From ec5aa60f9b025915510871c7bbbeb4ca56b885ae Mon Sep 17 00:00:00 2001
From: Lewis Gaul <lewis.gaul@gmail.com>
Date: Tue, 12 Nov 2019 11:24:05 +0000
Subject: [PATCH 01/33] Attempt at implementing channel_list_interpreters()

---
 Modules/_xxsubinterpretersmodule.c | 87 ++++++++++++++++++++++++++++++
 1 file changed, 87 insertions(+)

diff --git a/Modules/_xxsubinterpretersmodule.c b/Modules/_xxsubinterpretersmodule.c
index 4a6ffdd3266783..19aca1ad70f7df 100644
--- a/Modules/_xxsubinterpretersmodule.c
+++ b/Modules/_xxsubinterpretersmodule.c
@@ -627,6 +627,30 @@ _channelends_associate(_channelends *ends, int64_t interp, int send)
     return 0;
 }
 
+static int64_t *
+_channelends_list_interpreters(_channelends *ends, int64_t *count, int send)
+{
+    int64_t *ids = NULL;
+    int64_t numopen = send ? ends->numsendopen : ends->numrecvopen;
+    if (numopen >= PY_SSIZE_T_MAX) {
+        PyErr_SetString(PyExc_RuntimeError,
+                        "too many interpreters using the channel");
+        goto done;
+    }
+    ids = PyMem_NEW(int64_t, (Py_ssize_t)numopen);
+    if (ids == NULL) {
+        goto done;
+    }
+    _channelend *ref = send ? ends->send : ends->recv;
+    for (int64_t i=0; ref != NULL; ref = ref->next, i++) {
+        ids[i] = ref->interp;
+    }
+    *count = numopen;
+
+done:
+    return ids;
+}
+
 static int
 _channelends_is_open(_channelends *ends)
 {
@@ -2323,6 +2347,67 @@ PyDoc_STRVAR(channel_list_all_doc,
 \n\
 Return the list of all IDs for active channels.");
 
+
+static PyObject *
+channel_list_interpreters(PyObject *self, PyObject *args, PyObject *kwds)
+{
+    static char *kwlist[] = {"cid", NULL};
+    int64_t cid;
+    _PyChannelState *chan;
+    int send = 1;           /* Send or receive ends? */
+    int64_t count;          /* Number of interpreters to return */
+    int64_t *ids;           /* Array of interpreter IDs to return */
+    PyObject *id_obj;
+    PyObject *ret;          /* Python list of interpreter IDs */
+
+    if (!PyArg_ParseTupleAndKeywords(args, kwds, "O&:channel_list_interps",
+                                     kwlist, channel_id_converter, &cid)) {
+        return NULL;
+    }
+
+    chan = _channels_lookup(&_globals.channels, cid, NULL);
+    ids = _channelends_list_interpreters(chan->ends, &count, send);
+    
+    ret = PyList_New((Py_ssize_t)count);
+    if (ret == NULL) {
+        goto finally;
+    }
+
+    for (int64_t i=0; i < count; i++) {
+        PyInterpreterState *interp = PyInterpreterState_Head();
+        while (interp != NULL) {
+            id_obj = _PyInterpreterState_GetIDObject(interp);
+            if (id_obj == NULL) {
+                goto except;
+            }
+            if (ids[i] == PyInterpreterState_GetID(interp)) {
+                int rc = PyList_SetItem(ret, i, id_obj);
+                Py_DECREF(id_obj);
+                if (rc < 0) {
+                    goto except;
+                }
+                break;
+            }
+            interp = PyInterpreterState_Next(interp);
+        }
+    }
+
+except:
+    Py_XDECREF(ret);
+    ret = NULL;
+
+finally:
+    PyMem_Free(ids);
+    return ret;
+}
+
+PyDoc_STRVAR(channel_list_interpreters_doc,
+"channel_list_interpreters(cid) -> [id]\n\
+\n\
+Return the list of all interpreter IDs associated with the channel\n\
+XXX in the send direction.");
+
+
 static PyObject *
 channel_send(PyObject *self, PyObject *args, PyObject *kwds)
 {
@@ -2476,6 +2561,8 @@ static PyMethodDef module_functions[] = {
      METH_VARARGS | METH_KEYWORDS, channel_destroy_doc},
     {"channel_list_all",          channel_list_all,
      METH_NOARGS, channel_list_all_doc},
+    {"channel_list_interpreters", (PyCFunction)(void(*)(void))channel_list_interpreters,
+     METH_VARARGS | METH_KEYWORDS, channel_list_interpreters_doc},
     {"channel_send",              (PyCFunction)(void(*)(void))channel_send,
      METH_VARARGS | METH_KEYWORDS, channel_send_doc},
     {"channel_recv",              (PyCFunction)(void(*)(void))channel_recv,

