gh-116167: Allow disabling the GIL with `PYTHON_GIL=0` or `-X gil=0` (#116338)

In free-threaded builds, running with `PYTHON_GIL=0` will now disable the
GIL. Follow-up issues track work to re-enable the GIL when loading an
incompatible extension, and to disable the GIL by default.

In order to support re-enabling the GIL at runtime, all GIL-related data
structures are initialized as usual, and disabling the GIL simply sets a flag
that causes `take_gil()` and `drop_gil()` to return early.

4 files changed, 50 insertions, 1 deletions

diff --git a/Lib/subprocess.py b/Lib/subprocess.py
index 20db774..1437bf8 100644
--- a/Lib/subprocess.py
+++ b/Lib/subprocess.py
@@ -350,7 +350,7 @@ def _args_from_interpreter_flags():
     if dev_mode:
         args.extend(('-X', 'dev'))
     for opt in ('faulthandler', 'tracemalloc', 'importtime',
-                'frozen_modules', 'showrefcount', 'utf8'):
+                'frozen_modules', 'showrefcount', 'utf8', 'gil'):
         if opt in xoptions:
             value = xoptions[opt]
             if value is True:
diff --git a/Lib/test/_test_embed_set_config.py b/Lib/test/_test_embed_set_config.py
index 75b6b7d..5ff5218 100644
--- a/Lib/test/_test_embed_set_config.py
+++ b/Lib/test/_test_embed_set_config.py
@@ -9,6 +9,7 @@ import _testinternalcapi
 import os
 import sys
 import unittest
+from test import support
 from test.support import MS_WINDOWS
 
 
@@ -211,6 +212,19 @@ class SetConfigTests(unittest.TestCase):
         self.set_config(use_hash_seed=1, hash_seed=123)
         self.assertEqual(sys.flags.hash_randomization, 1)
 
+        if support.Py_GIL_DISABLED:
+            self.set_config(enable_gil=-1)
+            self.assertEqual(sys.flags.gil, None)
+            self.set_config(enable_gil=0)
+            self.assertEqual(sys.flags.gil, 0)
+            self.set_config(enable_gil=1)
+            self.assertEqual(sys.flags.gil, 1)
+        else:
+            # Builds without Py_GIL_DISABLED don't have
+            # PyConfig.enable_gil. sys.flags.gil is always defined to 1, for
+            # consistency.
+            self.assertEqual(sys.flags.gil, 1)
+
     def test_options(self):
         self.check(warnoptions=[])
         self.check(warnoptions=["default", "ignore"])
diff --git a/Lib/test/test_cmd_line.py b/Lib/test/test_cmd_line.py
index 6796dc6..c633f64 100644
--- a/Lib/test/test_cmd_line.py
+++ b/Lib/test/test_cmd_line.py
@@ -869,6 +869,39 @@ class CmdLineTest(unittest.TestCase):
         self.assertEqual(proc.stdout.rstrip(), 'True')
         self.assertEqual(proc.returncode, 0, proc)
 
+    @unittest.skipUnless(support.Py_GIL_DISABLED,
+                         "PYTHON_GIL and -X gil only supported in Py_GIL_DISABLED builds")
+    def test_python_gil(self):
+        cases = [
+            # (env, opt, expected, msg)
+            (None, None, 'None', "no options set"),
+            ('0', None, '0', "PYTHON_GIL=0"),
+            ('1', None, '1', "PYTHON_GIL=1"),
+            ('1', '0', '0', "-X gil=0 overrides PYTHON_GIL=1"),
+            (None, '0', '0', "-X gil=0"),
+            (None, '1', '1', "-X gil=1"),
+        ]
+
+        code = "import sys; print(sys.flags.gil)"
+        environ = dict(os.environ)
+
+        for env, opt, expected, msg in cases:
+            with self.subTest(msg, env=env, opt=opt):
+                environ.pop('PYTHON_GIL', None)
+                if env is not None:
+                    environ['PYTHON_GIL'] = env
+                extra_args = []
+                if opt is not None:
+                    extra_args = ['-X', f'gil={opt}']
+
+                proc = subprocess.run([sys.executable, *extra_args, '-c', code],
+                                      stdout=subprocess.PIPE,
+                                      stderr=subprocess.PIPE,
+                                      text=True, env=environ)
+                self.assertEqual(proc.returncode, 0, proc)
+                self.assertEqual(proc.stdout.rstrip(), expected)
+                self.assertEqual(proc.stderr, '')
+
     @unittest.skipUnless(sys.platform == 'win32',
                          'bpo-32457 only applies on Windows')
     def test_argv0_normalization(self):
diff --git a/Lib/test/test_embed.py b/Lib/test/test_embed.py
index 55d3acf..ab1d579 100644
--- a/Lib/test/test_embed.py
+++ b/Lib/test/test_embed.py
@@ -523,6 +523,8 @@ class InitConfigTests(EmbeddingTestsMixin, unittest.TestCase):
         CONFIG_COMPAT['_pystats'] = 0
     if support.Py_DEBUG:
         CONFIG_COMPAT['run_presite'] = None
+    if support.Py_GIL_DISABLED:
+        CONFIG_COMPAT['enable_gil'] = -1
     if MS_WINDOWS:
         CONFIG_COMPAT.update({
             'legacy_windows_stdio': 0,