gh-101659: Isolate "obmalloc" State to Each Interpreter (gh-101660)

This is strictly about moving the "obmalloc" runtime state from
`_PyRuntimeState` to `PyInterpreterState`.  Doing so improves isolation
between interpreters, specifically most of the memory (incl. objects)
allocated for each interpreter's use.  This is important for a
per-interpreter GIL, but such isolation is valuable even without it.

FWIW, a per-interpreter obmalloc is the proverbial
canary-in-the-coalmine when it comes to the isolation of objects between
interpreters.  Any object that leaks (unintentionally) to another
interpreter is highly likely to cause a crash (on debug builds at
least).  That's a useful thing to know, relative to interpreter
isolation.

1 files changed, 27 insertions, 6 deletions

diff --git a/Lib/test/test_capi/test_misc.py b/Lib/test/test_capi/test_misc.py
index 637adc0..eab6930 100644
--- a/Lib/test/test_capi/test_misc.py
+++ b/Lib/test/test_capi/test_misc.py
@@ -1211,20 +1211,25 @@ class SubinterpreterTest(unittest.TestCase):
         """
         import json
 
+        OBMALLOC = 1<<5
         EXTENSIONS = 1<<8
         THREADS = 1<<10
         DAEMON_THREADS = 1<<11
         FORK = 1<<15
         EXEC = 1<<16
 
-        features = ['fork', 'exec', 'threads', 'daemon_threads', 'extensions']
+        features = ['obmalloc', 'fork', 'exec', 'threads', 'daemon_threads',
+                    'extensions']
         kwlist = [f'allow_{n}' for n in features]
+        kwlist[0] = 'use_main_obmalloc'
         kwlist[-1] = 'check_multi_interp_extensions'
+
+        # expected to work
         for config, expected in {
-            (True, True, True, True, True):
-                FORK | EXEC | THREADS | DAEMON_THREADS | EXTENSIONS,
-            (False, False, False, False, False): 0,
-            (False, False, True, False, True): THREADS | EXTENSIONS,
+            (True, True, True, True, True, True):
+                OBMALLOC | FORK | EXEC | THREADS | DAEMON_THREADS | EXTENSIONS,
+            (True, False, False, False, False, False): OBMALLOC,
+            (False, False, False, True, False, True): THREADS | EXTENSIONS,
         }.items():
             kwargs = dict(zip(kwlist, config))
             expected = {
@@ -1246,6 +1251,20 @@ class SubinterpreterTest(unittest.TestCase):
 
                 self.assertEqual(settings, expected)
 
+        # expected to fail
+        for config in [
+            (False, False, False, False, False, False),
+        ]:
+            kwargs = dict(zip(kwlist, config))
+            with self.subTest(config):
+                script = textwrap.dedent(f'''
+                    import _testinternalcapi
+                    _testinternalcapi.get_interp_settings()
+                    raise NotImplementedError('unreachable')
+                    ''')
+                with self.assertRaises(RuntimeError):
+                    support.run_in_subinterp_with_config(script, **kwargs)
+
     @unittest.skipIf(_testsinglephase is None, "test requires _testsinglephase module")
     @unittest.skipUnless(hasattr(os, "pipe"), "requires os.pipe()")
     def test_overridden_setting_extensions_subinterp_check(self):
@@ -1257,13 +1276,15 @@ class SubinterpreterTest(unittest.TestCase):
         """
         import json
 
+        OBMALLOC = 1<<5
         EXTENSIONS = 1<<8
         THREADS = 1<<10
         DAEMON_THREADS = 1<<11
         FORK = 1<<15
         EXEC = 1<<16
-        BASE_FLAGS = FORK | EXEC | THREADS | DAEMON_THREADS
+        BASE_FLAGS = OBMALLOC | FORK | EXEC | THREADS | DAEMON_THREADS
         base_kwargs = {
+            'use_main_obmalloc': True,
             'allow_fork': True,
             'allow_exec': True,
             'allow_threads': True,