From 8b209fd4f8a9bf9603888bda2c44b5cfd4ebf47a Mon Sep 17 00:00:00 2001
From: Eric Snow <ericsnowcurrently@gmail.com>
Date: Mon, 15 Jul 2024 13:43:59 -0600
Subject: gh-76785: Expand How Interpreter Channels Handle Interpreter
 Finalization (gh-121805)

See 6b98b274b6 for an explanation of the problem and solution.  Here I've applied the solution to channels.
---
 Lib/test/support/interpreters/_crossinterp.py | 102 ++++++++
 Lib/test/support/interpreters/channels.py     | 110 ++++++--
 Lib/test/support/interpreters/queues.py       |  60 ++---
 Lib/test/test__interpchannels.py              | 275 ++++++++++----------
 Lib/test/test_interpreters/test_channels.py   | 222 +++++++++++++++++
 Lib/test/test_interpreters/test_queues.py     |   4 +-
 Modules/_interpchannelsmodule.c               | 344 ++++++++++++++++++++------
 Modules/_interpqueuesmodule.c                 |  42 +---
 Modules/_interpreters_common.h                |  45 ++++
 9 files changed, 898 insertions(+), 306 deletions(-)
 create mode 100644 Lib/test/support/interpreters/_crossinterp.py

diff --git a/Lib/test/support/interpreters/_crossinterp.py b/Lib/test/support/interpreters/_crossinterp.py
new file mode 100644
index 0000000..544e197
--- /dev/null
+++ b/Lib/test/support/interpreters/_crossinterp.py
@@ -0,0 +1,102 @@
+"""Common code between queues and channels."""
+
+
+class ItemInterpreterDestroyed(Exception):
+    """Raised when trying to get an item whose interpreter was destroyed."""
+
+
+class classonly:
+    """A non-data descriptor that makes a value only visible on the class.
+
+    This is like the "classmethod" builtin, but does not show up on
+    instances of the class.  It may be used as a decorator.
+    """
+
+    def __init__(self, value):
+        self.value = value
+        self.getter = classmethod(value).__get__
+        self.name = None
+
+    def __set_name__(self, cls, name):
+        if self.name is not None:
+            raise TypeError('already used')
+        self.name = name
+
+    def __get__(self, obj, cls):
+        if obj is not None:
+            raise AttributeError(self.name)
+        # called on the class
+        return self.getter(None, cls)
+
+
+class UnboundItem:
+    """Represents a cross-interpreter item no longer bound to an interpreter.
+
+    An item is unbound when the interpreter that added it to the
+    cross-interpreter container is destroyed.
+    """
+
+    __slots__ = ()
+
+    @classonly
+    def singleton(cls, kind, module, name='UNBOUND'):
+        doc = cls.__doc__.replace('cross-interpreter container', kind)
+        doc = doc.replace('cross-interpreter', kind)
+        subclass = type(
+            f'Unbound{kind.capitalize()}Item',
+            (cls,),
+            dict(
+                _MODULE=module,
+                _NAME=name,
+                __doc__=doc,
+            ),
+        )
+        return object.__new__(subclass)
+
+    _MODULE = __name__
+    _NAME = 'UNBOUND'
+
+    def __new__(cls):
+        raise Exception(f'use {cls._MODULE}.{cls._NAME}')
+
+    def __repr__(self):
+        return f'{self._MODULE}.{self._NAME}'
+#        return f'interpreters.queues.UNBOUND'
+
+
+UNBOUND = object.__new__(UnboundItem)
+UNBOUND_ERROR = object()
+UNBOUND_REMOVE = object()
+
+_UNBOUND_CONSTANT_TO_FLAG = {
+    UNBOUND_REMOVE: 1,
+    UNBOUND_ERROR: 2,
+    UNBOUND: 3,
+}
+_UNBOUND_FLAG_TO_CONSTANT = {v: k
+                             for k, v in _UNBOUND_CONSTANT_TO_FLAG.items()}
+
+
+def serialize_unbound(unbound):
+    op = unbound
+    try:
+        flag = _UNBOUND_CONSTANT_TO_FLAG[op]
+    except KeyError:
+        raise NotImplementedError(f'unsupported unbound replacement op {op!r}')
+    return flag,
+
+
+def resolve_unbound(flag, exctype_destroyed):
+    try:
+        op = _UNBOUND_FLAG_TO_CONSTANT[flag]
+    except KeyError:
+        raise NotImplementedError(f'unsupported unbound replacement op {flag!r}')
+    if op is UNBOUND_REMOVE:
+        # "remove" not possible here
+        raise NotImplementedError
+    elif op is UNBOUND_ERROR:
+        raise exctype_destroyed("item's original interpreter destroyed")
+    elif op is UNBOUND:
+        return UNBOUND
+    else:
+        raise NotImplementedError(repr(op))
diff --git a/Lib/test/support/interpreters/channels.py b/Lib/test/support/interpreters/channels.py
index fbae7e6..d2bd93d 100644
--- a/Lib/test/support/interpreters/channels.py
+++ b/Lib/test/support/interpreters/channels.py
@@ -2,35 +2,68 @@
 
 import time
 import _interpchannels as _channels
+from . import _crossinterp
 
 # aliases:
 from _interpchannels import (
     ChannelError, ChannelNotFoundError, ChannelClosedError,
     ChannelEmptyError, ChannelNotEmptyError,
 )
+from ._crossinterp import (
+    UNBOUND_ERROR, UNBOUND_REMOVE,
+)
 
 
 __all__ = [
+    'UNBOUND', 'UNBOUND_ERROR', 'UNBOUND_REMOVE',
     'create', 'list_all',
     'SendChannel', 'RecvChannel',
     'ChannelError', 'ChannelNotFoundError', 'ChannelEmptyError',
+    'ItemInterpreterDestroyed',
 ]
 
 
-def create():
+class ItemInterpreterDestroyed(ChannelError,
+                               _crossinterp.ItemInterpreterDestroyed):
+    """Raised from get() and get_nowait()."""
+
+
+UNBOUND = _crossinterp.UnboundItem.singleton('queue', __name__)
+
+
+def _serialize_unbound(unbound):
+    if unbound is UNBOUND:
+        unbound = _crossinterp.UNBOUND
+    return _crossinterp.serialize_unbound(unbound)
+
+
+def _resolve_unbound(flag):
+    resolved = _crossinterp.resolve_unbound(flag, ItemInterpreterDestroyed)
+    if resolved is _crossinterp.UNBOUND:
+        resolved = UNBOUND
+    return resolved
+
+
+def create(*, unbounditems=UNBOUND):
     """Return (recv, send) for a new cross-interpreter channel.
 
     The channel may be used to pass data safely between interpreters.
+
+    "unbounditems" sets the default for the send end of the channel.
+    See SendChannel.send() for supported values.  The default value
+    is UNBOUND, which replaces the unbound item when received.
     """
-    cid = _channels.create()
-    recv, send = RecvChannel(cid), SendChannel(cid)
+    unbound = _serialize_unbound(unbounditems)
+    unboundop, = unbound
+    cid = _channels.create(unboundop)
+    recv, send = RecvChannel(cid), SendChannel(cid, _unbound=unbound)
     return recv, send
 
 
 def list_all():
     """Return a list of (recv, send) for all open channels."""
-    return [(RecvChannel(cid), SendChannel(cid))
-            for cid in _channels.list_all()]
+    return [(RecvChannel(cid), SendChannel(cid, _unbound=unbound))
+            for cid, unbound in _channels.list_all()]
 
 
 class _ChannelEnd:
@@ -106,12 +139,15 @@ class RecvChannel(_ChannelEnd):
             if timeout < 0:
                 raise ValueError(f'timeout value must be non-negative')
             end = time.time() + timeout
-        obj = _channels.recv(self._id, _sentinel)
+        obj, unboundop = _channels.recv(self._id, _sentinel)
         while obj is _sentinel:
             time.sleep(_delay)
             if timeout is not None and time.time() >= end:
                 raise TimeoutError
-            obj = _channels.recv(self._id, _sentinel)
+            obj, unboundop = _channels.recv(self._id, _sentinel)
+        if unboundop is not None:
+            assert obj is None, repr(obj)
+            return _resolve_unbound(unboundop)
         return obj
 
     def recv_nowait(self, default=_NOT_SET):
@@ -122,9 +158,13 @@ class RecvChannel(_ChannelEnd):
         is the same as recv().
         """
         if default is _NOT_SET:
-            return _channels.recv(self._id)
+            obj, unboundop = _channels.recv(self._id)
         else:
-            return _channels.recv(self._id, default)
+            obj, unboundop = _channels.recv(self._id, default)
+        if unboundop is not None:
+            assert obj is None, repr(obj)
+            return _resolve_unbound(unboundop)
+        return obj
 
     def close(self):
         _channels.close(self._id, recv=True)
@@ -135,43 +175,79 @@ class SendChannel(_ChannelEnd):
 
     _end = 'send'
 
+    def __new__(cls, cid, *, _unbound=None):
+        if _unbound is None:
+            try:
+                op = _channels.get_channel_defaults(cid)
+                _unbound = (op,)
+            except ChannelNotFoundError:
+                _unbound = _serialize_unbound(UNBOUND)
+        self = super().__new__(cls, cid)
+        self._unbound = _unbound
+        return self
+
     @property
     def is_closed(self):
         info = self._info
         return info.closed or info.closing
 
-    def send(self, obj, timeout=None):
+    def send(self, obj, timeout=None, *,
+             unbound=None,
+             ):
         """Send the object (i.e. its data) to the channel's receiving end.
 
         This blocks until the object is received.
         """
-        _channels.send(self._id, obj, timeout=timeout, blocking=True)
+        if unbound is None:
+            unboundop, = self._unbound
+        else:
+            unboundop, = _serialize_unbound(unbound)
+        _channels.send(self._id, obj, unboundop, timeout=timeout, blocking=True)
 
-    def send_nowait(self, obj):
+    def send_nowait(self, obj, *,
+                    unbound=None,
+                    ):
         """Send the object to the channel's receiving end.
 
         If the object is immediately received then return True
         (else False).  Otherwise this is the same as send().
         """
+        if unbound is None:
+            unboundop, = self._unbound
+        else:
+            unboundop, = _serialize_unbound(unbound)
         # XXX Note that at the moment channel_send() only ever returns
         # None.  This should be fixed when channel_send_wait() is added.
         # See bpo-32604 and gh-19829.
-        return _channels.send(self._id, obj, blocking=False)
+        return _channels.send(self._id, obj, unboundop, blocking=False)
 
-    def send_buffer(self, obj, timeout=None):
+    def send_buffer(self, obj, timeout=None, *,
+                    unbound=None,
+                    ):
         """Send the object's buffer to the channel's receiving end.
 
         This blocks until the object is received.
         """
-        _channels.send_buffer(self._id, obj, timeout=timeout, blocking=True)
+        if unbound is None:
+            unboundop, = self._unbound
+        else:
+            unboundop, = _serialize_unbound(unbound)
+        _channels.send_buffer(self._id, obj, unboundop,
+                              timeout=timeout, blocking=True)
 
-    def send_buffer_nowait(self, obj):
+    def send_buffer_nowait(self, obj, *,
+                           unbound=None,
+                           ):
         """Send the object's buffer to the channel's receiving end.
 
