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 </pre><pre class="rust"><code><span class="kw">use </span>std::io;

 <span class="kw">use </span><span class="kw">crate</span>::codec::UserError;
 <span class="kw">use </span><span class="kw">crate</span>::frame::{<span class="self">self</span>, Reason, StreamId};
 <span class="kw">use </span><span class="kw">crate</span>::proto::{<span class="self">self</span>, Error, Initiator, PollReset};

 <span class="kw">use </span><span class="self">self</span>::Inner::<span class="kw-2">*</span>;
 <span class="kw">use </span><span class="self">self</span>::Peer::<span class="kw-2">*</span>;

 <span class="doccomment">/// Represents the state of an H2 stream
 ///
 /// ```not_rust
 ///                              +--------+
 ///                      send PP |        | recv PP
 ///                     ,--------|  idle  |--------.
 ///                    /         |        |         \
 ///                   v          +--------+          v
 ///            +----------+          |           +----------+
 ///            |          |          | send H /  |          |
 ///     ,------| reserved |          | recv H    | reserved |------.
 ///     |      | (local)  |          |           | (remote) |      |
 ///     |      +----------+          v           +----------+      |
 ///     |          |             +--------+             |          |
 ///     |          |     recv ES |        | send ES     |          |
 ///     |   send H |     ,-------|  open  |-------.     | recv H   |
 ///     |          |    /        |        |        \    |          |
 ///     |          v   v         +--------+         v   v          |
 ///     |      +----------+          |           +----------+      |
 ///     |      |   half   |          |           |   half   |      |
 ///     |      |  closed  |          | send R /  |  closed  |      |
 ///     |      | (remote) |          | recv R    | (local)  |      |
 ///     |      +----------+          |           +----------+      |
 ///     |           |                |                 |           |
 ///     |           | send ES /      |       recv ES / |           |
 ///     |           | send R /       v        send R / |           |
 ///     |           | recv R     +--------+   recv R   |           |
 ///     | send R /  `-----------&gt;|        |&lt;-----------&#39;  send R / |
 ///     | recv R                 | closed |               recv R   |
 ///     `-----------------------&gt;|        |&lt;----------------------&#39;
 ///                              +--------+
 ///
 ///        send:   endpoint sends this frame
 ///        recv:   endpoint receives this frame
 ///
 ///        H:  HEADERS frame (with implied CONTINUATIONs)
 ///        PP: PUSH_PROMISE frame (with implied CONTINUATIONs)
 ///        ES: END_STREAM flag
 ///        R:  RST_STREAM frame
 /// ```
 </span><span class="attribute">#[derive(Debug, Clone)]
 </span><span class="kw">pub struct </span>State {
     inner: Inner,
 }

 <span class="attribute">#[derive(Debug, Clone)]
 </span><span class="kw">enum </span>Inner {
     Idle,
     <span class="comment">// TODO: these states shouldn&#39;t count against concurrency limits:
     </span>ReservedLocal,
     ReservedRemote,
     Open { local: Peer, remote: Peer },
     HalfClosedLocal(Peer), <span class="comment">// TODO: explicitly name this value
     </span>HalfClosedRemote(Peer),
     Closed(Cause),
 }

 <span class="attribute">#[derive(Debug, Copy, Clone)]
 </span><span class="kw">enum </span>Peer {
     AwaitingHeaders,
     Streaming,
 }

 <span class="attribute">#[derive(Debug, Clone)]
 </span><span class="kw">enum </span>Cause {
     EndStream,
     Error(Error),

     <span class="doccomment">/// This indicates to the connection that a reset frame must be sent out
     /// once the send queue has been flushed.
     ///
     /// Examples of when this could happen:
     /// - User drops all references to a stream, so we want to CANCEL the it.
     /// - Header block size was too large, so we want to REFUSE, possibly
     ///   after sending a 431 response frame.
     </span>ScheduledLibraryReset(Reason),
 }

