowSize) {
+        log::trace!(
+            "dec_recv_window; sz={}; window={}, available={}",
+            sz,
+            self.window_size,
+            self.available
+        );
+        // This should not be able to overflow `window_size` from the bottom.
+        self.window_size -= sz;
+        self.available -= sz;
+    }
+
     /// Decrements the window reflecting data has actually been sent. The caller
     /// must ensure that the window has capacity.
     pub fn send_data(&mut self, sz: WindowSize) {

src/proto/streams/recv.rs

@@ -455,6 +455,67 @@ impl Recv {
         }
     }
 
+    pub(crate) fn apply_local_settings(
+        &mut self,
+        settings: &frame::Settings,
+        store: &mut Store,
+    ) -> Result<(), RecvError> {
+        let target = if let Some(val) = settings.initial_window_size() {
+            val
+        } else {
+            return Ok(());
+        };
+
+        let old_sz = self.init_window_sz;
+        self.init_window_sz = target;
+
+        log::trace!("update_initial_window_size; new={}; old={}", target, old_sz,);
+
+        // Per RFC 7540 §6.9.2:
+        //
+        // In addition to changing the flow-control window for streams that are
+        // not yet active, a SETTINGS frame can alter the initial flow-control
+        // window size for streams with active flow-control windows (that is,
+        // streams in the "open" or "half-closed (remote)" state). When the
+        // value of SETTINGS_INITIAL_WINDOW_SIZE changes, a receiver MUST adjust
+        // the size of all stream flow-control windows that it maintains by the
+        // difference between the new value and the old value.
+        //
+        // A change to `SETTINGS_INITIAL_WINDOW_SIZE` can cause the available
+        // space in a flow-control window to become negative. A sender MUST
+        // track the negative flow-control window and MUST NOT send new
+        // flow-controlled frames until it receives WINDOW_UPDATE frames that
+        // cause the flow-control window to become positive.
+
+        if target < old_sz {
+            // We must decrease the (local) window on every open stream.
+            let dec = old_sz - target;
+            log::trace!("decrementing all windows; dec={}", dec);
+
+            store.for_each(|mut stream| {
+                stream.recv_flow.dec_recv_window(dec);
+                Ok(())
+            })
+        } else if target > old_sz {
+            // We must increase the (local) window on every open stream.
+            let inc = target - old_sz;
+            log::trace!("incrementing all windows; inc={}", inc);
+            store.for_each(|mut stream| {
+                // XXX: Shouldn't the peer have already noticed our
+                // overflow and sent us a GOAWAY?
+                stream
+                    .recv_flow
+                    .inc_window(inc)
+                    .map_err(RecvError::Connection)?;
+                stream.recv_flow.assign_capacity(inc);
+                Ok(())
+            })
+        } else {
+            // size is the same... so do nothing
+            Ok(())
+        }
+    }
+
     pub fn is_end_stream(&self, stream: &store::Ptr) -> bool {
         if !stream.state.is_recv_closed() {
             return false;

src/proto/streams/send.rs

@@ -419,7 +419,7 @@ impl Send {
                 store.for_each(|mut stream| {
                     let stream = &mut *stream;
 
-                    stream.send_flow.dec_window(dec);
+                    stream.send_flow.dec_send_window(dec);
 
                     // It's possible that decreasing the window causes
                     // `window_size` (the stream-specific window) to fall below

src/proto/streams/streams.rs

@@ -664,6 +664,13 @@ where
         )
     }
 
+    pub fn apply_local_settings(&mut self, frame: &frame::Settings) -> Result<(), RecvError> {
+        let mut me = self.inner.lock().unwrap();
+        let me = &mut *me;
+
+        me.actions.recv.apply_local_settings(frame, &mut me.store)
+    }
+
     pub fn send_request(
         &mut self,
         request: Request<()>,

src/server.rs

@@ -441,6 +441,25 @@ where
         self.connection.set_target_window_size(size);
     }
 
+    /// Set a new `INITIAL_WINDOW_SIZE` setting (in octets) for stream-level
+    /// flow control for received data.
+    ///
+    /// The `SETTINGS` will be sent to the remote, and only applied once the
+    /// remote acknowledges the change.
+    ///
+    /// This can be used to increase or decrease the window size for existing
+    /// streams.
+    ///
+    /// # Errors
+    ///
+    /// Returns an error if a previous call is still pending acknowledgement
+    /// from the remote endpoint.
+    pub fn set_initial_window_size(&mut self, size: u32) -> Result<(), crate::Error> {
+        assert!(size <= proto::MAX_WINDOW_SIZE);
+        self.connection.set_initial_window_size(size)?;
+        Ok(())
+    }
+
     /// Returns `Ready` when the underlying connection has closed.
     ///
     /// If any new inbound streams are received during a call to `poll_closed`,