brontide: set read deadlines on socket during initial handshake

This commit fixes a lingering issue within lnd, which can cause a
server to freeze up, and not handle any incoming connections properly,
or cause clients to freeze and not return in a timely manner from a
failed connection attempt.

To avoid this, each time we need to read from the socket during the
initial brontide handshake, we add a 15 second read deadline. If we
don’t successfully read from the buffer during that time frame, then
the Read method will return a timeout error.

With this in place, we ensure that the main listener goroutine will
never be blocked waiting on a remote party to write ActOne.

brontide/conn.go

@@ -55,6 +55,11 @@ func Dial(localPriv *btcec.PrivateKey, netAddr *lnwire.NetAddress) (*Conn, error
 		return nil, err
 	}
 
+	// We'll ensure that we get ActTwo from the remote peer in a timely
+	// manner. If they don't respond within 15 seconds, then we'll kill the
+	// connection.
+	conn.SetReadDeadline(time.Now().Add(time.Second * 15))
+
 	// If the first act was successful (we know that address is actually
 	// remotePub), then read the second act after which we'll be able to
 	// send our static public key to the remote peer with strong forward
@@ -81,6 +86,10 @@ func Dial(localPriv *btcec.PrivateKey, netAddr *lnwire.NetAddress) (*Conn, error
 		return nil, err
 	}
 
+	// We'll reset the deadline as it's no longer critical beyond the
+	// initial handshake.
+	conn.SetReadDeadline(time.Time{})
+
 	return b, nil
 }