Make use of NLL to clean up handshaking logic (#576)

.github/workflows/CI.yml

@@ -83,3 +83,32 @@ jobs:
       - name: Check minimal versions
         run: cargo clean; cargo update -Zminimal-versions; cargo check
         if: matrix.rust == 'nightly'
+
+  msrv:
+    name: Check MSRV (${{ matrix.rust }})
+    needs: [style]
+    strategy:
+      matrix:
+        rust:
+          - 1.46 # never go past Hyper's own MSRV
+
+        os:
+          - ubuntu-latest
+
+    runs-on: ${{ matrix.os }}
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v1
+
+      - name: Install Rust (${{ matrix.rust }})
+        uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: ${{ matrix.rust }}
+          override: true
+
+      - name: Check
+        uses: actions-rs/cargo@v1
+        with:
+          command: check

src/server.rs

@@ -126,7 +126,7 @@ use std::future::Future;
 use std::pin::Pin;
 use std::task::{Context, Poll};
 use std::time::Duration;
-use std::{convert, fmt, io, mem};
+use std::{fmt, io};
 use tokio::io::{AsyncRead, AsyncWrite, ReadBuf};
 use tracing::instrument::{Instrument, Instrumented};
 
@@ -301,8 +301,8 @@ enum Handshaking<T, B: Buf> {
     Flushing(Instrumented<Flush<T, Prioritized<B>>>),
     /// State 2. Connection is waiting for the client preface.
     ReadingPreface(Instrumented<ReadPreface<T, Prioritized<B>>>),
-    /// Dummy state for `mem::replace`.
-    Empty,
+    /// State 3. Handshake is done, polling again would panic.
+    Done,
 }
 
 /// Flush a Sink
@@ -387,7 +387,8 @@ where
             .expect("invalid SETTINGS frame");
 
         // Create the handshake future.
-        let state = Handshaking::from(codec);
+        let state =
+            Handshaking::Flushing(Flush::new(codec).instrument(tracing::trace_span!("flush")));
 
         drop(entered);
 
@@ -1269,9 +1270,10 @@ where
         let span = self.span.clone(); // XXX(eliza): T_T
         let _e = span.enter();
         tracing::trace!(state = ?self.state);
-        use crate::server::Handshaking::*;
 
-        self.state = if let Flushing(ref mut flush) = self.state {
+        loop {
+            match &mut self.state {
+                Handshaking::Flushing(flush) => {
                     // We're currently flushing a pending SETTINGS frame. Poll the
                     // flush future, and, if it's completed, advance our state to wait
                     // for the client preface.
@@ -1285,27 +1287,15 @@ where
                             flushed
                         }
                     };
-            Handshaking::from(ReadPreface::new(codec))
-        } else {
-            // Otherwise, we haven't actually advanced the state, but we have
-            // to replace it with itself, because we have to return a value.
-            // (note that the assignment to `self.state` has to be outside of
-            // the `if let` block above in order to placate the borrow checker).
-            mem::replace(&mut self.state, Handshaking::Empty)
-        };
-        let poll = if let ReadingPreface(ref mut read) = self.state {
-            // We're now waiting for the client preface. Poll the `ReadPreface`
-            // future. If it has completed, we will create a `Connection` handle
-            // for the connection.
-            Pin::new(read).poll(cx)
-        // Actually creating the `Connection` has to occur outside of this
-        // `if let` block, because we've borrowed `self` mutably in order
-        // to poll the state and won't be able to borrow the SETTINGS frame
-        // as well until we release the borrow for `poll()`.
-        } else {
-            unreachable!("Handshake::poll() state was not advanced completely!")
-        };
-        poll?.map(|codec| {
+                    self.state = Handshaking::ReadingPreface(
+                        ReadPreface::new(codec).instrument(tracing::trace_span!("read_preface")),
+                    );
+                }
+                Handshaking::ReadingPreface(read) => {
+                    let codec = ready!(Pin::new(read).poll(cx)?);
+
+                    self.state = Handshaking::Done;
+
                     let connection = proto::Connection::new(
                         codec,
                         Config {
@@ -1324,8 +1314,14 @@ where
                     if let Some(sz) = self.builder.initial_target_connection_window_size {
                         c.set_target_window_size(sz);
                     }
-            Ok(c)
-        })
+
+                    return Poll::Ready(Ok(c));
+                }
+                Handshaking::Done => {
+                    panic!("Handshaking::poll() called again after handshaking was complete")
+                }
+            }
+        }
     }
 }
 
@@ -1548,42 +1544,9 @@ where
     #[inline]
     fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
         match *self {
-            Handshaking::Flushing(_) => write!(f, "Handshaking::Flushing(_)"),
-            Handshaking::ReadingPreface(_) => write!(f, "Handshaking::ReadingPreface(_)"),
-            Handshaking::Empty => write!(f, "Handshaking::Empty"),
+            Handshaking::Flushing(_) => f.write_str("Flushing(_)"),
+            Handshaking::ReadingPreface(_) => f.write_str("ReadingPreface(_)"),
+            Handshaking::Done => f.write_str("Done"),
         }
     }
 }
-
-impl<T, B> convert::From<Flush<T, Prioritized<B>>> for Handshaking<T, B>
-where
-    T: AsyncRead + AsyncWrite,
-    B: Buf,
-{
-    #[inline]
-    fn from(flush: Flush<T, Prioritized<B>>) -> Self {
-        Handshaking::Flushing(flush.instrument(tracing::trace_span!("flush")))
-    }
-}
-
-impl<T, B> convert::From<ReadPreface<T, Prioritized<B>>> for Handshaking<T, B>
-where
-    T: AsyncRead + AsyncWrite,
-    B: Buf,
-{
-    #[inline]
-    fn from(read: ReadPreface<T, Prioritized<B>>) -> Self {
-        Handshaking::ReadingPreface(read.instrument(tracing::trace_span!("read_preface")))
-    }
-}
-
-impl<T, B> convert::From<Codec<T, Prioritized<B>>> for Handshaking<T, B>
-where
-    T: AsyncRead + AsyncWrite,
-    B: Buf,
-{
-    #[inline]
-    fn from(codec: Codec<T, Prioritized<B>>) -> Self {
-        Handshaking::from(Flush::new(codec))
-    }
-}