         If the object is immediately received then return True
         (else False).  Otherwise this is the same as send().
         """
-        return _channels.send_buffer(self._id, obj, blocking=False)
+        if unbound is None:
+            unboundop, = self._unbound
+        else:
+            unboundop, = _serialize_unbound(unbound)
+        return _channels.send_buffer(self._id, obj, unboundop, blocking=False)
 
     def close(self):
         _channels.close(self._id, send=True)
diff --git a/Lib/test/support/interpreters/queues.py b/Lib/test/support/interpreters/queues.py
index 402ceff..deb8e86 100644
--- a/Lib/test/support/interpreters/queues.py
+++ b/Lib/test/support/interpreters/queues.py
@@ -5,11 +5,15 @@ import queue
 import time
 import weakref
 import _interpqueues as _queues
+from . import _crossinterp
 
 # aliases:
 from _interpqueues import (
     QueueError, QueueNotFoundError,
 )
+from ._crossinterp import (
+    UNBOUND_ERROR, UNBOUND_REMOVE,
+)
 
 __all__ = [
     'UNBOUND', 'UNBOUND_ERROR', 'UNBOUND_REMOVE',
@@ -34,7 +38,8 @@ class QueueFull(QueueError, queue.Full):
     """
 
 
-class ItemInterpreterDestroyed(QueueError):
+class ItemInterpreterDestroyed(QueueError,
+                               _crossinterp.ItemInterpreterDestroyed):
     """Raised from get() and get_nowait()."""
 
 
@@ -42,57 +47,20 @@ _SHARED_ONLY = 0
 _PICKLED = 1
 
 
-class UnboundItem:
-    """Represents a Queue item no longer bound to an interpreter.
-
-    An item is unbound when the interpreter that added it to the queue
-    is destroyed.
-    """
-
-    __slots__ = ()
-
-    def __new__(cls):
-        return UNBOUND
-
-    def __repr__(self):
-        return f'interpreters.queues.UNBOUND'
-
-
-UNBOUND = object.__new__(UnboundItem)
-UNBOUND_ERROR = object()
-UNBOUND_REMOVE = object()
+UNBOUND = _crossinterp.UnboundItem.singleton('queue', __name__)
 
-_UNBOUND_CONSTANT_TO_FLAG = {
-    UNBOUND_REMOVE: 1,
-    UNBOUND_ERROR: 2,
-    UNBOUND: 3,
-}
-_UNBOUND_FLAG_TO_CONSTANT = {v: k
-                             for k, v in _UNBOUND_CONSTANT_TO_FLAG.items()}
 
 def _serialize_unbound(unbound):
-    op = unbound
-    try:
-        flag = _UNBOUND_CONSTANT_TO_FLAG[op]
-    except KeyError:
-        raise NotImplementedError(f'unsupported unbound replacement op {op!r}')
-    return flag,
+    if unbound is UNBOUND:
+        unbound = _crossinterp.UNBOUND
+    return _crossinterp.serialize_unbound(unbound)
 
 
 def _resolve_unbound(flag):
-    try:
-        op = _UNBOUND_FLAG_TO_CONSTANT[flag]
-    except KeyError:
-        raise NotImplementedError(f'unsupported unbound replacement op {flag!r}')
-    if op is UNBOUND_REMOVE:
-        # "remove" not possible here
-        raise NotImplementedError
-    elif op is UNBOUND_ERROR:
-        raise ItemInterpreterDestroyed("item's original interpreter destroyed")
-    elif op is UNBOUND:
-        return UNBOUND
-    else:
-        raise NotImplementedError(repr(op))
+    resolved = _crossinterp.resolve_unbound(flag, ItemInterpreterDestroyed)
+    if resolved is _crossinterp.UNBOUND:
+        resolved = UNBOUND
+    return resolved
 
 
 def create(maxsize=0, *, syncobj=False, unbounditems=UNBOUND):
diff --git a/Lib/test/test__interpchannels.py b/Lib/test/test__interpchannels.py
index b76c589..4a7f04b 100644
--- a/Lib/test/test__interpchannels.py
+++ b/Lib/test/test__interpchannels.py
@@ -8,6 +8,8 @@ import unittest
 
 from test.support import import_helper
 
+_channels = import_helper.import_module('_interpchannels')
+from test.support.interpreters import _crossinterp
 from test.test__interpreters import (
     _interpreters,
     _run_output,
@@ -15,7 +17,7 @@ from test.test__interpreters import (
 )
 
 
-_channels = import_helper.import_module('_interpchannels')
+REPLACE = _crossinterp._UNBOUND_CONSTANT_TO_FLAG[_crossinterp.UNBOUND]
 
 
 # Additional tests are found in Lib/test/test_interpreters/test_channels.py.
@@ -29,9 +31,19 @@ _channels = import_helper.import_module('_interpchannels')
 def recv_wait(cid):
     while True:
         try:
-            return _channels.recv(cid)
+            obj, unboundop = _channels.recv(cid)
         except _channels.ChannelEmptyError:
             time.sleep(0.1)
+        else:
+            assert unboundop is None, repr(unboundop)
+            return obj
+
+
+def recv_nowait(cid, *args, unbound=False):
+    obj, unboundop = _channels.recv(cid, *args)
+    assert (unboundop is None) != unbound, repr(unboundop)
+    return obj
+
 
 #@contextmanager
 #def run_threaded(id, source, **shared):
@@ -212,7 +224,7 @@ def _run_action(cid, action, end, state):
                 else:
                     raise Exception('expected ChannelEmptyError')
             else:
-                _channels.recv(cid)
+                recv_nowait(cid)
                 return state.decr()
         else:
             raise ValueError(end)
@@ -235,7 +247,7 @@ def _run_action(cid, action, end, state):
 
 
 def clean_up_channels():
-    for cid in _channels.list_all():
+    for cid, _ in _channels.list_all():
         try:
             _channels.destroy(cid)
         except _channels.ChannelNotFoundError:
@@ -297,7 +309,7 @@ class ChannelIDTests(TestBase):
             _channels._channel_id(10, send=False, recv=False)
 
     def test_does_not_exist(self):
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         with self.assertRaises(_channels.ChannelNotFoundError):
             _channels._channel_id(int(cid) + 1)  # unforced
 
@@ -319,9 +331,9 @@ class ChannelIDTests(TestBase):
         self.assertEqual(repr(cid), 'ChannelID(10)')
 
     def test_equality(self):
-        cid1 = _channels.create()
+        cid1 = _channels.create(REPLACE)
         cid2 = _channels._channel_id(int(cid1))
-        cid3 = _channels.create()
+        cid3 = _channels.create(REPLACE)
 
         self.assertTrue(cid1 == cid1)
         self.assertTrue(cid1 == cid2)
@@ -341,11 +353,11 @@ class ChannelIDTests(TestBase):
         self.assertTrue(cid1 != cid3)
 
     def test_shareable(self):
-        chan = _channels.create()
+        chan = _channels.create(REPLACE)
 
-        obj = _channels.create()
+        obj = _channels.create(REPLACE)
         _channels.send(chan, obj, blocking=False)
-        got = _channels.recv(chan)
+        got = recv_nowait(chan)
 
         self.assertEqual(got, obj)
         self.assertIs(type(got), type(obj))
@@ -356,15 +368,15 @@ class ChannelIDTests(TestBase):
 class ChannelTests(TestBase):
 
     def test_create_cid(self):
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         self.assertIsInstance(cid, _channels.ChannelID)
 
     def test_sequential_ids(self):
-        before = _channels.list_all()
-        id1 = _channels.create()
-        id2 = _channels.create()
-        id3 = _channels.create()
-        after = _channels.list_all()
+        before = [cid for cid, _ in _channels.list_all()]
+        id1 = _channels.create(REPLACE)
+        id2 = _channels.create(REPLACE)
+        id3 = _channels.create(REPLACE)
+        after = [cid for cid, _ in _channels.list_all()]
 
         self.assertEqual(id2, int(id1) + 1)
         self.assertEqual(id3, int(id2) + 1)
@@ -374,7 +386,7 @@ class ChannelTests(TestBase):
         id1 = _interpreters.create()
         out = _run_output(id1, dedent("""
             import _interpchannels as _channels
-            cid = _channels.create()
+            cid = _channels.create(3)
             print(cid)
             """))
         cid1 = int(out.strip())
@@ -382,7 +394,7 @@ class ChannelTests(TestBase):
         id2 = _interpreters.create()
         out = _run_output(id2, dedent("""
             import _interpchannels as _channels
-            cid = _channels.create()
+            cid = _channels.create(3)
             print(cid)
             """))
         cid2 = int(out.strip())
@@ -392,7 +404,7 @@ class ChannelTests(TestBase):
     def test_channel_list_interpreters_none(self):
         """Test listing interpreters for a channel with no associations."""
         # Test for channel with no associated _interpreters.
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         send_interps = _channels.list_interpreters(cid, send=True)
         recv_interps = _channels.list_interpreters(cid, send=False)
         self.assertEqual(send_interps, [])
@@ -401,7 +413,7 @@ class ChannelTests(TestBase):
     def test_channel_list_interpreters_basic(self):
         """Test basic listing channel _interpreters."""
         interp0, *_ = _interpreters.get_main()
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         _channels.send(cid, "send", blocking=False)
         # Test for a channel that has one end associated to an interpreter.
         send_interps = _channels.list_interpreters(cid, send=True)
@@ -412,7 +424,7 @@ class ChannelTests(TestBase):
         interp1 = _interpreters.create()
         _run_output(interp1, dedent(f"""
             import _interpchannels as _channels
-            obj = _channels.recv({cid})
+            _channels.recv({cid})
             """))
         # Test for channel that has both ends associated to an interpreter.
         send_interps = _channels.list_interpreters(cid, send=True)
@@ -426,7 +438,7 @@ class ChannelTests(TestBase):
         interp1 = _interpreters.create()
         interp2 = _interpreters.create()
         interp3 = _interpreters.create()
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
 
