diff -r 624316fbb39d Lib/http/client.py
--- a/Lib/http/client.py        Tue Dec 22 08:22:05 2015 +0200
+++ b/Lib/http/client.py        Tue Dec 22 22:58:22 2015 +0000
@@ -75,6 +75,7 @@
 import os
 import re
 import socket
+import select
 import collections
 from urllib.parse import urlsplit

@@ -928,6 +929,11 @@
                     break
                 if encode:
                     datablock = datablock.encode("iso-8859-1")
+                (rlist, wlist, xlist) = select.select((self.sock,), (), (), 0)
+                if rlist:
+                    # server gave response before we finished sending. Break
+                    # and interpret the response
+                    break
                 self.sock.sendall(datablock)
             return
         try:
diff -r 624316fbb39d Lib/test/test_httplib.py
--- a/Lib/test/test_httplib.py  Tue Dec 22 08:22:05 2015 +0200
+++ b/Lib/test/test_httplib.py  Tue Dec 22 22:58:22 2015 +0000
@@ -74,6 +74,9 @@
     def setsockopt(self, level, optname, value):
         pass

+    def fileno(self):
+        return 0
+
 class EPipeSocket(FakeSocket):

     def __init__(self, text, pipe_trigger):