SETTINGS_MAX_HEADER_LIST_SIZE (#206)

This, uh, grew into something far bigger than expected, but it turns out, all of it was needed to eventually support this correctly.

- Adds configuration to client and server to set [SETTINGS_MAX_HEADER_LIST_SIZE](http://httpwg.org/specs/rfc7540.html#SETTINGS_MAX_HEADER_LIST_SIZE)
- If not set, a "sane default" of 16 MB is used (taken from golang's http2)
- Decoding header blocks now happens as they are received, instead of buffering up possibly forever until the last continuation frame is parsed.
- As each field is decoded, it's undecoded size is added to the total. Whenever a header block goes over the maximum size, the `frame` will be marked as such.
- Whenever a header block is deemed over max limit, decoding will still continue, but new fields will not be appended to `HeaderMap`. This is also can save wasted hashing.
- To protect against enormous string literals, such that they span multiple continuation frames, a check is made that the combined encoded bytes is less than the max allowed size. While technically not exactly what the spec suggests (counting decoded size instead), this should hopefully only happen when someone is indeed malicious. If found, a `GOAWAY` of `COMPRESSION_ERROR` is sent, and the connection shut down.
- After an oversize header block frame is finished decoding, the streams state machine will notice it is oversize, and handle that.
  - If the local peer is a server, a 431 response is sent, as suggested by the spec.
  - A `REFUSED_STREAM` reset is sent, since we cannot actually give the stream to the user.
- In order to be able to send both the 431 headers frame, and a reset frame afterwards, the scheduled `Canceled` machinery was made more general to a `Scheduled(Reason)` state instead.

Closes #18 
Closes #191

src/proto/streams/recv.rs

@@ -1,4 +1,5 @@
 use super::*;
+use super::store::Resolve;
 use {frame, proto};
 use codec::{RecvError, UserError};
 use frame::{Reason, DEFAULT_INITIAL_WINDOW_SIZE};
@@ -54,6 +55,12 @@ pub(super) enum Event {
     Trailers(HeaderMap),
 }
 
+#[derive(Debug)]
+pub(super) enum RecvHeaderBlockError<T> {
+    Oversize(T),
+    State(RecvError),
+}
+
 #[derive(Debug, Clone, Copy)]
 struct Indices {
     head: store::Key,
@@ -133,7 +140,7 @@ impl Recv {
         frame: frame::Headers,
         stream: &mut store::Ptr,
         counts: &mut Counts,
-    ) -> Result<(), RecvError> {
+    ) -> Result<(), RecvHeaderBlockError<Option<frame::Headers>>> {
         trace!("opening stream; init_window={}", self.init_window_sz);
         let is_initial = stream.state.recv_open(frame.is_end_stream())?;
 
@@ -158,7 +165,7 @@ impl Recv {
                         return Err(RecvError::Stream {
                             id: stream.id,
                             reason: Reason::PROTOCOL_ERROR,
-                        })
+                        }.into())
                     },
                 };
 
@@ -166,6 +173,32 @@ impl Recv {
             }
         }
 
+        if frame.is_over_size() {
+            // A frame is over size if the decoded header block was bigger than
+            // SETTINGS_MAX_HEADER_LIST_SIZE.
+            //
+            // > A server that receives a larger header block than it is willing
+            // > to handle can send an HTTP 431 (Request Header Fields Too
+            // > Large) status code [RFC6585]. A client can discard responses
+            // > that it cannot process.
+            //
+            // So, if peer is a server, we'll send a 431. In either case,
+            // an error is recorded, which will send a REFUSED_STREAM,
+            // since we don't want any of the data frames either.
+            trace!("recv_headers; frame for {:?} is over size", stream.id);
+            return if counts.peer().is_server() && is_initial {
+                let mut res = frame::Headers::new(
+                    stream.id,
+                    frame::Pseudo::response(::http::StatusCode::REQUEST_HEADER_FIELDS_TOO_LARGE),
+                    HeaderMap::new()
+                );
+                res.set_end_stream();
+                Err(RecvHeaderBlockError::Oversize(Some(res)))
+            } else {
+                Err(RecvHeaderBlockError::Oversize(None))
+            };
+        }
+
         let message = counts.peer().convert_poll_message(frame)?;
 
         // Push the frame onto the stream's recv buffer
@@ -517,15 +550,20 @@ impl Recv {
         );
 
         new_stream.state.reserve_remote()?;
+        // Store the stream
+        let new_stream = store.insert(frame.promised_id(), new_stream).key();
+
+
+        if frame.is_over_size() {
+            trace!("recv_push_promise; frame for {:?} is over size", frame.promised_id());
+            return Err(RecvError::Stream {
+                id: frame.promised_id(),
+                reason: Reason::REFUSED_STREAM,
+            });
+        }
 
         let mut ppp = store[stream].pending_push_promises.take();
-
-        {
-            // Store the stream
-            let mut new_stream = store.insert(frame.promised_id(), new_stream);
-
-            ppp.push(&mut new_stream);
-        }
+        ppp.push(&mut store.resolve(new_stream));
 
         let stream = &mut store[stream];
 
@@ -609,9 +647,7 @@ impl Recv {
         stream: &mut store::Ptr,
         counts: &mut Counts,
     ) {
-        assert!(stream.state.is_local_reset());
-
-        if stream.is_pending_reset_expiration() {
+        if !stream.state.is_local_reset() || stream.is_pending_reset_expiration() {
             return;
         }
 
@@ -842,6 +878,14 @@ impl Event {
     }
 }
 
+// ===== impl RecvHeaderBlockError =====
+
+impl<T> From<RecvError> for RecvHeaderBlockError<T> {
+    fn from(err: RecvError) -> Self {
+        RecvHeaderBlockError::State(err)
+    }
+}
+
 // ===== util =====
 
 fn parse_u64(src: &[u8]) -> Result<u64, ()> {