         _channels.send(cid, "send", blocking=False)
         _run_output(interp1, dedent(f"""
@@ -435,11 +447,11 @@ class ChannelTests(TestBase):
             """))
         _run_output(interp2, dedent(f"""
             import _interpchannels as _channels
-            obj = _channels.recv({cid})
+            _channels.recv({cid})
             """))
         _run_output(interp3, dedent(f"""
             import _interpchannels as _channels
-            obj = _channels.recv({cid})
+            _channels.recv({cid})
             """))
         send_interps = _channels.list_interpreters(cid, send=True)
         recv_interps = _channels.list_interpreters(cid, send=False)
@@ -450,11 +462,11 @@ class ChannelTests(TestBase):
         """Test listing channel interpreters with a destroyed interpreter."""
         interp0, *_ = _interpreters.get_main()
         interp1 = _interpreters.create()
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         _channels.send(cid, "send", blocking=False)
         _run_output(interp1, dedent(f"""
             import _interpchannels as _channels
-            obj = _channels.recv({cid})
+            _channels.recv({cid})
             """))
         # Should be one interpreter associated with each end.
         send_interps = _channels.list_interpreters(cid, send=True)
@@ -476,16 +488,16 @@ class ChannelTests(TestBase):
         interp0, *_ = _interpreters.get_main()
         interp1 = _interpreters.create()
         interp2 = _interpreters.create()
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         _channels.send(cid, "data", blocking=False)
         _run_output(interp1, dedent(f"""
             import _interpchannels as _channels
-            obj = _channels.recv({cid})
+            _channels.recv({cid})
             """))
         _channels.send(cid, "data", blocking=False)
         _run_output(interp2, dedent(f"""
             import _interpchannels as _channels
-            obj = _channels.recv({cid})
+            _channels.recv({cid})
             """))
         # Check the setup.
         send_interps = _channels.list_interpreters(cid, send=True)
@@ -516,7 +528,7 @@ class ChannelTests(TestBase):
         """Test listing channel interpreters with a closed channel."""
         interp0, *_ = _interpreters.get_main()
         interp1 = _interpreters.create()
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         # Put something in the channel so that it's not empty.
         _channels.send(cid, "send", blocking=False)
 
@@ -538,7 +550,7 @@ class ChannelTests(TestBase):
         """Test listing channel interpreters with a channel's send end closed."""
         interp0, *_ = _interpreters.get_main()
         interp1 = _interpreters.create()
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         # Put something in the channel so that it's not empty.
         _channels.send(cid, "send", blocking=False)
 
@@ -570,7 +582,7 @@ class ChannelTests(TestBase):
             _channels.list_interpreters(cid, send=False)
 
     def test_allowed_types(self):
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         objects = [
             None,
             'spam',
@@ -580,7 +592,7 @@ class ChannelTests(TestBase):
         for obj in objects:
             with self.subTest(obj):
                 _channels.send(cid, obj, blocking=False)
-                got = _channels.recv(cid)
+                got = recv_nowait(cid)
 
                 self.assertEqual(got, obj)
                 self.assertIs(type(got), type(obj))
@@ -589,7 +601,7 @@ class ChannelTests(TestBase):
                 # XXX What about between interpreters?
 
     def test_run_string_arg_unresolved(self):
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         interp = _interpreters.create()
 
         _interpreters.set___main___attrs(interp, dict(cid=cid.send))
@@ -598,7 +610,7 @@ class ChannelTests(TestBase):
             print(cid.end)
             _channels.send(cid, b'spam', blocking=False)
             """))
-        obj = _channels.recv(cid)
+        obj = recv_nowait(cid)
 
         self.assertEqual(obj, b'spam')
         self.assertEqual(out.strip(), 'send')
@@ -608,7 +620,7 @@ class ChannelTests(TestBase):
     # Note: this test caused crashes on some buildbots (bpo-33615).
     @unittest.skip('disabled until high-level channels exist')
     def test_run_string_arg_resolved(self):
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         cid = _channels._channel_id(cid, _resolve=True)
         interp = _interpreters.create()
 
@@ -618,7 +630,7 @@ class ChannelTests(TestBase):
             _channels.send(chan.id, b'spam', blocking=False)
             """),
             dict(chan=cid.send))
-        obj = _channels.recv(cid)
+        obj = recv_nowait(cid)
 
         self.assertEqual(obj, b'spam')
         self.assertEqual(out.strip(), 'send')
@@ -627,10 +639,10 @@ class ChannelTests(TestBase):
     # send/recv
 
     def test_send_recv_main(self):
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         orig = b'spam'
         _channels.send(cid, orig, blocking=False)
-        obj = _channels.recv(cid)
+        obj = recv_nowait(cid)
 
         self.assertEqual(obj, orig)
         self.assertIsNot(obj, orig)
@@ -639,27 +651,27 @@ class ChannelTests(TestBase):
         id1 = _interpreters.create()
         out = _run_output(id1, dedent("""
             import _interpchannels as _channels
-            cid = _channels.create()
+            cid = _channels.create(REPLACE)
             orig = b'spam'
             _channels.send(cid, orig, blocking=False)
-            obj = _channels.recv(cid)
+            obj, _ = _channels.recv(cid)
             assert obj is not orig
             assert obj == orig
             """))
 
     def test_send_recv_different_interpreters(self):
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         id1 = _interpreters.create()
         out = _run_output(id1, dedent(f"""
             import _interpchannels as _channels
             _channels.send({cid}, b'spam', blocking=False)
             """))
-        obj = _channels.recv(cid)
+        obj = recv_nowait(cid)
 
         self.assertEqual(obj, b'spam')
 
     def test_send_recv_different_threads(self):
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
 
         def f():
             obj = recv_wait(cid)
@@ -674,7 +686,7 @@ class ChannelTests(TestBase):
         self.assertEqual(obj, b'spam')
 
     def test_send_recv_different_interpreters_and_threads(self):
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         id1 = _interpreters.create()
         out = None
 
@@ -685,7 +697,7 @@ class ChannelTests(TestBase):
                 import _interpchannels as _channels
                 while True:
                     try:
-                        obj = _channels.recv({cid})
+                        obj, _ = _channels.recv({cid})
                         break
                     except _channels.ChannelEmptyError:
                         time.sleep(0.1)
@@ -710,23 +722,23 @@ class ChannelTests(TestBase):
             _channels.recv(10)
 
     def test_recv_empty(self):
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         with self.assertRaises(_channels.ChannelEmptyError):
             _channels.recv(cid)
 
     def test_recv_default(self):
         default = object()
-        cid = _channels.create()
-        obj1 = _channels.recv(cid, default)
+        cid = _channels.create(REPLACE)
+        obj1 = recv_nowait(cid, default)
         _channels.send(cid, None, blocking=False)
         _channels.send(cid, 1, blocking=False)
         _channels.send(cid, b'spam', blocking=False)
         _channels.send(cid, b'eggs', blocking=False)
-        obj2 = _channels.recv(cid, default)
-        obj3 = _channels.recv(cid, default)
-        obj4 = _channels.recv(cid)
-        obj5 = _channels.recv(cid, default)
-        obj6 = _channels.recv(cid, default)
+        obj2 = recv_nowait(cid, default)
+        obj3 = recv_nowait(cid, default)
+        obj4 = recv_nowait(cid)
+        obj5 = recv_nowait(cid, default)
+        obj6 = recv_nowait(cid, default)
 
         self.assertIs(obj1, default)
         self.assertIs(obj2, None)
@@ -737,7 +749,7 @@ class ChannelTests(TestBase):
 
     def test_recv_sending_interp_destroyed(self):
         with self.subTest('closed'):
-            cid1 = _channels.create()
+            cid1 = _channels.create(REPLACE)
             interp = _interpreters.create()
             _interpreters.run_string(interp, dedent(f"""
                 import _interpchannels as _channels
@@ -750,7 +762,7 @@ class ChannelTests(TestBase):
                 _channels.recv(cid1)
             del cid1
         with self.subTest('still open'):
-            cid2 = _channels.create()
+            cid2 = _channels.create(REPLACE)
             interp = _interpreters.create()
             _interpreters.run_string(interp, dedent(f"""
                 import _interpchannels as _channels
@@ -759,7 +771,8 @@ class ChannelTests(TestBase):
             _channels.send(cid2, b'eggs', blocking=False)
             _interpreters.destroy(interp)
 
-            _channels.recv(cid2)
+            recv_nowait(cid2, unbound=True)
+            recv_nowait(cid2, unbound=False)
             with self.assertRaisesRegex(RuntimeError,
                                         f'channel {cid2} is empty'):
                 _channels.recv(cid2)
@@ -770,9 +783,9 @@ class ChannelTests(TestBase):
 
     def test_send_buffer(self):
         buf = bytearray(b'spamspamspam')
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         _channels.send_buffer(cid, buf, blocking=False)
-        obj = _channels.recv(cid)
+        obj = recv_nowait(cid)
 
         self.assertIsNot(obj, buf)
         self.assertIsInstance(obj, memoryview)
@@ -794,12 +807,12 @@ class ChannelTests(TestBase):
         else:
             send = _channels.send
 
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         try:
             started = time.monotonic()
             send(cid, obj, blocking=False)
             stopped = time.monotonic()
-            _channels.recv(cid)
+            recv_nowait(cid)
         finally:
             _channels.destroy(cid)
         delay = stopped - started  # seconds
@@ -813,7 +826,7 @@ class ChannelTests(TestBase):
         received = None
         obj = b'spam'
         wait = self.build_send_waiter(obj)
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         def f():
             nonlocal received
             wait()
@@ -829,7 +842,7 @@ class ChannelTests(TestBase):
         received = None
         obj = bytearray(b'spam')
         wait = self.build_send_waiter(obj, buffer=True)
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         def f():
             nonlocal received
             wait()
@@ -844,7 +857,7 @@ class ChannelTests(TestBase):
     def test_send_blocking_no_wait(self):
         received = None
         obj = b'spam'
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         def f():
             nonlocal received
             received = recv_wait(cid)
@@ -858,7 +871,7 @@ class ChannelTests(TestBase):
     def test_send_buffer_blocking_no_wait(self):
         received = None
         obj = bytearray(b'spam')
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         def f():
             nonlocal received
             received = recv_wait(cid)
@@ -873,20 +886,20 @@ class ChannelTests(TestBase):
         obj = b'spam'
 
         with self.subTest('non-blocking with timeout'):
-            cid = _channels.create()
+            cid = _channels.create(REPLACE)
             with self.assertRaises(ValueError):
                 _channels.send(cid, obj, blocking=False, timeout=0.1)
 
         with self.subTest('timeout hit'):
-            cid = _channels.create()
+            cid = _channels.create(REPLACE)
             with self.assertRaises(TimeoutError):
                 _channels.send(cid, obj, blocking=True, timeout=0.1)
             with self.assertRaises(_channels.ChannelEmptyError):
-                received = _channels.recv(cid)
+                received = recv_nowait(cid)
                 print(repr(received))
 
         with self.subTest('timeout not hit'):
-            cid = _channels.create()
+            cid = _channels.create(REPLACE)
             def f():
                 recv_wait(cid)
             t = threading.Thread(target=f)
@@ -910,20 +923,20 @@ class ChannelTests(TestBase):
         obj = bytearray(b'spam')
 
         with self.subTest('non-blocking with timeout'):
-            cid = _channels.create()
+            cid = _channels.create(REPLACE)
             with self.assertRaises(ValueError):
                 _channels.send_buffer(cid, obj, blocking=False, timeout=0.1)
 
         with self.subTest('timeout hit'):
-            cid = _channels.create()
+            cid = _channels.create(REPLACE)
             with self.assertRaises(TimeoutError):
                 _channels.send_buffer(cid, obj, blocking=True, timeout=0.1)
             with self.assertRaises(_channels.ChannelEmptyError):
-                received = _channels.recv(cid)
+                received = recv_nowait(cid)
                 print(repr(received))
 
         with self.subTest('timeout not hit'):
-            cid = _channels.create()
+            cid = _channels.create(REPLACE)
             def f():
                 recv_wait(cid)
             t = threading.Thread(target=f)
@@ -936,7 +949,7 @@ class ChannelTests(TestBase):
         wait = self.build_send_waiter(obj)
 