 <span class="kw">impl </span>State {
     <span class="doccomment">/// Opens the send-half of a stream if it is not already open.
     </span><span class="kw">pub fn </span>send_open(<span class="kw-2">&amp;mut </span><span class="self">self</span>, eos: bool) -&gt; <span class="prelude-ty">Result</span>&lt;(), UserError&gt; {
         <span class="kw">let </span>local = Streaming;

         <span class="self">self</span>.inner = <span class="kw">match </span><span class="self">self</span>.inner {
             Idle =&gt; {
                 <span class="kw">if </span>eos {
                     HalfClosedLocal(AwaitingHeaders)
                 } <span class="kw">else </span>{
                     Open {
                         local,
                         remote: AwaitingHeaders,
                     }
                 }
             }
             Open {
                 local: AwaitingHeaders,
                 remote,
             } =&gt; {
                 <span class="kw">if </span>eos {
                     HalfClosedLocal(remote)
                 } <span class="kw">else </span>{
                     Open { local, remote }
                 }
             }
             HalfClosedRemote(AwaitingHeaders) | ReservedLocal =&gt; {
                 <span class="kw">if </span>eos {
                     Closed(Cause::EndStream)
                 } <span class="kw">else </span>{
                     HalfClosedRemote(local)
                 }
             }
             <span class="kw">_ </span>=&gt; {
                 <span class="comment">// All other transitions result in a protocol error
                 </span><span class="kw">return </span><span class="prelude-val">Err</span>(UserError::UnexpectedFrameType);
             }
         };

         <span class="prelude-val">Ok</span>(())
     }

     <span class="doccomment">/// Opens the receive-half of the stream when a HEADERS frame is received.
     ///
     /// Returns true if this transitions the state to Open.
     </span><span class="kw">pub fn </span>recv_open(<span class="kw-2">&amp;mut </span><span class="self">self</span>, frame: <span class="kw-2">&amp;</span>frame::Headers) -&gt; <span class="prelude-ty">Result</span>&lt;bool, Error&gt; {
         <span class="kw">let </span><span class="kw-2">mut </span>initial = <span class="bool-val">false</span>;
         <span class="kw">let </span>eos = frame.is_end_stream();

         <span class="self">self</span>.inner = <span class="kw">match </span><span class="self">self</span>.inner {
             Idle =&gt; {
                 initial = <span class="bool-val">true</span>;

                 <span class="kw">if </span>eos {
                     HalfClosedRemote(AwaitingHeaders)
                 } <span class="kw">else </span>{
                     Open {
                         local: AwaitingHeaders,
                         remote: <span class="kw">if </span>frame.is_informational() {
                             <span class="macro">tracing::trace!</span>(<span class="string">&quot;skipping 1xx response headers&quot;</span>);
                             AwaitingHeaders
                         } <span class="kw">else </span>{
                             Streaming
                         },
                     }
                 }
             }
             ReservedRemote =&gt; {
                 initial = <span class="bool-val">true</span>;

                 <span class="kw">if </span>eos {
                     Closed(Cause::EndStream)
                 } <span class="kw">else if </span>frame.is_informational() {
                     <span class="macro">tracing::trace!</span>(<span class="string">&quot;skipping 1xx response headers&quot;</span>);
                     ReservedRemote
                 } <span class="kw">else </span>{
                     HalfClosedLocal(Streaming)
                 }
             }
             Open {
                 local,
                 remote: AwaitingHeaders,
             } =&gt; {
                 <span class="kw">if </span>eos {
                     HalfClosedRemote(local)
                 } <span class="kw">else </span>{
                     Open {
                         local,
                         remote: <span class="kw">if </span>frame.is_informational() {
                             <span class="macro">tracing::trace!</span>(<span class="string">&quot;skipping 1xx response headers&quot;</span>);
                             AwaitingHeaders
                         } <span class="kw">else </span>{
                             Streaming
                         },
                     }
                 }
             }
             HalfClosedLocal(AwaitingHeaders) =&gt; {
                 <span class="kw">if </span>eos {
                     Closed(Cause::EndStream)
                 } <span class="kw">else if </span>frame.is_informational() {
                     <span class="macro">tracing::trace!</span>(<span class="string">&quot;skipping 1xx response headers&quot;</span>);
                     HalfClosedLocal(AwaitingHeaders)
                 } <span class="kw">else </span>{
                     HalfClosedLocal(Streaming)
                 }
             }
             <span class="kw-2">ref </span>state =&gt; {
                 <span class="comment">// All other transitions result in a protocol error
                 </span><span class="macro">proto_err!</span>(conn: <span class="string">&quot;recv_open: in unexpected state {:?}&quot;</span>, state);
                 <span class="kw">return </span><span class="prelude-val">Err</span>(Error::library_go_away(Reason::PROTOCOL_ERROR));
             }
         };

