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.

diff --git a/Lib/test/test_subprocess.py b/Lib/test/test_subprocess.py
index ddee3b94fed95d5d41769efea5c018b8e7b4cfc5..2a766d7c92ad822bd612de21f905be4991df0411 100644 (file)
--- a/Lib/test/test_subprocess.py
+++ b/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")

diff --git a/Misc/NEWS.d/next/Library/2018-02-28-13-08-00.bpo-32844.u8tnAe.rst b/Misc/NEWS.d/next/Library/2018-02-28-13-08-00.bpo-32844.u8tnAe.rst
new file mode 100644 (file)
index 0000000..67412fe
--- /dev/null
+++ b/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.

diff --git a/Modules/_posixsubprocess.c b/Modules/_posixsubprocess.c
index dc43ffc2e19dbc5aa03f4de0ab1d3de6f856617f..0150fcb0970c0e5a6635a782edbfecc9f49999ec 100644 (file)
--- a/Modules/_posixsubprocess.c
+++ b/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.