From 38e3c9de0f3713208ca0b71fd58febf4b7770fba Mon Sep 17 00:00:00 2001
From: Tony Garnock-Jones <tonygarnockjones@gmail.com>
Date: Wed, 27 Jan 2016 21:46:20 -0500
Subject: [PATCH] Avoid double-fin (!).

---
 tcp.rkt | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/tcp.rkt b/tcp.rkt
index 1aa461d..d726f31 100644
--- a/tcp.rkt
+++ b/tcp.rkt
@@ -536,10 +536,13 @@
 
   ;; ConnState -> Transition
   (define (close-outbound-stream s)
+    (define b (conn-state-outbound s))
     (transition
-     (struct-copy conn-state s
-       [outbound (struct-copy buffer (buffer-push (conn-state-outbound s) #"!") ;; dummy FIN byte
-		   [finished? #t])])
+     (if (buffer-finished? b)
+         s
+         (struct-copy conn-state s
+                      [outbound (struct-copy buffer (buffer-push b #"!") ;; dummy FIN byte
+                                             [finished? #t])]))
      '()))
 
   (define (state-vector-behavior e s)