         with self.subTest('without timeout'):
-            cid = _channels.create()
+            cid = _channels.create(REPLACE)
             def f():
                 wait()
                 _channels.close(cid, force=True)
@@ -947,7 +960,7 @@ class ChannelTests(TestBase):
             t.join()
 
         with self.subTest('with timeout'):
-            cid = _channels.create()
+            cid = _channels.create(REPLACE)
             def f():
                 wait()
                 _channels.close(cid, force=True)
@@ -974,7 +987,7 @@ class ChannelTests(TestBase):
         wait = self.build_send_waiter(obj, buffer=True)
 
         with self.subTest('without timeout'):
-            cid = _channels.create()
+            cid = _channels.create(REPLACE)
             def f():
                 wait()
                 _channels.close(cid, force=True)
@@ -985,7 +998,7 @@ class ChannelTests(TestBase):
             t.join()
 
         with self.subTest('with timeout'):
-            cid = _channels.create()
+            cid = _channels.create(REPLACE)
             def f():
                 wait()
                 _channels.close(cid, force=True)
@@ -999,9 +1012,9 @@ class ChannelTests(TestBase):
     # close
 
     def test_close_single_user(self):
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         _channels.send(cid, b'spam', blocking=False)
-        _channels.recv(cid)
+        recv_nowait(cid)
         _channels.close(cid)
 
         with self.assertRaises(_channels.ChannelClosedError):
@@ -1010,7 +1023,7 @@ class ChannelTests(TestBase):
             _channels.recv(cid)
 
     def test_close_multiple_users(self):
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         id1 = _interpreters.create()
         id2 = _interpreters.create()
         _interpreters.run_string(id1, dedent(f"""
@@ -1034,9 +1047,9 @@ class ChannelTests(TestBase):
         self.assertEqual(excsnap.type.__name__, 'ChannelClosedError')
 
     def test_close_multiple_times(self):
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         _channels.send(cid, b'spam', blocking=False)
-        _channels.recv(cid)
+        recv_nowait(cid)
         _channels.close(cid)
 
         with self.assertRaises(_channels.ChannelClosedError):
@@ -1051,9 +1064,9 @@ class ChannelTests(TestBase):
             ]
         for send, recv in tests:
             with self.subTest((send, recv)):
-                cid = _channels.create()
+                cid = _channels.create(REPLACE)
                 _channels.send(cid, b'spam', blocking=False)
-                _channels.recv(cid)
+                recv_nowait(cid)
                 _channels.close(cid, send=send, recv=recv)
 
                 with self.assertRaises(_channels.ChannelClosedError):
@@ -1062,56 +1075,56 @@ class ChannelTests(TestBase):
                     _channels.recv(cid)
 
     def test_close_defaults_with_unused_items(self):
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         _channels.send(cid, b'spam', blocking=False)
         _channels.send(cid, b'ham', blocking=False)
 
         with self.assertRaises(_channels.ChannelNotEmptyError):
             _channels.close(cid)
-        _channels.recv(cid)
+        recv_nowait(cid)
         _channels.send(cid, b'eggs', blocking=False)
 
     def test_close_recv_with_unused_items_unforced(self):
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         _channels.send(cid, b'spam', blocking=False)
         _channels.send(cid, b'ham', blocking=False)
 
         with self.assertRaises(_channels.ChannelNotEmptyError):
             _channels.close(cid, recv=True)
-        _channels.recv(cid)
+        recv_nowait(cid)
         _channels.send(cid, b'eggs', blocking=False)
-        _channels.recv(cid)
-        _channels.recv(cid)
+        recv_nowait(cid)
+        recv_nowait(cid)
         _channels.close(cid, recv=True)
 
     def test_close_send_with_unused_items_unforced(self):
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         _channels.send(cid, b'spam', blocking=False)
         _channels.send(cid, b'ham', blocking=False)
         _channels.close(cid, send=True)
 
         with self.assertRaises(_channels.ChannelClosedError):
             _channels.send(cid, b'eggs')
-        _channels.recv(cid)
-        _channels.recv(cid)
+        recv_nowait(cid)
+        recv_nowait(cid)
         with self.assertRaises(_channels.ChannelClosedError):
             _channels.recv(cid)
 
     def test_close_both_with_unused_items_unforced(self):
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         _channels.send(cid, b'spam', blocking=False)
         _channels.send(cid, b'ham', blocking=False)
 
         with self.assertRaises(_channels.ChannelNotEmptyError):
             _channels.close(cid, recv=True, send=True)
-        _channels.recv(cid)
+        recv_nowait(cid)
         _channels.send(cid, b'eggs', blocking=False)
-        _channels.recv(cid)
-        _channels.recv(cid)
+        recv_nowait(cid)
+        recv_nowait(cid)
         _channels.close(cid, recv=True)
 
     def test_close_recv_with_unused_items_forced(self):
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         _channels.send(cid, b'spam', blocking=False)
         _channels.send(cid, b'ham', blocking=False)
         _channels.close(cid, recv=True, force=True)
@@ -1122,7 +1135,7 @@ class ChannelTests(TestBase):
             _channels.recv(cid)
 
     def test_close_send_with_unused_items_forced(self):
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         _channels.send(cid, b'spam', blocking=False)
         _channels.send(cid, b'ham', blocking=False)
         _channels.close(cid, send=True, force=True)
@@ -1133,7 +1146,7 @@ class ChannelTests(TestBase):
             _channels.recv(cid)
 
     def test_close_both_with_unused_items_forced(self):
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         _channels.send(cid, b'spam', blocking=False)
         _channels.send(cid, b'ham', blocking=False)
         _channels.close(cid, send=True, recv=True, force=True)
@@ -1144,7 +1157,7 @@ class ChannelTests(TestBase):
             _channels.recv(cid)
 
     def test_close_never_used(self):
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         _channels.close(cid)
 
         with self.assertRaises(_channels.ChannelClosedError):
@@ -1153,7 +1166,7 @@ class ChannelTests(TestBase):
             _channels.recv(cid)
 
     def test_close_by_unassociated_interp(self):
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         _channels.send(cid, b'spam', blocking=False)
         interp = _interpreters.create()
         _interpreters.run_string(interp, dedent(f"""
@@ -1166,11 +1179,11 @@ class ChannelTests(TestBase):
             _channels.close(cid)
 
     def test_close_used_multiple_times_by_single_user(self):
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         _channels.send(cid, b'spam', blocking=False)
         _channels.send(cid, b'spam', blocking=False)
         _channels.send(cid, b'spam', blocking=False)
-        _channels.recv(cid)
+        recv_nowait(cid)
         _channels.close(cid, force=True)
 
         with self.assertRaises(_channels.ChannelClosedError):
@@ -1179,7 +1192,7 @@ class ChannelTests(TestBase):
             _channels.recv(cid)
 
     def test_channel_list_interpreters_invalid_channel(self):
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         # Test for invalid channel ID.
         with self.assertRaises(_channels.ChannelNotFoundError):
             _channels.list_interpreters(1000, send=True)
@@ -1191,7 +1204,7 @@ class ChannelTests(TestBase):
 
     def test_channel_list_interpreters_invalid_args(self):
         # Tests for invalid arguments passed to the API.
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         with self.assertRaises(TypeError):
             _channels.list_interpreters(cid)
 
@@ -1240,9 +1253,9 @@ class ChannelReleaseTests(TestBase):
     """
 
     def test_single_user(self):
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         _channels.send(cid, b'spam', blocking=False)
-        _channels.recv(cid)
+        recv_nowait(cid)
         _channels.release(cid, send=True, recv=True)
 
         with self.assertRaises(_channels.ChannelClosedError):
@@ -1251,7 +1264,7 @@ class ChannelReleaseTests(TestBase):
             _channels.recv(cid)
 
     def test_multiple_users(self):
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         id1 = _interpreters.create()
         id2 = _interpreters.create()
         _interpreters.run_string(id1, dedent(f"""
@@ -1260,7 +1273,7 @@ class ChannelReleaseTests(TestBase):
             """))
         out = _run_output(id2, dedent(f"""
             import _interpchannels as _channels
-            obj = _channels.recv({cid})
+            obj, _ = _channels.recv({cid})
             _channels.release({cid})
             print(repr(obj))
             """))
@@ -1271,9 +1284,9 @@ class ChannelReleaseTests(TestBase):
         self.assertEqual(out.strip(), "b'spam'")
 
     def test_no_kwargs(self):
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         _channels.send(cid, b'spam', blocking=False)
-        _channels.recv(cid)
+        recv_nowait(cid)
         _channels.release(cid)
 
         with self.assertRaises(_channels.ChannelClosedError):
@@ -1282,16 +1295,16 @@ class ChannelReleaseTests(TestBase):
             _channels.recv(cid)
 
     def test_multiple_times(self):
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         _channels.send(cid, b'spam', blocking=False)
-        _channels.recv(cid)
+        recv_nowait(cid)
         _channels.release(cid, send=True, recv=True)
 
         with self.assertRaises(_channels.ChannelClosedError):
             _channels.release(cid, send=True, recv=True)
 
     def test_with_unused_items(self):
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         _channels.send(cid, b'spam', blocking=False)
         _channels.send(cid, b'ham', blocking=False)
         _channels.release(cid, send=True, recv=True)
@@ -1300,7 +1313,7 @@ class ChannelReleaseTests(TestBase):
             _channels.recv(cid)
 
     def test_never_used(self):
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         _channels.release(cid)
 
         with self.assertRaises(_channels.ChannelClosedError):
@@ -1309,14 +1322,14 @@ class ChannelReleaseTests(TestBase):
             _channels.recv(cid)
 
     def test_by_unassociated_interp(self):
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         _channels.send(cid, b'spam', blocking=False)
         interp = _interpreters.create()
         _interpreters.run_string(interp, dedent(f"""
             import _interpchannels as _channels
             _channels.release({cid})
             """))
-        obj = _channels.recv(cid)
+        obj = recv_nowait(cid)
         _channels.release(cid)
 
         with self.assertRaises(_channels.ChannelClosedError):
@@ -1325,7 +1338,7 @@ class ChannelReleaseTests(TestBase):
 
     def test_close_if_unassociated(self):
         # XXX Something's not right with this test...
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         interp = _interpreters.create()
         _interpreters.run_string(interp, dedent(f"""
             import _interpchannels as _channels
@@ -1338,21 +1351,21 @@ class ChannelReleaseTests(TestBase):
 
     def test_partially(self):
         # XXX Is partial close too weird/confusing?
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         _channels.send(cid, None, blocking=False)
-        _channels.recv(cid)
+        recv_nowait(cid)
         _channels.send(cid, b'spam', blocking=False)
         _channels.release(cid, send=True)
-        obj = _channels.recv(cid)
+        obj = recv_nowait(cid)
 
         self.assertEqual(obj, b'spam')
 
     def test_used_multiple_times_by_single_user(self):
-        cid = _channels.create()
+        cid = _channels.create(REPLACE)
         _channels.send(cid, b'spam', blocking=False)
         _channels.send(cid, b'spam', blocking=False)
         _channels.send(cid, b'spam', blocking=False)
-        _channels.recv(cid)
+        recv_nowait(cid)
         _channels.release(cid, send=True, recv=True)
 
