bpo-34521: Fix tests in test_multiprocessing_spawn to use correctly CMSG_SPACE (GH-9613) (GH-9619)

miss-islington · pablogsal · vstinner · commit 007fda436f70 · 2018-09-28T06:07:56.000-07:00
After some failures in AMD64 FreeBSD CURRENT Debug 3.x buildbots regarding tests in test_multiprocessing_spawn and after examining similar failures in test_socket, some errors in the calculation of ancillary data buffers were found in multiprocessing.reduction. CMSG_LEN() can often be used as the buffer size for recvmsg() to receive a single item of ancillary data, but RFC 3542 requires portable applications to use CMSG_SPACE() and thus include space for padding, even when the item will be the last in the buffer. The failures we experience are due to the usage of CMSG_LEN() instead of CMSG_SPACE(). (cherry picked from commit 077061a) Co-authored-by: Pablo Galindo <Pablogsal@gmail.com>
diff --git a/Lib/multiprocessing/reduction.py b/Lib/multiprocessing/reduction.py
@@ -150,7 +150,7 @@ def recvfds(sock, size):
         '''Receive an array of fds over an AF_UNIX socket.'''
         a = array.array('i')
         bytes_size = a.itemsize * size
-        msg, ancdata, flags, addr = sock.recvmsg(1, socket.CMSG_LEN(bytes_size))
+        msg, ancdata, flags, addr = sock.recvmsg(1, socket.CMSG_SPACE(bytes_size))
         if not msg and not ancdata:
             raise EOFError
         try:
