gh-112334: Regression test that vfork is used when expected. (#112734)

Regression test that vfork is used when expected by subprocess.

This is written integration test style, it uses strace if it is present and appears to work to find out what system call actually gets used in different scenarios.

Test coverage is added for the default behavior and that of each of the specific arguments that must disable the use of vfork.  obviously not an entire test matrix, but it covers the most important aspects.

If there are ever issues with this test being flaky or failing on new platforms, rather than try and adapt it for all possible platforms, feel free to narrow the range it gets tested on when appropriate. That is not likely to reduce coverage.

.github/workflows/posix-deps-apt.sh

@@ -21,6 +21,7 @@ apt-get -yq install \
     libssl-dev \
     lzma \
     lzma-dev \
+    strace \
     tk-dev \
     uuid-dev \
     xvfb \

Lib/test/support/script_helper.py

@@ -92,13 +92,28 @@ def fail(self, cmd_line):
 # Executing the interpreter in a subprocess
 @support.requires_subprocess()
 def run_python_until_end(*args, **env_vars):
+    """Used to implement assert_python_*.
+
+    *args are the command line flags to pass to the python interpreter.
+    **env_vars keyword arguments are environment variables to set on the process.
+
+    If __run_using_command= is supplied, it must be a list of
+    command line arguments to prepend to the command line used.
+    Useful when you want to run another command that should launch the
+    python interpreter via its own arguments. ["/bin/echo", "--"] for
+    example could print the unquoted python command line instead of
+    run it.
+    """
     env_required = interpreter_requires_environment()
+    run_using_command = env_vars.pop('__run_using_command', None)
     cwd = env_vars.pop('__cwd', None)
     if '__isolated' in env_vars:
         isolated = env_vars.pop('__isolated')
     else:
         isolated = not env_vars and not env_required
     cmd_line = [sys.executable, '-X', 'faulthandler']
+    if run_using_command:
+        cmd_line = run_using_command + cmd_line
     if isolated:
         # isolated mode: ignore Python environment variables, ignore user
         # site-packages, and don't add the current directory to sys.path

Lib/test/test_subprocess.py

@@ -1561,21 +1561,6 @@ def test_class_getitems(self):
         self.assertIsInstance(subprocess.Popen[bytes], types.GenericAlias)
         self.assertIsInstance(subprocess.CompletedProcess[str], types.GenericAlias)
 
-    @unittest.skipIf(not sysconfig.get_config_var("HAVE_VFORK"),
-                     "vfork() not enabled by configure.")
-    @mock.patch("subprocess._fork_exec")
-    def test__use_vfork(self, mock_fork_exec):
-        self.assertTrue(subprocess._USE_VFORK)  # The default value regardless.
-        mock_fork_exec.side_effect = RuntimeError("just testing args")
-        with self.assertRaises(RuntimeError):
-            subprocess.run([sys.executable, "-c", "pass"])
-        mock_fork_exec.assert_called_once()
-        self.assertTrue(mock_fork_exec.call_args.args[-1])
-        with mock.patch.object(subprocess, '_USE_VFORK', False):
-            with self.assertRaises(RuntimeError):
-                subprocess.run([sys.executable, "-c", "pass"])
-            self.assertFalse(mock_fork_exec.call_args_list[-1].args[-1])
-
 
 class RunFuncTestCase(BaseTestCase):
     def run_python(self, code, **kwargs):
@@ -3360,6 +3345,89 @@ def exit_handler():
         self.assertEqual(out, b'')
         self.assertIn(b"preexec_fn not supported at interpreter shutdown", err)
 
+    @unittest.skipIf(not sysconfig.get_config_var("HAVE_VFORK"),
+                     "vfork() not enabled by configure.")
+    @mock.patch("subprocess._fork_exec")
+    def test__use_vfork(self, mock_fork_exec):
+        self.assertTrue(subprocess._USE_VFORK)  # The default value regardless.
+        mock_fork_exec.side_effect = RuntimeError("just testing args")
+        with self.assertRaises(RuntimeError):
+            subprocess.run([sys.executable, "-c", "pass"])
+        mock_fork_exec.assert_called_once()
+        # NOTE: These assertions are *ugly* as they require the last arg
+        # to remain the have_vfork boolean. We really need to refactor away
+        # from the giant "wall of args" internal C extension API.
+        self.assertTrue(mock_fork_exec.call_args.args[-1])
+        with mock.patch.object(subprocess, '_USE_VFORK', False):
+            with self.assertRaises(RuntimeError):
+                subprocess.run([sys.executable, "-c", "pass"])
+            self.assertFalse(mock_fork_exec.call_args_list[-1].args[-1])
+
+    @unittest.skipIf(not sysconfig.get_config_var("HAVE_VFORK"),
+                     "vfork() not enabled by configure.")
+    @unittest.skipIf(sys.platform != "linux", "Linux only, requires strace.")
+    def test_vfork_used_when_expected(self):
+        # This is a performance regression test to ensure we default to using
+        # vfork() when possible.
+        strace_binary = "/usr/bin/strace"
+        # The only system calls we are interested in.
+        strace_filter = "--trace=clone,clone2,clone3,fork,vfork,exit,exit_group"
+        true_binary = "/bin/true"
+        strace_command = [strace_binary, strace_filter]
+
+        try:
+            does_strace_work_process = subprocess.run(
+                    strace_command + [true_binary],
+                    stderr=subprocess.PIPE,
+                    stdout=subprocess.DEVNULL,
+            )
+            rc = does_strace_work_process.returncode
+            stderr = does_strace_work_process.stderr
+        except OSError:
+            rc = -1
+            stderr = ""
+        if rc or (b"+++ exited with 0 +++" not in stderr):
+            self.skipTest("strace not found or not working as expected.")
+
+        with self.subTest(name="default_is_vfork"):
+            vfork_result = assert_python_ok(
+                    "-c",
+                    textwrap.dedent(f"""\
+                    import subprocess
+                    subprocess.check_call([{true_binary!r}])"""),
+                    __run_using_command=strace_command,
+            )
+            # Match both vfork() and clone(..., flags=...|CLONE_VFORK|...)
+            self.assertRegex(vfork_result.err, br"(?i)vfork")
+            # Do NOT check that fork() or other clones did not happen.
+            # If the OS denys the vfork it'll fallback to plain fork().
+
+        # Test that each individual thing that would disable the use of vfork
+        # actually disables it.
+        for sub_name, preamble, sp_kwarg, expect_permission_error in (
+                ("!use_vfork", "subprocess._USE_VFORK = False", "", False),
+                ("preexec", "", "preexec_fn=lambda: None", False),
+                ("setgid", "", f"group={os.getgid()}", True),
+                ("setuid", "", f"user={os.getuid()}", True),
+                ("setgroups", "", "extra_groups=[]", True),
+        ):
+            with self.subTest(name=sub_name):
+                non_vfork_result = assert_python_ok(
+                    "-c",
+                    textwrap.dedent(f"""\
+                    import subprocess
+                    {preamble}
+                    try:
+                        subprocess.check_call(
+                                [{true_binary!r}], **dict({sp_kwarg}))
+                    except PermissionError:
+                        if not {expect_permission_error}:
+                            raise"""),
+                    __run_using_command=strace_command,
+                )
+                # Ensure neither vfork() or clone(..., flags=...|CLONE_VFORK|...).
+                self.assertNotRegex(non_vfork_result.err, br"(?i)vfork")
+
 
 @unittest.skipUnless(mswindows, "Windows specific tests")
 class Win32ProcessTestCase(BaseTestCase):

Misc/NEWS.d/next/Tests/2023-12-04-15-56-11.gh-issue-112334.FFc9Ti.rst

@@ -0,0 +1,2 @@
+Adds a regression test to verify that ``vfork()`` is used when expected by
+:mod:`subprocess` on vfork enabled POSIX systems (Linux).