gh-99741: Clean Up the _xxsubinterpreters Module (gh-99940)

This cleanup up resolves a few subtle bugs and makes the implementation for multi-phase init much cleaner.

https://github.com/python/cpython/issues/99741

Lib/test/test__xxsubinterpreters.py

@@ -386,7 +386,6 @@ class ShareableTypeTests(unittest.TestCase):
         self._assert_values([
             b'spam',
             9999,
-            self.cid,
             ])
 
     def test_bytes(self):
@@ -1213,6 +1212,18 @@ class ChannelIDTests(TestBase):
         self.assertFalse(cid1 != cid2)
         self.assertTrue(cid1 != cid3)
 
+    def test_shareable(self):
+        chan = interpreters.channel_create()
+
+        obj = interpreters.channel_create()
+        interpreters.channel_send(chan, obj)
+        got = interpreters.channel_recv(chan)
+
+        self.assertEqual(got, obj)
+        self.assertIs(type(got), type(obj))
+        # XXX Check the following in the channel tests?
+        #self.assertIsNot(got, obj)
+
 
 class ChannelTests(TestBase):
 
@@ -1545,6 +1556,19 @@ class ChannelTests(TestBase):
         self.assertEqual(obj5, b'eggs')
         self.assertIs(obj6, default)
 
+    def test_recv_sending_interp_destroyed(self):
+        cid = interpreters.channel_create()
+        interp = interpreters.create()
+        interpreters.run_string(interp, dedent(f"""
+            import _xxsubinterpreters as _interpreters
+            _interpreters.channel_send({cid}, b'spam')
+            """))
+        interpreters.destroy(interp)
+
+        with self.assertRaisesRegex(RuntimeError,
+                                    'unrecognized interpreter ID'):
+            interpreters.channel_recv(cid)
+
     def test_run_string_arg_unresolved(self):
         cid = interpreters.channel_create()
         interp = interpreters.create()