         <span class="prelude-val">Ok</span>(initial)
     }

     <span class="doccomment">/// Transition from Idle -&gt; ReservedRemote
     </span><span class="kw">pub fn </span>reserve_remote(<span class="kw-2">&amp;mut </span><span class="self">self</span>) -&gt; <span class="prelude-ty">Result</span>&lt;(), Error&gt; {
         <span class="kw">match </span><span class="self">self</span>.inner {
             Idle =&gt; {
                 <span class="self">self</span>.inner = ReservedRemote;
                 <span class="prelude-val">Ok</span>(())
             }
             <span class="kw-2">ref </span>state =&gt; {
                 <span class="macro">proto_err!</span>(conn: <span class="string">&quot;reserve_remote: in unexpected state {:?}&quot;</span>, state);
                 <span class="prelude-val">Err</span>(Error::library_go_away(Reason::PROTOCOL_ERROR))
             }
         }
     }

     <span class="doccomment">/// Transition from Idle -&gt; ReservedLocal
     </span><span class="kw">pub fn </span>reserve_local(<span class="kw-2">&amp;mut </span><span class="self">self</span>) -&gt; <span class="prelude-ty">Result</span>&lt;(), UserError&gt; {
         <span class="kw">match </span><span class="self">self</span>.inner {
             Idle =&gt; {
                 <span class="self">self</span>.inner = ReservedLocal;
                 <span class="prelude-val">Ok</span>(())
             }
             <span class="kw">_ </span>=&gt; <span class="prelude-val">Err</span>(UserError::UnexpectedFrameType),
         }
     }

     <span class="doccomment">/// Indicates that the remote side will not send more data to the local.
     </span><span class="kw">pub fn </span>recv_close(<span class="kw-2">&amp;mut </span><span class="self">self</span>) -&gt; <span class="prelude-ty">Result</span>&lt;(), Error&gt; {
         <span class="kw">match </span><span class="self">self</span>.inner {
             Open { local, .. } =&gt; {
                 <span class="comment">// The remote side will continue to receive data.
                 </span><span class="macro">tracing::trace!</span>(<span class="string">&quot;recv_close: Open =&gt; HalfClosedRemote({:?})&quot;</span>, local);
                 <span class="self">self</span>.inner = HalfClosedRemote(local);
                 <span class="prelude-val">Ok</span>(())
             }
             HalfClosedLocal(..) =&gt; {
                 <span class="macro">tracing::trace!</span>(<span class="string">&quot;recv_close: HalfClosedLocal =&gt; Closed&quot;</span>);
                 <span class="self">self</span>.inner = Closed(Cause::EndStream);
                 <span class="prelude-val">Ok</span>(())
             }
             <span class="kw-2">ref </span>state =&gt; {
                 <span class="macro">proto_err!</span>(conn: <span class="string">&quot;recv_close: in unexpected state {:?}&quot;</span>, state);
                 <span class="prelude-val">Err</span>(Error::library_go_away(Reason::PROTOCOL_ERROR))
             }
         }
     }

