bpo-32844: Fix a subprocess misredirection of a low fd (GH5689)

bpo-32844: subprocess: Fix a potential misredirection of a low fd to stderr.

When redirecting, subprocess attempts to achieve the following state:
each fd to be redirected to is less than or equal to the fd
it is redirected from, which is necessary because redirection
occurs in the ascending order of destination descriptors.
It fails to do so in a couple of corner cases,
for example, if 1 is redirected to 2 and 0 is closed in the parent.

Lib/test/test_subprocess.py

@@ -6,6 +6,7 @@ import sys
 import platform
 import signal
 import io
+import itertools
 import os
 import errno
 import tempfile
@@ -2134,6 +2135,55 @@ class POSIXProcessTestCase(BaseTestCase):
         self.check_swap_fds(2, 0, 1)
         self.check_swap_fds(2, 1, 0)
 
+    def _check_swap_std_fds_with_one_closed(self, from_fds, to_fds):
+        saved_fds = self._save_fds(range(3))
+        try:
+            for from_fd in from_fds:
+                with tempfile.TemporaryFile() as f:
+                    os.dup2(f.fileno(), from_fd)
+
+            fd_to_close = (set(range(3)) - set(from_fds)).pop()
+            os.close(fd_to_close)
+
+            arg_names = ['stdin', 'stdout', 'stderr']
+            kwargs = {}
+            for from_fd, to_fd in zip(from_fds, to_fds):
+                kwargs[arg_names[to_fd]] = from_fd
+
+            code = textwrap.dedent(r'''
+                import os, sys
+                skipped_fd = int(sys.argv[1])
+                for fd in range(3):
+                    if fd != skipped_fd:
+                        os.write(fd, str(fd).encode('ascii'))
+            ''')
+
+            skipped_fd = (set(range(3)) - set(to_fds)).pop()
+
+            rc = subprocess.call([sys.executable, '-c', code, str(skipped_fd)],
+                                 **kwargs)
+            self.assertEqual(rc, 0)
+
+            for from_fd, to_fd in zip(from_fds, to_fds):
+                os.lseek(from_fd, 0, os.SEEK_SET)
+                read_bytes = os.read(from_fd, 1024)
+                read_fds = list(map(int, read_bytes.decode('ascii')))
+                msg = textwrap.dedent(f"""
+                    When testing {from_fds} to {to_fds} redirection,
+                    parent descriptor {from_fd} got redirected
+                    to descriptor(s) {read_fds} instead of descriptor {to_fd}.
+                """)
+                self.assertEqual([to_fd], read_fds, msg)
+        finally:
+            self._restore_fds(saved_fds)
+
+    # Check that subprocess can remap std fds correctly even
+    # if one of them is closed (#32844).
+    def test_swap_std_fds_with_one_closed(self):
+        for from_fds in itertools.combinations(range(3), 2):
+            for to_fds in itertools.permutations(range(3), 2):
+                self._check_swap_std_fds_with_one_closed(from_fds, to_fds)
+
     def test_surrogates_error_message(self):
         def prepare():
             raise ValueError("surrogate:\uDCff")

Misc/NEWS.d/next/Library/2018-02-28-13-08-00.bpo-32844.u8tnAe.rst

@@ -0,0 +1,2 @@
+Fix wrong redirection of a low descriptor (0 or 1) to stderr in subprocess
+if another low descriptor is closed.

Modules/_posixsubprocess.c

@@ -424,7 +424,7 @@ child_exec(char *const exec_array[],
        either 0, 1 or 2, it is possible that it is overwritten (#12607). */
     if (c2pwrite == 0)
         POSIX_CALL(c2pwrite = dup(c2pwrite));
-    if (errwrite == 0 || errwrite == 1)
+    while (errwrite == 0 || errwrite == 1)
         POSIX_CALL(errwrite = dup(errwrite));
 
     /* Dup fds for child.