         with self.assertRaises(_channels.ChannelClosedError):
@@ -1428,9 +1441,9 @@ class ChannelCloseFixture(namedtuple('ChannelCloseFixture',
 
     def _new_channel(self, creator):
         if creator.name == 'main':
-            return _channels.create()
+            return _channels.create(REPLACE)
         else:
-            ch = _channels.create()
+            ch = _channels.create(REPLACE)
             run_interp(creator.id, f"""
                 import _interpreters
                 cid = _xxsubchannels.create()
@@ -1439,7 +1452,7 @@ class ChannelCloseFixture(namedtuple('ChannelCloseFixture',
                 _xxsubchannels.send({ch}, int(cid), blocking=False)
                 del _interpreters
                 """)
-            self._cid = _channels.recv(ch)
+            self._cid = recv_nowait(ch)
         return self._cid
 
     def _get_interpreter(self, interp):
@@ -1657,7 +1670,7 @@ class ExhaustiveChannelTests(TestBase):
                 )
             fix.record_action(action, result)
         else:
-            _cid = _channels.create()
+            _cid = _channels.create(REPLACE)
             run_interp(interp.id, f"""
                 result = helpers.run_action(
                     {fix.cid},
@@ -1670,8 +1683,8 @@ class ExhaustiveChannelTests(TestBase):
                 _channels.send({_cid}, b'X' if result.closed else b'', blocking=False)
                 """)
             result = ChannelState(
-                pending=int.from_bytes(_channels.recv(_cid), 'little'),
-                closed=bool(_channels.recv(_cid)),
+                pending=int.from_bytes(recv_nowait(_cid), 'little'),
+                closed=bool(recv_nowait(_cid)),
                 )
             fix.record_action(action, result)
 
@@ -1729,7 +1742,7 @@ class ExhaustiveChannelTests(TestBase):
         self.assertTrue(fix.state.closed)
 
         for _ in range(fix.state.pending):
-            _channels.recv(fix.cid)
+            recv_nowait(fix.cid)
         self._assert_closed_in_interp(fix)
 
         for interp in ('same', 'other'):
diff --git a/Lib/test/test_interpreters/test_channels.py b/Lib/test/test_interpreters/test_channels.py
index 6c37754..eada18f 100644
--- a/Lib/test/test_interpreters/test_channels.py
+++ b/Lib/test/test_interpreters/test_channels.py
@@ -372,6 +372,228 @@ class TestSendRecv(TestBase):
         obj[4:8] = b'ham.'
         self.assertEqual(obj, buf)
 
+    def test_send_cleared_with_subinterpreter(self):
+        def common(rch, sch, unbound=None, presize=0):
+            if not unbound:
+                extraargs = ''
+            elif unbound is channels.UNBOUND:
+                extraargs = ', unbound=channels.UNBOUND'
+            elif unbound is channels.UNBOUND_ERROR:
+                extraargs = ', unbound=channels.UNBOUND_ERROR'
+            elif unbound is channels.UNBOUND_REMOVE:
+                extraargs = ', unbound=channels.UNBOUND_REMOVE'
+            else:
+                raise NotImplementedError(repr(unbound))
+            interp = interpreters.create()
+
+            _run_output(interp, dedent(f"""
+                from test.support.interpreters import channels
+                sch = channels.SendChannel({sch.id})
+                obj1 = b'spam'
+                obj2 = b'eggs'
+                sch.send_nowait(obj1{extraargs})
+                sch.send_nowait(obj2{extraargs})
+                """))
+            self.assertEqual(
+                _channels.get_count(rch.id),
+                presize + 2,
+            )
+
+            if presize == 0:
+                obj1 = rch.recv()
+                self.assertEqual(obj1, b'spam')
+                self.assertEqual(
+                    _channels.get_count(rch.id),
+                    presize + 1,
+                )
+
+            return interp
+
+        with self.subTest('default'):  # UNBOUND
+            rch, sch = channels.create()
+            interp = common(rch, sch)
+            del interp
+            self.assertEqual(_channels.get_count(rch.id), 1)
+            obj1 = rch.recv()
+            self.assertEqual(_channels.get_count(rch.id), 0)
+            self.assertIs(obj1, channels.UNBOUND)
+            self.assertEqual(_channels.get_count(rch.id), 0)
+            with self.assertRaises(channels.ChannelEmptyError):
+                rch.recv_nowait()
+
+        with self.subTest('UNBOUND'):
+            rch, sch = channels.create()
+            interp = common(rch, sch, channels.UNBOUND)
+            del interp
+            self.assertEqual(_channels.get_count(rch.id), 1)
+            obj1 = rch.recv()
+            self.assertIs(obj1, channels.UNBOUND)
+            self.assertEqual(_channels.get_count(rch.id), 0)
+            with self.assertRaises(channels.ChannelEmptyError):
+                rch.recv_nowait()
+
+        with self.subTest('UNBOUND_ERROR'):
+            rch, sch = channels.create()
+            interp = common(rch, sch, channels.UNBOUND_ERROR)
+
+            del interp
+            self.assertEqual(_channels.get_count(rch.id), 1)
+            with self.assertRaises(channels.ItemInterpreterDestroyed):
+                rch.recv()
+
+            self.assertEqual(_channels.get_count(rch.id), 0)
+            with self.assertRaises(channels.ChannelEmptyError):
+                rch.recv_nowait()
+
+        with self.subTest('UNBOUND_REMOVE'):
+            rch, sch = channels.create()
+
+            interp = common(rch, sch, channels.UNBOUND_REMOVE)
+            del interp
+            self.assertEqual(_channels.get_count(rch.id), 0)
+            with self.assertRaises(channels.ChannelEmptyError):
+                rch.recv_nowait()
+
+            sch.send_nowait(b'ham', unbound=channels.UNBOUND_REMOVE)
+            self.assertEqual(_channels.get_count(rch.id), 1)
+            interp = common(rch, sch, channels.UNBOUND_REMOVE, 1)
+            self.assertEqual(_channels.get_count(rch.id), 3)
+            sch.send_nowait(42, unbound=channels.UNBOUND_REMOVE)
+            self.assertEqual(_channels.get_count(rch.id), 4)
+            del interp
+            self.assertEqual(_channels.get_count(rch.id), 2)
+            obj1 = rch.recv()
+            obj2 = rch.recv()
+            self.assertEqual(obj1, b'ham')
+            self.assertEqual(obj2, 42)
+            self.assertEqual(_channels.get_count(rch.id), 0)
+            with self.assertRaises(channels.ChannelEmptyError):
+                rch.recv_nowait()
+
+    def test_send_cleared_with_subinterpreter_mixed(self):
+        rch, sch = channels.create()
+        interp = interpreters.create()
+
+        # If we don't associate the main interpreter with the channel
+        # then the channel will be automatically closed when interp
+        # is destroyed.
+        sch.send_nowait(None)
+        rch.recv()
+        self.assertEqual(_channels.get_count(rch.id), 0)
+
+        _run_output(interp, dedent(f"""
+            from test.support.interpreters import channels
+            sch = channels.SendChannel({sch.id})
+            sch.send_nowait(1, unbound=channels.UNBOUND)
+            sch.send_nowait(2, unbound=channels.UNBOUND_ERROR)
+            sch.send_nowait(3)
+            sch.send_nowait(4, unbound=channels.UNBOUND_REMOVE)
+            sch.send_nowait(5, unbound=channels.UNBOUND)
+            """))
+        self.assertEqual(_channels.get_count(rch.id), 5)
+
+        del interp
+        self.assertEqual(_channels.get_count(rch.id), 4)
+
+        obj1 = rch.recv()
+        self.assertIs(obj1, channels.UNBOUND)
+        self.assertEqual(_channels.get_count(rch.id), 3)
+
+        with self.assertRaises(channels.ItemInterpreterDestroyed):
+            rch.recv()
+        self.assertEqual(_channels.get_count(rch.id), 2)
+
+        obj2 = rch.recv()
+        self.assertIs(obj2, channels.UNBOUND)
+        self.assertEqual(_channels.get_count(rch.id), 1)
+
+        obj3 = rch.recv()
+        self.assertIs(obj3, channels.UNBOUND)
+        self.assertEqual(_channels.get_count(rch.id), 0)
+
+    def test_send_cleared_with_subinterpreter_multiple(self):
+        rch, sch = channels.create()
+        interp1 = interpreters.create()
+        interp2 = interpreters.create()
+
+        sch.send_nowait(1)
+        _run_output(interp1, dedent(f"""
+            from test.support.interpreters import channels
+            rch = channels.RecvChannel({rch.id})
+            sch = channels.SendChannel({sch.id})
+            obj1 = rch.recv()
+            sch.send_nowait(2, unbound=channels.UNBOUND)
+            sch.send_nowait(obj1, unbound=channels.UNBOUND_REMOVE)
+            """))
+        _run_output(interp2, dedent(f"""
+            from test.support.interpreters import channels
+            rch = channels.RecvChannel({rch.id})
+            sch = channels.SendChannel({sch.id})
+            obj2 = rch.recv()
+            obj1 = rch.recv()
+            """))
+        self.assertEqual(_channels.get_count(rch.id), 0)
+        sch.send_nowait(3)
+        _run_output(interp1, dedent("""
+            sch.send_nowait(4, unbound=channels.UNBOUND)
+            # interp closed here
+            sch.send_nowait(5, unbound=channels.UNBOUND_REMOVE)
+            sch.send_nowait(6, unbound=channels.UNBOUND)
+            """))
+        _run_output(interp2, dedent("""
+            sch.send_nowait(7, unbound=channels.UNBOUND_ERROR)
+            # interp closed here
+            sch.send_nowait(obj1, unbound=channels.UNBOUND_ERROR)
+            sch.send_nowait(obj2, unbound=channels.UNBOUND_REMOVE)
+            sch.send_nowait(8, unbound=channels.UNBOUND)
+            """))
+        _run_output(interp1, dedent("""
+            sch.send_nowait(9, unbound=channels.UNBOUND_REMOVE)
+            sch.send_nowait(10, unbound=channels.UNBOUND)
+            """))
+        self.assertEqual(_channels.get_count(rch.id), 10)
+
+        obj3 = rch.recv()
+        self.assertEqual(obj3, 3)
+        self.assertEqual(_channels.get_count(rch.id), 9)
+
+        obj4 = rch.recv()
+        self.assertEqual(obj4, 4)
+        self.assertEqual(_channels.get_count(rch.id), 8)
+
+        del interp1
+        self.assertEqual(_channels.get_count(rch.id), 6)
+
+        # obj5 was removed
+
+        obj6 = rch.recv()
+        self.assertIs(obj6, channels.UNBOUND)
+        self.assertEqual(_channels.get_count(rch.id), 5)
+
+        obj7 = rch.recv()
+        self.assertEqual(obj7, 7)
+        self.assertEqual(_channels.get_count(rch.id), 4)
+
+        del interp2
+        self.assertEqual(_channels.get_count(rch.id), 3)
+
+        # obj1
+        with self.assertRaises(channels.ItemInterpreterDestroyed):
+            rch.recv()
+        self.assertEqual(_channels.get_count(rch.id), 2)
+
+        # obj2 was removed
+
+        obj8 = rch.recv()
+        self.assertIs(obj8, channels.UNBOUND)
+        self.assertEqual(_channels.get_count(rch.id), 1)
+
+        # obj9 was removed
+
+        obj10 = rch.recv()
+        self.assertIs(obj10, channels.UNBOUND)
+        self.assertEqual(_channels.get_count(rch.id), 0)
+
 
 if __name__ == '__main__':
     # Test needs to be a package, so we can do relative imports.
diff --git a/Lib/test/test_interpreters/test_queues.py b/Lib/test/test_interpreters/test_queues.py
index 30d58a5..18f83d0 100644
--- a/Lib/test/test_interpreters/test_queues.py
+++ b/Lib/test/test_interpreters/test_queues.py
@@ -8,11 +8,11 @@ from test.support import import_helper, Py_DEBUG
 # Raise SkipTest if subinterpreters not supported.
 _queues = import_helper.import_module('_interpqueues')
 from test.support import interpreters
-from test.support.interpreters import queues
+from test.support.interpreters import queues, _crossinterp
 from .utils import _run_output, TestBase as _TestBase
 