From ee781339735d021ae30feb8995523b5eb5bc8439 Mon Sep 17 00:00:00 2001
From: Lewis Gaul <lewis.gaul@gmail.com>
Date: Tue, 12 Nov 2019 16:56:12 +0000
Subject: [PATCH 02/33] Fix some error cases

---
 Modules/_xxsubinterpretersmodule.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/Modules/_xxsubinterpretersmodule.c b/Modules/_xxsubinterpretersmodule.c
index 19aca1ad70f7df..b265c7d87d2eaa 100644
--- a/Modules/_xxsubinterpretersmodule.c
+++ b/Modules/_xxsubinterpretersmodule.c
@@ -2356,21 +2356,25 @@ channel_list_interpreters(PyObject *self, PyObject *args, PyObject *kwds)
     _PyChannelState *chan;
     int send = 1;           /* Send or receive ends? */
     int64_t count;          /* Number of interpreters to return */
-    int64_t *ids;           /* Array of interpreter IDs to return */
-    PyObject *id_obj;
-    PyObject *ret;          /* Python list of interpreter IDs */
+    int64_t *ids = NULL;    /* Array of interpreter IDs to return */
+    PyObject *id_obj = NULL;
+    PyObject *ret = NULL;   /* Python list of interpreter IDs */
 
-    if (!PyArg_ParseTupleAndKeywords(args, kwds, "O&:channel_list_interps",
+    if (!PyArg_ParseTupleAndKeywords(args, kwds, "O&:channel_list_interpreters",
                                      kwlist, channel_id_converter, &cid)) {
         return NULL;
     }
 
     chan = _channels_lookup(&_globals.channels, cid, NULL);
+    if (chan == NULL) {
+        goto except;
+    }
+
     ids = _channelends_list_interpreters(chan->ends, &count, send);
     
     ret = PyList_New((Py_ssize_t)count);
     if (ret == NULL) {
-        goto finally;
+        goto except;
     }
 
     for (int64_t i=0; i < count; i++) {
@@ -2392,6 +2396,8 @@ channel_list_interpreters(PyObject *self, PyObject *args, PyObject *kwds)
         }
     }
 
+    goto finally;
+
 except:
     Py_XDECREF(ret);
     ret = NULL;

From 433a5b0aef9fdbf873d350a741a21fbf60f4f1ad Mon Sep 17 00:00:00 2001
From: Lewis Gaul <lewis.gaul@gmail.com>
Date: Tue, 12 Nov 2019 17:46:43 +0000
Subject: [PATCH 03/33] Fix issue with decreasing list items' references

---
 Modules/_xxsubinterpretersmodule.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/Modules/_xxsubinterpretersmodule.c b/Modules/_xxsubinterpretersmodule.c
index b265c7d87d2eaa..7eb67cec93baeb 100644
--- a/Modules/_xxsubinterpretersmodule.c
+++ b/Modules/_xxsubinterpretersmodule.c
@@ -2382,14 +2382,11 @@ channel_list_interpreters(PyObject *self, PyObject *args, PyObject *kwds)
         while (interp != NULL) {
             id_obj = _PyInterpreterState_GetIDObject(interp);
             if (id_obj == NULL) {
+                Py_DECREF(id_obj);
                 goto except;
             }
             if (ids[i] == PyInterpreterState_GetID(interp)) {
-                int rc = PyList_SetItem(ret, i, id_obj);
-                Py_DECREF(id_obj);
-                if (rc < 0) {
-                    goto except;
-                }
+                PyList_SET_ITEM(ret, i, id_obj);
                 break;
             }
             interp = PyInterpreterState_Next(interp);

