gh-106931: Intern Statically Allocated Strings Globally (gh-107272)

We tried this before with a dict and for all interned strings.  That ran into problems due to interpreter isolation.  However, exclusively using a per-interpreter cache caused some inconsistency that can eliminate the benefit of interning.  Here we circle back to using a global cache, but only for statically allocated strings.  We also use a more-basic _Py_hashtable_t for that global cache instead of a dict.

Ideally we would only have the global cache, but the optional isolation of each interpreter's allocator means that a non-static string object must not outlive its interpreter.  Thus we would have to store a copy of each such interned string in the global cache, tied to the main interpreter.

Lib/test/test_sys.py

@@ -14,6 +14,7 @@ from test.support import os_helper
 from test.support.script_helper import assert_python_ok, assert_python_failure
 from test.support import threading_helper
 from test.support import import_helper
+from test.support import interpreters
 import textwrap
 import unittest
 import warnings
@@ -699,6 +700,35 @@ class SysModuleTest(unittest.TestCase):
 
         self.assertRaises(TypeError, sys.intern, S("abc"))
 
+    def test_subinterp_intern_dynamically_allocated(self):
+        global INTERN_NUMRUNS
+        INTERN_NUMRUNS += 1
+        s = "never interned before" + str(INTERN_NUMRUNS)
+        t = sys.intern(s)
+        self.assertIs(t, s)
+
+        interp = interpreters.create()
+        interp.run(textwrap.dedent(f'''
+            import sys
+            t = sys.intern({s!r})
+            assert id(t) != {id(s)}, (id(t), {id(s)})
+            assert id(t) != {id(t)}, (id(t), {id(t)})
+            '''))
+
+    def test_subinterp_intern_statically_allocated(self):
+        # See Tools/build/generate_global_objects.py for the list
+        # of strings that are always statically allocated.
+        s = '__init__'
+        t = sys.intern(s)
+
+        print('------------------------')
+        interp = interpreters.create()
+        interp.run(textwrap.dedent(f'''
+            import sys
+            t = sys.intern({s!r})
+            assert id(t) == {id(t)}, (id(t), {id(t)})
+            '''))
+
     def test_sys_flags(self):
         self.assertTrue(sys.flags)
         attrs = ("debug",