diff -r d38509db3616 Lib/subprocess.py
--- a/Lib/subprocess.py	Fri Aug 19 18:43:43 2011 +0200
+++ b/Lib/subprocess.py	Sat Aug 20 00:31:11 2011 +0200
@@ -1035,7 +1035,7 @@
             if stdin is None:
                 pass
             elif stdin == PIPE:
-                p2cread, p2cwrite = os.pipe()
+                p2cread, p2cwrite = self._pipe2()
             elif isinstance(stdin, int):
                 p2cread = stdin
             else:
@@ -1045,7 +1045,7 @@
             if stdout is None:
                 pass
             elif stdout == PIPE:
-                c2pread, c2pwrite = os.pipe()
+                c2pread, c2pwrite = self._pipe2()
             elif isinstance(stdout, int):
                 c2pwrite = stdout
             else:
@@ -1055,7 +1055,7 @@
             if stderr is None:
                 pass
             elif stderr == PIPE:
-                errread, errwrite = os.pipe()
+                errread, errwrite = self._pipe2()
             elif stderr == STDOUT:
                 errwrite = c2pwrite
             elif isinstance(stderr, int):
@@ -1082,6 +1082,16 @@
                 fcntl.fcntl(fd, fcntl.F_SETFD, old & ~cloexec_flag)
 
 
+        def _pipe2(self, cloexec=True):
+            """Ad-hoc pipe2() implementation."""
+            # This is not atomic, we would need pipe2() syscall for that.
+            r, w = os.pipe()
+            if cloexec:
+                self._set_cloexec_flag(r)
+                self._set_cloexec_flag(w)
+            return r, w
+
+
         def _close_fds(self, but):
             if hasattr(os, 'closerange'):
                 os.closerange(3, but)
@@ -1120,11 +1130,9 @@
             # For transferring possible exec failure from child to parent
             # The first char specifies the exception type: 0 means
             # OSError, 1 means some other error.
-            errpipe_read, errpipe_write = os.pipe()
+            errpipe_read, errpipe_write = self._pipe2()
             try:
                 try:
-                    self._set_cloexec_flag(errpipe_write)
-
                     gc_was_enabled = gc.isenabled()
                     # Disable gc to avoid bug where gc -> file_dealloc ->
                     # write to stderr -> hang.  http://bugs.python.org/issue1336
diff -r d38509db3616 Lib/test/test_subprocess.py
--- a/Lib/test/test_subprocess.py	Fri Aug 19 18:43:43 2011 +0200
+++ b/Lib/test/test_subprocess.py	Sat Aug 20 00:31:11 2011 +0200
@@ -995,6 +995,37 @@
         self.assertRaises(OSError, os.waitpid, pid, 0)
         self.assertNotIn(ident, [id(o) for o in subprocess._active])
 
+    def test_pipe_cloexec(self):
+        # Issue 12786: check that the parent-end FDs of the communication pipes
+        # are set CLOEXEC, and not inherited by another child process.
+        p1 = subprocess.Popen([sys.executable, "-c",
+                               'import os;'
+                               'os.read(0)'
+                              ],
+                              stdin=subprocess.PIPE, stdout=subprocess.PIPE,
+                              stderr=subprocess.PIPE)
+
+        p2 = subprocess.Popen([sys.executable, "-c", """if True:
+                               import os, errno, sys
+                               for fd in %r:
+                                   try:
+                                       os.close(fd)
+                                   except OSError as e:
+                                       if e.errno != errno.EBADF:
+                                           raise
+                                   else:
+                                       sys.exit(1)
+                               sys.exit(0)
+                               """ % [f.fileno() for f in (p1.stdin, p1.stdout,
+                                                           p1.stderr)]
+                              ],
+                              stdin=subprocess.PIPE, stdout=subprocess.PIPE,
+                              stderr=subprocess.PIPE, close_fds=False)
+        p1.communicate('foo')
+        _, stderr = p2.communicate()
+
+        self.assertEqual(p2.returncode, 0, "Unexpected error: " + repr(stderr))
+
 
 @unittest.skipUnless(mswindows, "Windows specific tests")
 class Win32ProcessTestCase(BaseTestCase):