From 64c1661854ea7488abaa14adfd2125ccb71fff18 Mon Sep 17 00:00:00 2001
From: Lewis Gaul <lewis.gaul@gmail.com>
Date: Wed, 20 Nov 2019 11:32:45 +0000
Subject: [PATCH 04/33] Fix issue with reference to interpreter

---
 Modules/_xxsubinterpretersmodule.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/Modules/_xxsubinterpretersmodule.c b/Modules/_xxsubinterpretersmodule.c
index 7eb67cec93baeb..eb65e10479c368 100644
--- a/Modules/_xxsubinterpretersmodule.c
+++ b/Modules/_xxsubinterpretersmodule.c
@@ -2388,6 +2388,8 @@ channel_list_interpreters(PyObject *self, PyObject *args, PyObject *kwds)
             if (ids[i] == PyInterpreterState_GetID(interp)) {
                 PyList_SET_ITEM(ret, i, id_obj);
                 break;
+            } else {
+                Py_DECREF(id_obj);
             }
             interp = PyInterpreterState_Next(interp);
         }

From 2c5c6df01629e304cef490814a05938c57c73b83 Mon Sep 17 00:00:00 2001
From: Lewis Gaul <lewis.gaul@gmail.com>
Date: Wed, 20 Nov 2019 11:34:07 +0000
Subject: [PATCH 05/33] Add ability to list interpreters on send or receive
 channels

---
 Modules/_xxsubinterpretersmodule.c | 22 ++++++++++++++--------
 1 file changed, 14 insertions(+), 8 deletions(-)

diff --git a/Modules/_xxsubinterpretersmodule.c b/Modules/_xxsubinterpretersmodule.c
index eb65e10479c368..c1d6f3f7d17394 100644
--- a/Modules/_xxsubinterpretersmodule.c
+++ b/Modules/_xxsubinterpretersmodule.c
@@ -2354,17 +2354,23 @@ channel_list_interpreters(PyObject *self, PyObject *args, PyObject *kwds)
     static char *kwlist[] = {"cid", NULL};
     int64_t cid;
     _PyChannelState *chan;
-    int send = 1;           /* Send or receive ends? */
-    int64_t count;          /* Number of interpreters to return */
-    int64_t *ids = NULL;    /* Array of interpreter IDs to return */
-    PyObject *id_obj = NULL;
+    int send = 0;           /* Send end? */
+    int recv = 0;           /* Receive end? */
+    int64_t *ids = NULL;    /* Array of interpreter IDs */
+    int64_t count = 0;      /* Number of interpreters to return */
     PyObject *ret = NULL;   /* Python list of interpreter IDs */
 
-    if (!PyArg_ParseTupleAndKeywords(args, kwds, "O&:channel_list_interpreters",
-                                     kwlist, channel_id_converter, &cid)) {
+    if (!PyArg_ParseTupleAndKeywords(args, kwds, "O&|$pp:channel_list_interpreters",
+                                     kwlist, channel_id_converter, &cid, &send, &recv)) {
         return NULL;
     }
 
+    if ((send && recv) || (!send && !recv)) {
+        PyErr_SetString(PyExc_RuntimeError,
+                        "Can only list interpreters for a single end of a channel");
+        goto except;
+    }
+
     chan = _channels_lookup(&_globals.channels, cid, NULL);
     if (chan == NULL) {
         goto except;
@@ -2380,7 +2386,7 @@ channel_list_interpreters(PyObject *self, PyObject *args, PyObject *kwds)
     for (int64_t i=0; i < count; i++) {
         PyInterpreterState *interp = PyInterpreterState_Head();
         while (interp != NULL) {
-            id_obj = _PyInterpreterState_GetIDObject(interp);
+            PyObject *id_obj = _PyInterpreterState_GetIDObject(interp);
             if (id_obj == NULL) {
                 Py_DECREF(id_obj);
                 goto except;
@@ -2407,7 +2413,7 @@ channel_list_interpreters(PyObject *self, PyObject *args, PyObject *kwds)
 }
 
