Index: Misc/NEWS
===================================================================
--- Misc/NEWS	(revision 87602)
+++ Misc/NEWS	(working copy)
@@ -20,6 +20,9 @@
 Library
 -------
 
+- Issue XXXXX: Fix issue in subprocess where performing dup() on a file
+  descriptor does not reset the close-on-exec flag. Patch by Ross Lagerwall.
+
 - Issue 6285: IDLE no longer crashes on missing help file; patch by Scott David
   Daniels.
 
Index: Lib/test/test_subprocess.py
===================================================================
--- Lib/test/test_subprocess.py	(revision 87602)
+++ Lib/test/test_subprocess.py	(working copy)
@@ -903,6 +903,33 @@
         self.assertStderrEqual(stderr, b'')
         self.assertEqual(p.wait(), -signal.SIGTERM)
 
+    def test_close_std_fds(self):
+        o0 = os.dup(0)
+        o1 = os.dup(1)
+        o2 = os.dup(2)
+        try:
+            os.close(0)
+            os.close(1)
+            os.close(2)
+
+            out, err = subprocess.Popen([sys.executable, "-c",
+                              'import sys;'
+                              'sys.stdout.write("apple");'
+                              'sys.stdout.flush();'
+                              'sys.stderr.write("orange")'],
+                       stdin=o0,
+                       stdout=subprocess.PIPE,
+                       stderr=subprocess.PIPE).communicate()
+            err = support.strip_python_stderr(err)
+            self.assertEqual((out, err), (b'apple', b'orange'))
+        finally:
+            os.dup2(o0, 0)
+            os.dup2(o1, 1)
+            os.dup2(o2, 2)
+            os.close(o0)
+            os.close(o1)
+            os.close(o2)
+
     def test_surrogates_error_message(self):
         def prepare():
             raise ValueError("surrogate:\uDCff")
Index: Lib/subprocess.py
===================================================================
--- Lib/subprocess.py	(revision 87602)
+++ Lib/subprocess.py	(working copy)
@@ -1194,11 +1194,25 @@
                                 os.close(errpipe_read)
 
                                 # Dup fds for child
-                                if p2cread != -1:
+                                try:
+                                    cloexec_flag = fcntl.FD_CLOEXEC
+                                except AttributeError:
+                                    cloexec_flag = 1
+
+                                if p2cread == 0:
+                                    old = fcntl.fcntl(p2cread, fcntl.F_GETFD)
+                                    fcntl.fcntl(p2cread, fcntl.F_SETFD, old & ~cloexec_flag)
+                                elif p2cread != -1:
                                     os.dup2(p2cread, 0)
-                                if c2pwrite != -1:
+                                if c2pwrite == 1:
+                                    old = fcntl.fcntl(c2pwrite, fcntl.F_GETFD)
+                                    fcntl.fcntl(c2pwrite, fcntl.F_SETFD, old & ~cloexec_flag)
+                                elif c2pwrite != -1:
                                     os.dup2(c2pwrite, 1)
-                                if errwrite != -1:
+                                if errwrite == 2:
+                                    old = fcntl.fcntl(errwrite, fcntl.F_GETFD)
+                                    fcntl.fcntl(errwrite, fcntl.F_SETFD, old & ~cloexec_flag)
+                                elif errwrite != -1:
                                     os.dup2(errwrite, 2)
 
                                 # Close pipe fds.  Make sure we don't close the
Index: Modules/_posixsubprocess.c
===================================================================
--- Modules/_posixsubprocess.c	(revision 87602)
+++ Modules/_posixsubprocess.c	(working copy)
@@ -70,13 +70,25 @@
     POSIX_CALL(close(errpipe_read));
 
     /* Dup fds for child. */
-    if (p2cread != -1) {
+    if (p2cread == 0) {
+        int old = fcntl(p2cread, F_GETFD);
+        if (old != -1)
+            fcntl(p2cread, F_SETFD, old & ~FD_CLOEXEC);
+    } else if (p2cread != -1) {
         POSIX_CALL(dup2(p2cread, 0));  /* stdin */
     }
-    if (c2pwrite != -1) {
+    if (c2pwrite == 1) {
+        int old = fcntl(c2pwrite, F_GETFD);
+        if (old != -1)
+            fcntl(c2pwrite, F_SETFD, old & ~FD_CLOEXEC);
+    } else if (c2pwrite != -1) {
         POSIX_CALL(dup2(c2pwrite, 1));  /* stdout */
     }
-    if (errwrite != -1) {
+    if (errwrite == 2) {
+        int old = fcntl(errwrite, F_GETFD);
+        if (old != -1)
+            fcntl(errwrite, F_SETFD, old & ~FD_CLOEXEC);
+    } else if (errwrite != -1) {
         POSIX_CALL(dup2(errwrite, 2));  /* stderr */
     }