     <span class="doccomment">/// The remote explicitly sent a RST_STREAM.
     ///
     /// # Arguments
     /// - `frame`: the received RST_STREAM frame.
     /// - `queued`: true if this stream has frames in the pending send queue.
     </span><span class="kw">pub fn </span>recv_reset(<span class="kw-2">&amp;mut </span><span class="self">self</span>, frame: frame::Reset, queued: bool) {
         <span class="kw">match </span><span class="self">self</span>.inner {
             <span class="comment">// If the stream is already in a `Closed` state, do nothing,
             // provided that there are no frames still in the send queue.
             </span>Closed(..) <span class="kw">if </span>!queued =&gt; {}
             <span class="comment">// A notionally `Closed` stream may still have queued frames in
             // the following cases:
             //
             // - if the cause is `Cause::Scheduled(..)` (i.e. we have not
             //   actually closed the stream yet).
             // - if the cause is `Cause::EndStream`: we transition to this
             //   state when an EOS frame is *enqueued* (so that it&#39;s invalid
             //   to enqueue more frames), not when the EOS frame is *sent*;
             //   therefore, there may still be frames ahead of the EOS frame
             //   in the send queue.
             //
             // In either of these cases, we want to overwrite the stream&#39;s
             // previous state with the received RST_STREAM, so that the queue
             // will be cleared by `Prioritize::pop_frame`.
             </span><span class="kw-2">ref </span>state =&gt; {
                 <span class="macro">tracing::trace!</span>(
                     <span class="string">&quot;recv_reset; frame={:?}; state={:?}; queued={:?}&quot;</span>,
                     frame,
                     state,
                     queued
                 );
                 <span class="self">self</span>.inner = Closed(Cause::Error(Error::remote_reset(
                     frame.stream_id(),
                     frame.reason(),
                 )));
             }
         }
     }

     <span class="doccomment">/// Handle a connection-level error.
     </span><span class="kw">pub fn </span>handle_error(<span class="kw-2">&amp;mut </span><span class="self">self</span>, err: <span class="kw-2">&amp;</span>proto::Error) {
         <span class="kw">match </span><span class="self">self</span>.inner {
             Closed(..) =&gt; {}
             <span class="kw">_ </span>=&gt; {
                 <span class="macro">tracing::trace!</span>(<span class="string">&quot;handle_error; err={:?}&quot;</span>, err);
                 <span class="self">self</span>.inner = Closed(Cause::Error(err.clone()));
             }
         }
     }

     <span class="kw">pub fn </span>recv_eof(<span class="kw-2">&amp;mut </span><span class="self">self</span>) {
         <span class="kw">match </span><span class="self">self</span>.inner {
             Closed(..) =&gt; {}
             <span class="kw-2">ref </span>state =&gt; {
                 <span class="macro">tracing::trace!</span>(<span class="string">&quot;recv_eof; state={:?}&quot;</span>, state);
                 <span class="self">self</span>.inner = Closed(Cause::Error(
                     io::Error::new(
                         io::ErrorKind::BrokenPipe,
                         <span class="string">&quot;stream closed because of a broken pipe&quot;</span>,
                     )
                     .into(),
                 ));
             }
         }
     }

     <span class="doccomment">/// Indicates that the local side will not send more data to the local.
     </span><span class="kw">pub fn </span>send_close(<span class="kw-2">&amp;mut </span><span class="self">self</span>) {
         <span class="kw">match </span><span class="self">self</span>.inner {
             Open { remote, .. } =&gt; {
                 <span class="comment">// The remote side will continue to receive data.
                 </span><span class="macro">tracing::trace!</span>(<span class="string">&quot;send_close: Open =&gt; HalfClosedLocal({:?})&quot;</span>, remote);
                 <span class="self">self</span>.inner = HalfClosedLocal(remote);
             }
             HalfClosedRemote(..) =&gt; {
                 <span class="macro">tracing::trace!</span>(<span class="string">&quot;send_close: HalfClosedRemote =&gt; Closed&quot;</span>);
                 <span class="self">self</span>.inner = Closed(Cause::EndStream);
             }
             <span class="kw-2">ref </span>state =&gt; <span class="macro">panic!</span>(<span class="string">&quot;send_close: unexpected state {:?}&quot;</span>, state),
         }
     }