 PyDoc_STRVAR(channel_list_interpreters_doc,
-"channel_list_interpreters(cid) -> [id]\n\
+"channel_list_interpreters(cid, *, send=False, recv=False) -> [id]\n\
 \n\
 Return the list of all interpreter IDs associated with the channel\n\
 XXX in the send direction.");

From 7f439fa8743d580f722103a34096ddd3c4df353f Mon Sep 17 00:00:00 2001
From: Lewis Gaul <lewis.gaul@gmail.com>
Date: Wed, 20 Nov 2019 11:45:41 +0000
Subject: [PATCH 06/33] Fix arg parsing in C code

---
 Modules/_xxsubinterpretersmodule.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Modules/_xxsubinterpretersmodule.c b/Modules/_xxsubinterpretersmodule.c
index c1d6f3f7d17394..b5c9af0bf93f92 100644
--- a/Modules/_xxsubinterpretersmodule.c
+++ b/Modules/_xxsubinterpretersmodule.c
@@ -2351,7 +2351,7 @@ Return the list of all IDs for active channels.");
 static PyObject *
 channel_list_interpreters(PyObject *self, PyObject *args, PyObject *kwds)
 {
-    static char *kwlist[] = {"cid", NULL};
+    static char *kwlist[] = {"cid", "send", "recv", NULL};
     int64_t cid;
     _PyChannelState *chan;
     int send = 0;           /* Send end? */

From d86ae971377dcae1e0bfffabd165c54e8a620f43 Mon Sep 17 00:00:00 2001
From: Lewis Gaul <lewis.gaul@gmail.com>
Date: Wed, 20 Nov 2019 11:58:49 +0000
Subject: [PATCH 07/33] Minor changes

---
 Modules/_xxsubinterpretersmodule.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/Modules/_xxsubinterpretersmodule.c b/Modules/_xxsubinterpretersmodule.c
index b5c9af0bf93f92..ca227b5760880e 100644
--- a/Modules/_xxsubinterpretersmodule.c
+++ b/Modules/_xxsubinterpretersmodule.c
@@ -2360,14 +2360,15 @@ channel_list_interpreters(PyObject *self, PyObject *args, PyObject *kwds)
     int64_t count = 0;      /* Number of interpreters to return */
     PyObject *ret = NULL;   /* Python list of interpreter IDs */
 
-    if (!PyArg_ParseTupleAndKeywords(args, kwds, "O&|$pp:channel_list_interpreters",
-                                     kwlist, channel_id_converter, &cid, &send, &recv)) {
+    if (!PyArg_ParseTupleAndKeywords(
+            args, kwds, "O&|$pp:channel_list_interpreters",
+            kwlist, channel_id_converter, &cid, &send, &recv)) {
         return NULL;
     }
 
     if ((send && recv) || (!send && !recv)) {
-        PyErr_SetString(PyExc_RuntimeError,
-                        "Can only list interpreters for a single end of a channel");
+        PyErr_SetString(PyExc_ValueError,
+                        "Specify exactly one of send or recv");
         goto except;
     }
 

From c529c8814085200494224fd4c0a9039e84d07cef Mon Sep 17 00:00:00 2001
From: Benjamin Edwards <benedwar@cisco.com>
Date: Thu, 21 Nov 2019 02:58:04 -0800
Subject: [PATCH 08/33] Add channel_list_interpreters() tests

---
 Lib/test/test__xxsubinterpreters.py | 75 ++++++++++++++++++++++++++++-
 1 file changed, 74 insertions(+), 1 deletion(-)

diff --git a/Lib/test/test__xxsubinterpreters.py b/Lib/test/test__xxsubinterpreters.py
index 207b5db5d8fb9b..e5494212d4eada 100644
--- a/Lib/test/test__xxsubinterpreters.py
+++ b/Lib/test/test__xxsubinterpreters.py
@@ -1202,6 +1202,58 @@ def test_ids_global(self):
         cid2 = int(out.strip())
 
