gh-105436: The environment block should end with two null wchar_t values (GH-105495)

Lib/test/test_subprocess.py

@@ -1692,6 +1692,13 @@ class RunFuncTestCase(BaseTestCase):
         res = subprocess.run(args)
         self.assertEqual(res.returncode, 57)
 
+    @unittest.skipUnless(mswindows, "Maybe test trigger a leak on Ubuntu")
+    def test_run_with_an_empty_env(self):
+        # gh-105436: fix subprocess.run(..., env={}) broken on Windows
+        args = [sys.executable, "-c", 'import sys; sys.exit(57)']
+        res = subprocess.run(args, env={})
+        self.assertEqual(res.returncode, 57)
+
     def test_capture_output(self):
         cp = self.run_python(("import sys;"
                               "sys.stdout.write('BDFL'); "

Misc/NEWS.d/next/Windows/2023-06-08-11-30-17.gh-issue-105436.1qlDxw.rst

@@ -0,0 +1,2 @@
+Ensure that an empty environment block is terminated by two null characters,
+as is required by Windows.

Modules/_winapi.c

@@ -796,6 +796,17 @@ getenvironment(PyObject* environment)
     }
 
     envsize = PyList_GET_SIZE(keys);
+
+    if (envsize == 0) {
+        // A environment block must be terminated by two null characters --
+        // one for the last string and one for the block.
+        buffer = PyMem_Calloc(2, sizeof(wchar_t));
+        if (!buffer) {
+            PyErr_NoMemory();
+        }
+        goto cleanup;
+    }
+
     if (PyList_GET_SIZE(values) != envsize) {
         PyErr_SetString(PyExc_RuntimeError,
             "environment changed size during iteration");
@@ -869,7 +880,8 @@ getenvironment(PyObject* environment)
     *p++ = L'\0';
     assert(p == end);
 
- error:
+cleanup:
+error:
     Py_XDECREF(keys);
     Py_XDECREF(values);
     return buffer;