     <span class="doccomment">/// Set the stream state to reset locally.
     </span><span class="kw">pub fn </span>set_reset(<span class="kw-2">&amp;mut </span><span class="self">self</span>, stream_id: StreamId, reason: Reason, initiator: Initiator) {
         <span class="self">self</span>.inner = Closed(Cause::Error(Error::Reset(stream_id, reason, initiator)));
     }

     <span class="doccomment">/// Set the stream state to a scheduled reset.
     </span><span class="kw">pub fn </span>set_scheduled_reset(<span class="kw-2">&amp;mut </span><span class="self">self</span>, reason: Reason) {
         <span class="macro">debug_assert!</span>(!<span class="self">self</span>.is_closed());
         <span class="self">self</span>.inner = Closed(Cause::ScheduledLibraryReset(reason));
     }

     <span class="kw">pub fn </span>get_scheduled_reset(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; <span class="prelude-ty">Option</span>&lt;Reason&gt; {
         <span class="kw">match </span><span class="self">self</span>.inner {
             Closed(Cause::ScheduledLibraryReset(reason)) =&gt; <span class="prelude-val">Some</span>(reason),
             <span class="kw">_ </span>=&gt; <span class="prelude-val">None</span>,
         }
     }

     <span class="kw">pub fn </span>is_scheduled_reset(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; bool {
         <span class="macro">matches!</span>(<span class="self">self</span>.inner, Closed(Cause::ScheduledLibraryReset(..)))
     }

     <span class="kw">pub fn </span>is_local_error(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; bool {
         <span class="kw">match </span><span class="self">self</span>.inner {
             Closed(Cause::Error(<span class="kw-2">ref </span>e)) =&gt; e.is_local(),
             Closed(Cause::ScheduledLibraryReset(..)) =&gt; <span class="bool-val">true</span>,
             <span class="kw">_ </span>=&gt; <span class="bool-val">false</span>,
         }
     }

     <span class="kw">pub fn </span>is_remote_reset(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; bool {
         <span class="kw">match </span><span class="self">self</span>.inner {
             Closed(Cause::Error(Error::Reset(<span class="kw">_</span>, <span class="kw">_</span>, Initiator::Remote))) =&gt; <span class="bool-val">true</span>,
             <span class="kw">_ </span>=&gt; <span class="bool-val">false</span>,
         }
     }

     <span class="doccomment">/// Returns true if the stream is already reset.
     </span><span class="kw">pub fn </span>is_reset(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; bool {
         <span class="kw">match </span><span class="self">self</span>.inner {
             Closed(Cause::EndStream) =&gt; <span class="bool-val">false</span>,
             Closed(<span class="kw">_</span>) =&gt; <span class="bool-val">true</span>,
             <span class="kw">_ </span>=&gt; <span class="bool-val">false</span>,
         }
     }

     <span class="kw">pub fn </span>is_send_streaming(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; bool {
         <span class="macro">matches!</span>(
             <span class="self">self</span>.inner,
             Open {
                 local: Streaming,
                 ..
             } | HalfClosedRemote(Streaming)
         )
     }

     <span class="doccomment">/// Returns true when the stream is in a state to receive headers
     </span><span class="kw">pub fn </span>is_recv_headers(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; bool {
         <span class="macro">matches!</span>(
             <span class="self">self</span>.inner,
             Idle | Open {
                 remote: AwaitingHeaders,
                 ..
             } | HalfClosedLocal(AwaitingHeaders)
                 | ReservedRemote
         )
     }

     <span class="kw">pub fn </span>is_recv_streaming(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; bool {
         <span class="macro">matches!</span>(
             <span class="self">self</span>.inner,
             Open {
                 remote: Streaming,
                 ..
             } | HalfClosedLocal(Streaming)
         )
     }

