handshake: Use read_exact, not read and checking len

read_exact has a loop in it, which we need.

This means we end up separating the two sites that generate the "not a
relay" error, so we need to fish out the error construction.

As per tpo/core/arti!249 (comment 2771023)

crates/tor-proto/src/channel/handshake.rs

@@ -117,9 +117,13 @@ impl<T: AsyncRead + AsyncWrite + Send + Unpin + 'static> OutboundClientHandshake
         let their_versions: msg::Versions = {
             // TODO: this could be turned into another function, I suppose.
             let mut hdr = [0_u8; 5];
-            let len = self.tls.read(&mut hdr).await?;
-            if len != hdr.len() || hdr[0..3] != [0, 0, ChanCmd::VERSIONS.into()] {
-                return Err(Error::ChanProto("Doesn't seem to be a tor relay".into()));
+            let not_relay = || Err(Error::ChanProto("Doesn't seem to be a tor relay".into()));
+            match self.tls.read_exact(&mut hdr).await {
+                Err(e) if e.kind() == std::io::ErrorKind::UnexpectedEof => return not_relay(),
+                otherwise => otherwise,
+            }?;
+            if hdr[0..3] != [0, 0, ChanCmd::VERSIONS.into()] {
+                return not_relay();
             }
             let msglen = u16::from_be_bytes(*array_ref![hdr, 3, 2]);
             let mut msg = vec![0; msglen as usize];