bpo-34044: subprocess.Popen copies startupinfo (GH-8090)

subprocess.Popen now copies the startupinfo argument to leave it
unchanged: it will modify the copy, so that the same STARTUPINFO
object can be used multiple times.

Add subprocess.STARTUPINFO.copy() method.

Lib/test/test_subprocess.py

@@ -2822,6 +2822,33 @@ class Win32ProcessTestCase(BaseTestCase):
         subprocess.call([sys.executable, "-c", "import sys; sys.exit(0)"],
                         startupinfo=startupinfo)
 
+    def test_startupinfo_copy(self):
+        # bpo-34044: Popen must not modify input STARTUPINFO structure
+        startupinfo = subprocess.STARTUPINFO()
+        startupinfo.dwFlags = subprocess.STARTF_USESHOWWINDOW
+        startupinfo.wShowWindow = subprocess.SW_HIDE
+
+        # Call Popen() twice with the same startupinfo object to make sure
+        # that it's not modified
+        for _ in range(2):
+            cmd = [sys.executable, "-c", "pass"]
+            with open(os.devnull, 'w') as null:
+                proc = subprocess.Popen(cmd,
+                                        stdout=null,
+                                        stderr=subprocess.STDOUT,
+                                        startupinfo=startupinfo)
+                with proc:
+                    proc.communicate()
+                self.assertEqual(proc.returncode, 0)
+
+            self.assertEqual(startupinfo.dwFlags,
+                             subprocess.STARTF_USESHOWWINDOW)
+            self.assertIsNone(startupinfo.hStdInput)
+            self.assertIsNone(startupinfo.hStdOutput)
+            self.assertIsNone(startupinfo.hStdError)
+            self.assertEqual(startupinfo.wShowWindow, subprocess.SW_HIDE)
+            self.assertEqual(startupinfo.lpAttributeList, {"handle_list": []})
+
     def test_creationflags(self):
         # creationflags argument
         CREATE_NEW_CONSOLE = 16