     <span class="kw">pub fn </span>is_closed(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; bool {
         <span class="macro">matches!</span>(<span class="self">self</span>.inner, Closed(<span class="kw">_</span>))
     }

     <span class="kw">pub fn </span>is_recv_closed(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; bool {
         <span class="macro">matches!</span>(
             <span class="self">self</span>.inner,
             Closed(..) | HalfClosedRemote(..) | ReservedLocal
         )
     }

     <span class="kw">pub fn </span>is_send_closed(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; bool {
         <span class="macro">matches!</span>(
             <span class="self">self</span>.inner,
             Closed(..) | HalfClosedLocal(..) | ReservedRemote
         )
     }

     <span class="kw">pub fn </span>is_idle(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; bool {
         <span class="macro">matches!</span>(<span class="self">self</span>.inner, Idle)
     }

     <span class="kw">pub fn </span>ensure_recv_open(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; <span class="prelude-ty">Result</span>&lt;bool, proto::Error&gt; {
         <span class="comment">// TODO: Is this correct?
         </span><span class="kw">match </span><span class="self">self</span>.inner {
             Closed(Cause::Error(<span class="kw-2">ref </span>e)) =&gt; <span class="prelude-val">Err</span>(e.clone()),
             Closed(Cause::ScheduledLibraryReset(reason)) =&gt; {
                 <span class="prelude-val">Err</span>(proto::Error::library_go_away(reason))
             }
             Closed(Cause::EndStream) | HalfClosedRemote(..) | ReservedLocal =&gt; <span class="prelude-val">Ok</span>(<span class="bool-val">false</span>),
             <span class="kw">_ </span>=&gt; <span class="prelude-val">Ok</span>(<span class="bool-val">true</span>),
         }
     }

     <span class="doccomment">/// Returns a reason if the stream has been reset.
     </span><span class="kw">pub</span>(<span class="kw">super</span>) <span class="kw">fn </span>ensure_reason(<span class="kw-2">&amp;</span><span class="self">self</span>, mode: PollReset) -&gt; <span class="prelude-ty">Result</span>&lt;<span class="prelude-ty">Option</span>&lt;Reason&gt;, <span class="kw">crate</span>::Error&gt; {
         <span class="kw">match </span><span class="self">self</span>.inner {
             Closed(Cause::Error(Error::Reset(<span class="kw">_</span>, reason, <span class="kw">_</span>)))
             | Closed(Cause::Error(Error::GoAway(<span class="kw">_</span>, reason, <span class="kw">_</span>)))
             | Closed(Cause::ScheduledLibraryReset(reason)) =&gt; <span class="prelude-val">Ok</span>(<span class="prelude-val">Some</span>(reason)),
             Closed(Cause::Error(<span class="kw-2">ref </span>e)) =&gt; <span class="prelude-val">Err</span>(e.clone().into()),
             Open {
                 local: Streaming, ..
             }
             | HalfClosedRemote(Streaming) =&gt; <span class="kw">match </span>mode {
                 PollReset::AwaitingHeaders =&gt; <span class="prelude-val">Err</span>(UserError::PollResetAfterSendResponse.into()),
                 PollReset::Streaming =&gt; <span class="prelude-val">Ok</span>(<span class="prelude-val">None</span>),
             },
             <span class="kw">_ </span>=&gt; <span class="prelude-val">Ok</span>(<span class="prelude-val">None</span>),
         }
     }
 }

 <span class="kw">impl </span>Default <span class="kw">for </span>State {
     <span class="kw">fn </span>default() -&gt; State {
         State { inner: Inner::Idle }
     }
 }

 <span class="kw">impl </span>Default <span class="kw">for </span>Peer {
     <span class="kw">fn </span>default() -&gt; <span class="self">Self </span>{
         AwaitingHeaders
     }
 }
 </code></pre></div>
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