Fixes Issue #15798 - subprocess.Popen() no longer fails if file

descriptor 0, 1 or 2 is closed.

Lib/subprocess.py

@@ -1361,7 +1361,10 @@ class Popen(object):
                         executable_list = tuple(
                             os.path.join(os.fsencode(dir), executable)
                             for dir in os.get_exec_path(env))
-                    fds_to_keep = set(pass_fds)
+                    # Never close stdin, stdout and stderr for the child.
+                    fds_to_keep = {0,1,2}
+                    fds_to_keep.update(pass_fds)
+                    # Our child uses this one to signal error before exec().
                     fds_to_keep.add(errpipe_write)
                     self.pid = _posixsubprocess.fork_exec(
                             args, executable_list,

Lib/test/test_subprocess.py

@@ -1559,6 +1559,27 @@ class POSIXProcessTestCase(BaseTestCase):
         # all standard fds closed.
         self.check_close_std_fds([0, 1, 2])
 
+    def test_small_errpipe_write_fd(self):
+        """Issue #15798: Popen should work when stdio fds are available."""
+        new_stdin = os.dup(0)
+        new_stdout = os.dup(1)
+        try:
+            os.close(0)
+            os.close(1)
+
+            # Side test: if errpipe_write fails to have its CLOEXEC
+            # flag set this should cause the parent to think the exec
+            # failed.  Extremely unlikely: everyone supports CLOEXEC.
+            subprocess.Popen([
+                    sys.executable, "-c",
+                    "print('AssertionError:0:CLOEXEC failure.')"]).wait()
+        finally:
+            # Restore original stdin and stdout
+            os.dup2(new_stdin, 0)
+            os.dup2(new_stdout, 1)
+            os.close(new_stdin)
+            os.close(new_stdout)
+
     def test_remapping_std_fds(self):
         # open up some temporary files
         temps = [mkstemp() for i in range(3)]

Misc/NEWS

@@ -18,6 +18,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #15798: Fixed subprocess.Popen() to no longer fail if file
+  descriptor 0, 1 or 2 is closed.
+
 - Issue #17897: Optimized unpickle prefetching.
 
 - Issue #3693: Make the error message more helpful when the array.array()

Modules/_posixsubprocess.c

@@ -458,7 +458,7 @@ child_exec(char *const exec_array[],
             local_max_fd = max_fd;
 #endif
         /* TODO HP-UX could use pstat_getproc() if anyone cares about it. */
-        _close_open_fd_range(3, local_max_fd, py_fds_to_keep);
+        _close_open_fd_range(0, local_max_fd, py_fds_to_keep);
     }
 
     /* This loop matches the Lib/os.py _execvpe()'s PATH search when */
@@ -535,10 +535,6 @@ subprocess_fork_exec(PyObject* self, PyObject *args)
             &restore_signals, &call_setsid, &preexec_fn))
         return NULL;
 
-    if (close_fds && errpipe_write < 3) {  /* precondition */
-        PyErr_SetString(PyExc_ValueError, "errpipe_write must be >= 3");
-        return NULL;
-    }
     if (PySequence_Length(py_fds_to_keep) < 0) {
         PyErr_SetString(PyExc_ValueError, "cannot get length of fds_to_keep");
         return NULL;