`subprocess.run(..., env={})` broken on Windows

[mandolaerik]

Bug report

When passing an empty dictionary as env to subprocess.run (or Popen), I get the following error:

Python 3.11.3 (tags/v3.11.3:f3909b8, Apr  4 2023, 23:49:59) [MSC v.1934 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import subprocess
>>> subprocess.run('c:\\msys64\\usr\\bin\\echo.exe', env={})
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\jhbaarnh\AppData\Local\Programs\Python\Python311\Lib\subprocess.py", line 548, in run
    with Popen(*popenargs, **kwargs) as process:
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\jhbaarnh\AppData\Local\Programs\Python\Python311\Lib\subprocess.py", line 1024, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "C:\Users\jhbaarnh\AppData\Local\Programs\Python\Python311\Lib\subprocess.py", line 1509, in _execute_child
    hp, ht, pid, tid = _winapi.CreateProcess(executable, args,
                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
OSError: [WinError 87] The parameter is incorrect
>>>

Environment

• Reproduced on the following CPython versions, all native windows builds:
  • vanilla python 3.11.3
  • vanilla Python 3.9.6
  • Intel Python 3.9.16
• I can not reproduce the problem on any of the following:
  • python 3.7.2, native Windows build and MSYS build (did not try newer MSYS)
  • any linux

Linked PRs

• gh-105436： the environment block should end with two zero-valued wchar_t values #105495
• [3.12] gh-105436: The environment block should end with two null wchar_t values (GH-105495) #105700
• [3.11] gh-105436: The environment block should end with two null wchar_t values (GH-105495) #105701
• gh-105436: Ignore unrelated errors when checking empty env #105742
• [3.12] gh-105436: Ignore unrelated errors when checking empty env (GH-105742) #105756
• [3.11] gh-105436: Ignore unrelated errors when checking empty env (GH-105742) #105757

[sku2000]

The error occurs in the _winapi_CreateProcess_impl function of cpython/Modules/_winapi.c.
When the CreateProcessW function is called, the wenvironment argument ends with a single 0 byte.
In the CreateProcessW document, the ANSI environment block ends with two zero bytes:
Note that an ANSI environment block is terminated by two zero bytes

[eryksun]

@sku2000

Yes, CreateProcessW() fails with ERROR_INVALID_PARAMETER (87) if the environment block doesn't end with two nulls (i.e. two zero-valued wchar_t values, which is four zero-valued bytes), one for the last variable and one for the block. Thus even an empty block has to have an empty variable assignment. Currently if the env mapping is empty, we create a block that only has a single null. Probably the simplest fix would be to special case the result of getenvironment() in "Modules/_winapi.c" when envsize is 0.

cpython/Modules/_winapi.c

Lines 798 to 806 in 7279fb6

	envsize = PyList_GET_SIZE(keys);
	if (PyList_GET_SIZE(values) != envsize) {
	PyErr_SetString(PyExc_RuntimeError,
	"environment changed size during iteration");
	goto error;
	}
	totalsize = 1; /* trailing null character */
	for (i = 0; i < envsize; i++) {

[zooba]

Thanks for the report and to @sku2000 for the fix!