 
-REPLACE = queues._UNBOUND_CONSTANT_TO_FLAG[queues.UNBOUND]
+REPLACE = _crossinterp._UNBOUND_CONSTANT_TO_FLAG[_crossinterp.UNBOUND]
 
 
 def get_num_queues():
diff --git a/Modules/_interpchannelsmodule.c b/Modules/_interpchannelsmodule.c
index f044747..a8b4a8d 100644
--- a/Modules/_interpchannelsmodule.c
+++ b/Modules/_interpchannelsmodule.c
@@ -18,7 +18,9 @@
 #endif
 
 #define REGISTERS_HEAP_TYPES
+#define HAS_UNBOUND_ITEMS
 #include "_interpreters_common.h"
+#undef HAS_UNBOUND_ITEMS
 #undef REGISTERS_HEAP_TYPES
 
 
@@ -511,8 +513,14 @@ _waiting_finish_releasing(_waiting_t *waiting)
 struct _channelitem;
 
 typedef struct _channelitem {
+    /* The interpreter that added the item to the queue.
+       The actual bound interpid is found in item->data.
+       This is necessary because item->data might be NULL,
+       meaning the interpreter has been destroyed. */
+    int64_t interpid;
     _PyCrossInterpreterData *data;
     _waiting_t *waiting;
+    int unboundop;
     struct _channelitem *next;
 } _channelitem;
 
@@ -524,11 +532,22 @@ _channelitem_ID(_channelitem *item)
 
 static void
 _channelitem_init(_channelitem *item,
-                  _PyCrossInterpreterData *data, _waiting_t *waiting)
+                  int64_t interpid, _PyCrossInterpreterData *data,
+                  _waiting_t *waiting, int unboundop)
 {
+    if (interpid < 0) {
+        interpid = _get_interpid(data);
+    }
+    else {
+        assert(data == NULL
+               || _PyCrossInterpreterData_INTERPID(data) < 0
+               || interpid == _PyCrossInterpreterData_INTERPID(data));
+    }
     *item = (_channelitem){
+        .interpid = interpid,
         .data = data,
         .waiting = waiting,
+        .unboundop = unboundop,
     };
     if (waiting != NULL) {
         waiting->itemid = _channelitem_ID(item);
@@ -536,17 +555,15 @@ _channelitem_init(_channelitem *item,
 }
 
 static void
-_channelitem_clear(_channelitem *item)
+_channelitem_clear_data(_channelitem *item, int removed)
 {
-    item->next = NULL;
-
     if (item->data != NULL) {
         // It was allocated in channel_send().
         (void)_release_xid_data(item->data, XID_IGNORE_EXC & XID_FREE);
         item->data = NULL;
     }
 
-    if (item->waiting != NULL) {
+    if (item->waiting != NULL && removed) {
         if (item->waiting->status == WAITING_ACQUIRED) {
             _waiting_release(item->waiting, 0);
         }
@@ -554,15 +571,23 @@ _channelitem_clear(_channelitem *item)
     }
 }
 
+static void
+_channelitem_clear(_channelitem *item)
+{
+    item->next = NULL;
+    _channelitem_clear_data(item, 1);
+}
+
 static _channelitem *
-_channelitem_new(_PyCrossInterpreterData *data, _waiting_t *waiting)
+_channelitem_new(int64_t interpid, _PyCrossInterpreterData *data,
+                 _waiting_t *waiting, int unboundop)
 {
     _channelitem *item = GLOBAL_MALLOC(_channelitem);
     if (item == NULL) {
         PyErr_NoMemory();
         return NULL;
     }
-    _channelitem_init(item, data, waiting);
+    _channelitem_init(item, interpid, data, waiting, unboundop);
     return item;
 }
 
@@ -585,17 +610,48 @@ _channelitem_free_all(_channelitem *item)
 
 static void
 _channelitem_popped(_channelitem *item,
-                    _PyCrossInterpreterData **p_data, _waiting_t **p_waiting)
+                    _PyCrossInterpreterData **p_data, _waiting_t **p_waiting,
+                    int *p_unboundop)
 {
     assert(item->waiting == NULL || item->waiting->status == WAITING_ACQUIRED);
     *p_data = item->data;
     *p_waiting = item->waiting;
+    *p_unboundop = item->unboundop;
     // We clear them here, so they won't be released in _channelitem_clear().
     item->data = NULL;
     item->waiting = NULL;
     _channelitem_free(item);
 }
 
+static int
+_channelitem_clear_interpreter(_channelitem *item)
+{
+    assert(item->interpid >= 0);
+    if (item->data == NULL) {
+        // Its interpreter was already cleared (or it was never bound).
+        // For UNBOUND_REMOVE it should have been freed at that time.
+        assert(item->unboundop != UNBOUND_REMOVE);
+        return 0;
+    }
+    assert(_PyCrossInterpreterData_INTERPID(item->data) == item->interpid);
+
+    switch (item->unboundop) {
+    case UNBOUND_REMOVE:
+        // The caller must free/clear it.
+        return 1;
+    case UNBOUND_ERROR:
+    case UNBOUND_REPLACE:
+        // We won't need the cross-interpreter data later
+        // so we completely throw it away.
+        _channelitem_clear_data(item, 0);
+        return 0;
+    default:
+        Py_FatalError("not reachable");
+        return -1;
+    }
+}
+
+
 typedef struct _channelqueue {
     int64_t count;
     _channelitem *first;
@@ -634,9 +690,10 @@ _channelqueue_free(_channelqueue *queue)
 
 static int
 _channelqueue_put(_channelqueue *queue,
-                  _PyCrossInterpreterData *data, _waiting_t *waiting)
+                  int64_t interpid, _PyCrossInterpreterData *data,
+                  _waiting_t *waiting, int unboundop)
 {
-    _channelitem *item = _channelitem_new(data, waiting);
+    _channelitem *item = _channelitem_new(interpid, data, waiting, unboundop);
     if (item == NULL) {
         return -1;
     }
@@ -659,7 +716,8 @@ _channelqueue_put(_channelqueue *queue,
 
 static int
 _channelqueue_get(_channelqueue *queue,
-                  _PyCrossInterpreterData **p_data, _waiting_t **p_waiting)
+                  _PyCrossInterpreterData **p_data, _waiting_t **p_waiting,
+                  int *p_unboundop)
 {
     _channelitem *item = queue->first;
     if (item == NULL) {
@@ -671,7 +729,7 @@ _channelqueue_get(_channelqueue *queue,
     }
     queue->count -= 1;
 
-    _channelitem_popped(item, p_data, p_waiting);
+    _channelitem_popped(item, p_data, p_waiting, p_unboundop);
     return 0;
 }
 
@@ -737,7 +795,8 @@ _channelqueue_remove(_channelqueue *queue, _channelitem_id_t itemid,
     }
     queue->count -= 1;
 
-    _channelitem_popped(item, p_data, p_waiting);
+    int unboundop;
+    _channelitem_popped(item, p_data, p_waiting, &unboundop);
 }
 
 static void
@@ -748,14 +807,17 @@ _channelqueue_clear_interpreter(_channelqueue *queue, int64_t interpid)
     while (next != NULL) {
         _channelitem *item = next;
         next = item->next;
-        if (_PyCrossInterpreterData_INTERPID(item->data) == interpid) {
+        int remove = (item->interpid == interpid)
+            ? _channelitem_clear_interpreter(item)
+            : 0;
+        if (remove) {
+            _channelitem_free(item);
             if (prev == NULL) {
-                queue->first = item->next;
+                queue->first = next;
             }
             else {
-                prev->next = item->next;
+                prev->next = next;
             }
-            _channelitem_free(item);
             queue->count -= 1;
         }
         else {
@@ -1018,12 +1080,15 @@ typedef struct _channel {
     PyThread_type_lock mutex;
     _channelqueue *queue;
     _channelends *ends;
+    struct {
+        int unboundop;
+    } defaults;
     int open;
     struct _channel_closing *closing;
 } _channel_state;
 
 static _channel_state *
-_channel_new(PyThread_type_lock mutex)
+_channel_new(PyThread_type_lock mutex, int unboundop)
 {
     _channel_state *chan = GLOBAL_MALLOC(_channel_state);
     if (chan == NULL) {
@@ -1041,6 +1106,7 @@ _channel_new(PyThread_type_lock mutex)
         GLOBAL_FREE(chan);
         return NULL;
     }
+    chan->defaults.unboundop = unboundop;
     chan->open = 1;
     chan->closing = NULL;
     return chan;
@@ -1061,7 +1127,8 @@ _channel_free(_channel_state *chan)
 
 static int
 _channel_add(_channel_state *chan, int64_t interpid,
-             _PyCrossInterpreterData *data, _waiting_t *waiting)
+             _PyCrossInterpreterData *data, _waiting_t *waiting,
+             int unboundop)
 {
     int res = -1;
     PyThread_acquire_lock(chan->mutex, WAIT_LOCK);
@@ -1075,7 +1142,7 @@ _channel_add(_channel_state *chan, int64_t interpid,
         goto done;
     }
 
-    if (_channelqueue_put(chan->queue, data, waiting) != 0) {
+    if (_channelqueue_put(chan->queue, interpid, data, waiting, unboundop) != 0) {
         goto done;
     }
     // Any errors past this point must cause a _waiting_release() call.
@@ -1088,7 +1155,8 @@ done:
 
 static int
 _channel_next(_channel_state *chan, int64_t interpid,
-              _PyCrossInterpreterData **p_data, _waiting_t **p_waiting)
+              _PyCrossInterpreterData **p_data, _waiting_t **p_waiting,
+              int *p_unboundop)
 {
     int err = 0;
     PyThread_acquire_lock(chan->mutex, WAIT_LOCK);
@@ -1102,11 +1170,15 @@ _channel_next(_channel_state *chan, int64_t interpid,
         goto done;
     }
 
-    int empty = _channelqueue_get(chan->queue, p_data, p_waiting);
-    assert(empty == 0 || empty == ERR_CHANNEL_EMPTY);
+    int empty = _channelqueue_get(chan->queue, p_data, p_waiting, p_unboundop);
     assert(!PyErr_Occurred());
-    if (empty && chan->closing != NULL) {
-        chan->open = 0;
+    if (empty) {
+        assert(empty == ERR_CHANNEL_EMPTY);
+        if (chan->closing != NULL) {
+            chan->open = 0;
+        }
+        err = ERR_CHANNEL_EMPTY;
+        goto done;
     }
 
 done:
@@ -1528,18 +1600,27 @@ done:
     PyThread_release_lock(channels->mutex);
 }
 
-static int64_t *
+struct channel_id_and_info {
+    int64_t id;
+    int unboundop;
+};
+
+static struct channel_id_and_info *
 _channels_list_all(_channels *channels, int64_t *count)
 {
-    int64_t *cids = NULL;
+    struct channel_id_and_info *cids = NULL;
     PyThread_acquire_lock(channels->mutex, WAIT_LOCK);
-    int64_t *ids = PyMem_NEW(int64_t, (Py_ssize_t)(channels->numopen));
+    struct channel_id_and_info *ids =
+        PyMem_NEW(struct channel_id_and_info, (Py_ssize_t)(channels->numopen));
     if (ids == NULL) {
         goto done;
     }
     _channelref *ref = channels->head;
     for (int64_t i=0; ref != NULL; ref = ref->next, i++) {
-        ids[i] = ref->cid;
+        ids[i] = (struct channel_id_and_info){
+            .id = ref->cid,
+            .unboundop = ref->chan->defaults.unboundop,
+        };
     }
     *count = channels->numopen;
 
@@ -1624,13 +1705,13 @@ _channel_finish_closing(_channel_state *chan) {
 
 // Create a new channel.
 static int64_t
-channel_create(_channels *channels)
+channel_create(_channels *channels, int unboundop)
 {
     PyThread_type_lock mutex = PyThread_allocate_lock();
     if (mutex == NULL) {
         return ERR_CHANNEL_MUTEX_INIT;
     }
-    _channel_state *chan = _channel_new(mutex);
+    _channel_state *chan = _channel_new(mutex, unboundop);
     if (chan == NULL) {
         PyThread_free_lock(mutex);
         return -1;
@@ -1662,7 +1743,7 @@ channel_destroy(_channels *channels, int64_t cid)
 // Optionally request to be notified when it is received.
 static int
 channel_send(_channels *channels, int64_t cid, PyObject *obj,
-             _waiting_t *waiting)
+             _waiting_t *waiting, int unboundop)
 {
     PyInterpreterState *interp = _get_current_interp();
     if (interp == NULL) {
@@ -1698,7 +1779,7 @@ channel_send(_channels *channels, int64_t cid, PyObject *obj,
     }
 
     // Add the data to the channel.
-    int res = _channel_add(chan, interpid, data, waiting);
+    int res = _channel_add(chan, interpid, data, waiting, unboundop);
     PyThread_release_lock(mutex);
     if (res != 0) {
         // We may chain an exception here:
@@ -1735,7 +1816,7 @@ channel_clear_sent(_channels *channels, int64_t cid, _waiting_t *waiting)
 // Like channel_send(), but strictly wait for the object to be received.
 static int
 channel_send_wait(_channels *channels, int64_t cid, PyObject *obj,
-                   PY_TIMEOUT_T timeout)
+                  int unboundop, PY_TIMEOUT_T timeout)
 {
     // We use a stack variable here, so we must ensure that &waiting
     // is not held by any channel item at the point this function exits.
@@ -1746,7 +1827,7 @@ channel_send_wait(_channels *channels, int64_t cid, PyObject *obj,
     }
 
     /* Queue up the object. */
-    int res = channel_send(channels, cid, obj, &waiting);
+    int res = channel_send(channels, cid, obj, &waiting, unboundop);
     if (res < 0) {
         assert(waiting.status == WAITING_NO_STATUS);
         goto finally;
@@ -1788,7 +1869,7 @@ finally:
 // The current interpreter gets associated with the recv end of the channel.
 // XXX Support a "wait" mutex?
 static int
-channel_recv(_channels *channels, int64_t cid, PyObject **res)
+channel_recv(_channels *channels, int64_t cid, PyObject **res, int *p_unboundop)
 {
     int err;
     *res = NULL;
@@ -1816,13 +1897,15 @@ channel_recv(_channels *channels, int64_t cid, PyObject **res)
     // Pop off the next item from the channel.
     _PyCrossInterpreterData *data = NULL;
     _waiting_t *waiting = NULL;
-    err = _channel_next(chan, interpid, &data, &waiting);
+    err = _channel_next(chan, interpid, &data, &waiting, p_unboundop);
     PyThread_release_lock(mutex);
     if (err != 0) {
         return err;
     }
     else if (data == NULL) {
+        // The item was unbound.
         assert(!PyErr_Occurred());
+        *res = NULL;
         return 0;
     }
 
@@ -1915,6 +1998,23 @@ channel_is_associated(_channels *channels, int64_t cid, int64_t interpid,
     return (end != NULL && end->open);
 }
 
+static int
+_channel_get_count(_channels *channels, int64_t cid, Py_ssize_t *p_count)
+{
+    PyThread_type_lock mutex = NULL;
+    _channel_state *chan = NULL;
+    int err = _channels_lookup(channels, cid, &mutex, &chan);
+    if (err != 0) {
+        return err;
+    }
+    assert(chan != NULL);
+    int64_t count = chan->queue->count;
+    PyThread_release_lock(mutex);
+
+    *p_count = (Py_ssize_t)count;
+    return 0;
+}
+
 
 /* channel info */
 
@@ -2767,9 +2867,22 @@ clear_interpreter(void *data)
 
 
 static PyObject *
-channelsmod_create(PyObject *self, PyObject *Py_UNUSED(ignored))
+channelsmod_create(PyObject *self, PyObject *args, PyObject *kwds)
 {
-    int64_t cid = channel_create(&_globals.channels);
+    static char *kwlist[] = {"unboundop", NULL};
+    int unboundop;
+    if (!PyArg_ParseTupleAndKeywords(args, kwds, "i:create", kwlist,
+                                     &unboundop))
+    {
+        return NULL;
+    }
+    if (!check_unbound(unboundop)) {
+        PyErr_Format(PyExc_ValueError,
+                     "unsupported unboundop %d", unboundop);
+        return NULL;
+    }
+
+    int64_t cid = channel_create(&_globals.channels, unboundop);
     if (cid < 0) {
         (void)handle_channel_error(-1, self, cid);
         return NULL;
@@ -2796,7 +2909,7 @@ channelsmod_create(PyObject *self, PyObject *Py_UNUSED(ignored))
 }
 
 PyDoc_STRVAR(channelsmod_create_doc,
-"channel_create() -> cid\n\
+"channel_create(unboundop) -> cid\n\
 \n\
 Create a new cross-interpreter channel and return a unique generated ID.");
 
@@ -2831,7 +2944,8 @@ static PyObject *
 channelsmod_list_all(PyObject *self, PyObject *Py_UNUSED(ignored))
 {
     int64_t count = 0;
-    int64_t *cids = _channels_list_all(&_globals.channels, &count);
+    struct channel_id_and_info *cids =
+        _channels_list_all(&_globals.channels, &count);
     if (cids == NULL) {
         if (count == 0) {
             return PyList_New(0);
@@ -2848,19 +2962,26 @@ channelsmod_list_all(PyObject *self, PyObject *Py_UNUSED(ignored))
         ids = NULL;
         goto finally;
     }
-    int64_t *cur = cids;
+    struct channel_id_and_info *cur = cids;
     for (int64_t i=0; i < count; cur++, i++) {
         PyObject *cidobj = NULL;
-        int err = newchannelid(state->ChannelIDType, *cur, 0,
+        int err = newchannelid(state->ChannelIDType, cur->id, 0,
                                &_globals.channels, 0, 0,
                                (channelid **)&cidobj);
-        if (handle_channel_error(err, self, *cur)) {
+        if (handle_channel_error(err, self, cur->id)) {
             assert(cidobj == NULL);
             Py_SETREF(ids, NULL);
             break;
         }
         assert(cidobj != NULL);
-        PyList_SET_ITEM(ids, (Py_ssize_t)i, cidobj);
+
+        PyObject *item = Py_BuildValue("Oi", cidobj, cur->unboundop);
+        Py_DECREF(cidobj);
+        if (item == NULL) {
+            Py_SETREF(ids, NULL);
+            break;
+        }
+        PyList_SET_ITEM(ids, (Py_ssize_t)i, item);
     }
 
 finally:
@@ -2942,16 +3063,24 @@ receive end.");
 static PyObject *
 channelsmod_send(PyObject *self, PyObject *args, PyObject *kwds)
 {
-    static char *kwlist[] = {"cid", "obj", "blocking", "timeout", NULL};
+    static char *kwlist[] = {"cid", "obj", "unboundop", "blocking", "timeout",
+                             NULL};
     struct channel_id_converter_data cid_data = {
         .module = self,
     };
     PyObject *obj;
+    int unboundop = UNBOUND_REPLACE;
     int blocking = 1;
     PyObject *timeout_obj = NULL;
-    if (!PyArg_ParseTupleAndKeywords(args, kwds, "O&O|$pO:channel_send", kwlist,
+    if (!PyArg_ParseTupleAndKeywords(args, kwds, "O&O|i$pO:channel_send", kwlist,
                                      channel_id_converter, &cid_data, &obj,
-                                     &blocking, &timeout_obj)) {
+                                     &unboundop, &blocking, &timeout_obj))
+    {
+        return NULL;
+    }
+    if (!check_unbound(unboundop)) {
+        PyErr_Format(PyExc_ValueError,
+                     "unsupported unboundop %d", unboundop);
         return NULL;
     }
 
@@ -2964,10 +3093,10 @@ channelsmod_send(PyObject *self, PyObject *args, PyObject *kwds)
     /* Queue up the object. */
     int err = 0;
     if (blocking) {
-        err = channel_send_wait(&_globals.channels, cid, obj, timeout);
+        err = channel_send_wait(&_globals.channels, cid, obj, unboundop, timeout);
     }
     else {
-        err = channel_send(&_globals.channels, cid, obj, NULL);
+        err = channel_send(&_globals.channels, cid, obj, NULL, unboundop);
     }
     if (handle_channel_error(err, self, cid)) {
         return NULL;
@@ -2985,17 +3114,24 @@ By default this waits for the object to be received.");
 static PyObject *
 channelsmod_send_buffer(PyObject *self, PyObject *args, PyObject *kwds)
 {
-    static char *kwlist[] = {"cid", "obj", "blocking", "timeout", NULL};
+    static char *kwlist[] = {"cid", "obj", "unboundop", "blocking", "timeout",
+                             NULL};
     struct channel_id_converter_data cid_data = {
         .module = self,
     };
     PyObject *obj;
+    int unboundop = UNBOUND_REPLACE;
     int blocking = 1;
     PyObject *timeout_obj = NULL;
     if (!PyArg_ParseTupleAndKeywords(args, kwds,
-                                     "O&O|$pO:channel_send_buffer", kwlist,
+                                     "O&O|i$pO:channel_send_buffer", kwlist,
                                      channel_id_converter, &cid_data, &obj,
-                                     &blocking, &timeout_obj)) {
+                                     &unboundop, &blocking, &timeout_obj)) {
+        return NULL;
+    }
+    if (!check_unbound(unboundop)) {
+        PyErr_Format(PyExc_ValueError,
+                     "unsupported unboundop %d", unboundop);
         return NULL;
     }
 
@@ -3013,10 +3149,11 @@ channelsmod_send_buffer(PyObject *self, PyObject *args, PyObject *kwds)
     /* Queue up the object. */
     int err = 0;
     if (blocking) {
-        err = channel_send_wait(&_globals.channels, cid, tempobj, timeout);
+        err = channel_send_wait(
+                &_globals.channels, cid, tempobj, unboundop, timeout);
     }
     else {
-        err = channel_send(&_globals.channels, cid, tempobj, NULL);
+        err = channel_send(&_globals.channels, cid, tempobj, NULL, unboundop);
     }
     Py_DECREF(tempobj);
     if (handle_channel_error(err, self, cid)) {
@@ -3048,25 +3185,28 @@ channelsmod_recv(PyObject *self, PyObject *args, PyObject *kwds)
     cid = cid_data.cid;
 
     PyObject *obj = NULL;
-    int err = channel_recv(&_globals.channels, cid, &obj);
-    if (handle_channel_error(err, self, cid)) {
-        return NULL;
-    }
-    Py_XINCREF(dflt);
-    if (obj == NULL) {
+    int unboundop = 0;
+    int err = channel_recv(&_globals.channels, cid, &obj, &unboundop);
+    if (err == ERR_CHANNEL_EMPTY && dflt != NULL) {
         // Use the default.
-        if (dflt == NULL) {
-            (void)handle_channel_error(ERR_CHANNEL_EMPTY, self, cid);
-            return NULL;
-        }
         obj = Py_NewRef(dflt);
+        err = 0;
     }
-    Py_XDECREF(dflt);
-    return obj;
+    else if (handle_channel_error(err, self, cid)) {
+        return NULL;
+    }
+    else if (obj == NULL) {
+        // The item was unbound.
+        return Py_BuildValue("Oi", Py_None, unboundop);
+    }
+
+    PyObject *res = Py_BuildValue("OO", obj, Py_None);
+    Py_DECREF(obj);
+    return res;
 }
 
 PyDoc_STRVAR(channelsmod_recv_doc,
-"channel_recv(cid, [default]) -> obj\n\
+"channel_recv(cid, [default]) -> (obj, unboundop)\n\
 \n\
 Return a new object from the data at the front of the channel's queue.\n\
 \n\
@@ -3168,6 +3308,34 @@ Close the channel for the current interpreter.  'send' and 'recv'\n\
 ends are closed.  Closing an already closed end is a noop.");
 
 static PyObject *
+channelsmod_get_count(PyObject *self, PyObject *args, PyObject *kwds)
+{
+    static char *kwlist[] = {"cid", NULL};
+    struct channel_id_converter_data cid_data = {
+        .module = self,
+    };
+    if (!PyArg_ParseTupleAndKeywords(args, kwds,
+                                     "O&:get_count", kwlist,
+                                     channel_id_converter, &cid_data)) {
+        return NULL;
+    }
+    int64_t cid = cid_data.cid;
+
+    Py_ssize_t count = -1;
+    int err = _channel_get_count(&_globals.channels, cid, &count);
+    if (handle_channel_error(err, self, cid)) {
+        return NULL;
+    }
+    assert(count >= 0);
+    return PyLong_FromSsize_t(count);
+}
+
+PyDoc_STRVAR(channelsmod_get_count_doc,
+"get_count(cid)\n\
+\n\
+Return the number of items in the channel.");
+
+static PyObject *
 channelsmod_get_info(PyObject *self, PyObject *args, PyObject *kwds)
 {
     static char *kwlist[] = {"cid", NULL};
@@ -3195,6 +3363,38 @@ PyDoc_STRVAR(channelsmod_get_info_doc,
 Return details about the channel.");
 
 static PyObject *
+channelsmod_get_channel_defaults(PyObject *self, PyObject *args, PyObject *kwds)
+{
+    static char *kwlist[] = {"cid", NULL};
+    struct channel_id_converter_data cid_data = {
+        .module = self,
+    };
+    if (!PyArg_ParseTupleAndKeywords(args, kwds,
+                                     "O&:get_channel_defaults", kwlist,
+                                     channel_id_converter, &cid_data)) {
+        return NULL;
+    }
+    int64_t cid = cid_data.cid;
+
+    PyThread_type_lock mutex = NULL;
+    _channel_state *channel = NULL;
+    int err = _channels_lookup(&_globals.channels, cid, &mutex, &channel);
+    if (handle_channel_error(err, self, cid)) {
+        return NULL;
+    }
+    int unboundop = channel->defaults.unboundop;
+    PyThread_release_lock(mutex);
+
+    PyObject *defaults = Py_BuildValue("i", unboundop);
+    return defaults;
+}
+
+PyDoc_STRVAR(channelsmod_get_channel_defaults_doc,
+"get_channel_defaults(cid)\n\
+\n\
+Return the channel's default values, set when it was created.");
+
+static PyObject *
 channelsmod__channel_id(PyObject *self, PyObject *args, PyObject *kwds)
 {
     module_state *state = get_module_state(self);
@@ -3240,8 +3440,8 @@ channelsmod__register_end_types(PyObject *self, PyObject *args, PyObject *kwds)
 }
 
 static PyMethodDef module_functions[] = {
-    {"create",                     channelsmod_create,
-     METH_NOARGS,                  channelsmod_create_doc},
+    {"create",                     _PyCFunction_CAST(channelsmod_create),
+     METH_VARARGS | METH_KEYWORDS, channelsmod_create_doc},
     {"destroy",                    _PyCFunction_CAST(channelsmod_destroy),
      METH_VARARGS | METH_KEYWORDS, channelsmod_destroy_doc},
     {"list_all",                   channelsmod_list_all,
@@ -3258,8 +3458,12 @@ static PyMethodDef module_functions[] = {
      METH_VARARGS | METH_KEYWORDS, channelsmod_close_doc},
     {"release",                    _PyCFunction_CAST(channelsmod_release),
      METH_VARARGS | METH_KEYWORDS, channelsmod_release_doc},
+    {"get_count",                   _PyCFunction_CAST(channelsmod_get_count),
+     METH_VARARGS | METH_KEYWORDS, channelsmod_get_count_doc},
     {"get_info",                   _PyCFunction_CAST(channelsmod_get_info),
      METH_VARARGS | METH_KEYWORDS, channelsmod_get_info_doc},
+    {"get_channel_defaults",       _PyCFunction_CAST(channelsmod_get_channel_defaults),
+     METH_VARARGS | METH_KEYWORDS, channelsmod_get_channel_defaults_doc},
     {"_channel_id",                _PyCFunction_CAST(channelsmod__channel_id),
      METH_VARARGS | METH_KEYWORDS, NULL},
     {"_register_end_types",        _PyCFunction_CAST(channelsmod__register_end_types),
diff --git a/Modules/_interpqueuesmodule.c b/Modules/_interpqueuesmodule.c
index 8e82789..5dec240 100644
--- a/Modules/_interpqueuesmodule.c
+++ b/Modules/_interpqueuesmodule.c
@@ -9,7 +9,9 @@
 #include "pycore_crossinterp.h"   // struct _xid
 
 #define REGISTERS_HEAP_TYPES
+#define HAS_UNBOUND_ITEMS
 #include "_interpreters_common.h"
+#undef HAS_UNBOUND_ITEMS
 #undef REGISTERS_HEAP_TYPES
 
 
@@ -58,20 +60,6 @@ _release_xid_data(_PyCrossInterpreterData *data, int flags)
     return res;
 }
 
-static inline int64_t
-_get_interpid(_PyCrossInterpreterData *data)
-{
-    int64_t interpid;
-    if (data != NULL) {
-        interpid = _PyCrossInterpreterData_INTERPID(data);
-        assert(!PyErr_Occurred());
-    }
-    else {
-        interpid = PyInterpreterState_GetID(PyInterpreterState_Get());
-    }
-    return interpid;
-}
-
 static PyInterpreterState *
 _get_current_interp(void)
 {
@@ -402,32 +390,6 @@ handle_queue_error(int err, PyObject *mod, int64_t qid)
 }
 
 
-/* unbound items ************************************************************/
-
-#define UNBOUND_REMOVE 1
-#define UNBOUND_ERROR 2
-#define UNBOUND_REPLACE 3
-
-// It would also be possible to add UNBOUND_REPLACE where the replacement
-// value is user-provided.  There would be some limitations there, though.
-// Another possibility would be something like UNBOUND_COPY, where the
-// object is released but the underlying data is copied (with the "raw"
-// allocator) and used when the item is popped off the queue.
-
-static int
-check_unbound(int unboundop)
-{
-    switch (unboundop) {
-    case UNBOUND_REMOVE:
-    case UNBOUND_ERROR:
-    case UNBOUND_REPLACE:
-        return 1;
-    default:
-        return 0;
-    }
-}
-
-
 /* the basic queue **********************************************************/
 
 struct _queueitem;
diff --git a/Modules/_interpreters_common.h b/Modules/_interpreters_common.h
index 07120f6..0d2e0c9 100644
--- a/Modules/_interpreters_common.h
+++ b/Modules/_interpreters_common.h
@@ -19,3 +19,48 @@ clear_xid_class(PyTypeObject *cls)
     return _PyCrossInterpreterData_UnregisterClass(cls);
 }
 #endif
+
+
+static inline int64_t
+_get_interpid(_PyCrossInterpreterData *data)
+{
+    int64_t interpid;
+    if (data != NULL) {
+        interpid = _PyCrossInterpreterData_INTERPID(data);
+        assert(!PyErr_Occurred());
+    }
+    else {
+        interpid = PyInterpreterState_GetID(PyInterpreterState_Get());
+    }
+    return interpid;
+}
+
+
+/* unbound items ************************************************************/
+
+#ifdef HAS_UNBOUND_ITEMS
+
+#define UNBOUND_REMOVE 1
+#define UNBOUND_ERROR 2
+#define UNBOUND_REPLACE 3
+
+// It would also be possible to add UNBOUND_REPLACE where the replacement
+// value is user-provided.  There would be some limitations there, though.
+// Another possibility would be something like UNBOUND_COPY, where the
+// object is released but the underlying data is copied (with the "raw"
+// allocator) and used when the item is popped off the queue.
+
+static int
+check_unbound(int unboundop)
+{
+    switch (unboundop) {
+    case UNBOUND_REMOVE:
+    case UNBOUND_ERROR:
+    case UNBOUND_REPLACE:
+        return 1;
+    default:
+        return 0;
+    }
+}
+
+#endif
-- 
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