         self.assertEqual(cid2, int(cid1) + 1)
+        
+    def _assert_interpreters_returned_are(self, cid, send, recv):
+        ilist = set(interpreters.channel_list_interpreters(cid, send=True))
+        self.assertEqual(ilist, set(send))
+        ilist = set(interpreters.channel_list_interpreters(cid, recv=True))
+        self.assertEqual(ilist, set(recv))
+        
+    def test_channel_list_interpreters_empty(self):
+        # Test for channel with no associated interpreters.
+        cid = interpreters.channel_create()
+        self._assert_interpreters_returned_are(cid, [], [])
+        
+    def test_channel_list_interpreters_mainline(self):
+        interp0 = interpreters.get_main()
+        cid = interpreters.channel_create()
+        interpreters.channel_send(cid, "send")
+        # Test for a channel that has one end associated to an interpreter.
+        self._assert_interpreters_returned_are(
+            cid, [interp0], [])
+        
+        interp1 = interpreters.create()
+        _run_output(interp1, dedent(f"""
+            import _interpreters
+            obj = _interpreters.channel_recv({cid})
+            """))
+        # Test for channel that has boths ends associated to an interpreter.
+        self._assert_interpreters_returned_are(
+            cid, [interp0], [interp1])
+        
+    def test_channel_list_interpreters_multiple(self):
+        # Test for channel with both ends associated to many interpreters.
+        interp0 = interpreters.get_main()
+        interp1 = interpreters.create()
+        interp2 = interpreters.create()
+        interp3 = interpreters.create()
+        cid = interpreters.channel_create()
+        
+        interpreters.channel_send(cid, "send")
+        _run_output(interp1, dedent(f"""
+            import _interpreters
+            obj = _interpreters.channel_send({cid}, "send")
+            """))
+        _run_output(interp2, dedent(f"""
+            import _interpreters
+            obj = _interpreters.channel_recv({cid})
+            """))
+        _run_output(interp3, dedent(f"""
+            import _interpreters
+            obj = _interpreters.channel_recv({cid})
+            """))
+        self._assert_interpreters_returned_are(
+            cid, [interp0, interp1], [interp3, interp2])
 
     ####################
 
@@ -1514,7 +1566,28 @@ def test_close_used_multiple_times_by_single_user(self):
             interpreters.channel_send(cid, b'eggs')
         with self.assertRaises(interpreters.ChannelClosedError):
             interpreters.channel_recv(cid)
-
+            
+    def test_channel_list_interpreters_invalid_channel(self):
+        cid = interpreters.channel_create()    
+        # Test for invalid channel ID.    
+        with self.assertRaises(interpreters.ChannelNotFoundError):
+            interpreters.channel_list_interpreters(1000, send=True)
+            
+        interpreters.channel_close(cid)
+        # Test for a channel that has been closed.
+        with self.assertRaises(interpreters.ChannelClosedError):
+            interpreters.channel_list_interpreters(cid, send=True)
+            
+    def test_channel_list_interpreters_invalid_args(self):
+        # Tests for invalid arguments passed to the API.
+        cid = interpreters.channel_create()
+        
+        with self.assertRaises(ValueError):
+            interpreters.channel_list_interpreters(cid)
+            
+        with self.assertRaises(ValueError):
+            interpreters.channel_list_interpreters(cid, send=True, recv=True)
+        
 
 class ChannelReleaseTests(TestBase):
 

From 168ebf36028ade4e7db6588982453905abb6c0d0 Mon Sep 17 00:00:00 2001
From: Lewis Gaul <lewis.gaul@gmail.com>
Date: Thu, 21 Nov 2019 11:58:49 +0000
Subject: [PATCH 09/33] Fix docstring

---
 Modules/_xxsubinterpretersmodule.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/Modules/_xxsubinterpretersmodule.c b/Modules/_xxsubinterpretersmodule.c
index ca227b5760880e..0c28946c57ff98 100644
--- a/Modules/_xxsubinterpretersmodule.c
+++ b/Modules/_xxsubinterpretersmodule.c
@@ -2417,7 +2417,8 @@ PyDoc_STRVAR(channel_list_interpreters_doc,
 "channel_list_interpreters(cid, *, send=False, recv=False) -> [id]\n\
 \n\
 Return the list of all interpreter IDs associated with the channel\n\
-XXX in the send direction.");
+Exactly one of 'send' or 'recv' should be True, corresponding to the end of\n\
+the channel to list interpreters for.");
 
 
 static PyObject *

From 2a7b508e5ba107dd0c77e55587e137bf0f9a1e22 Mon Sep 17 00:00:00 2001
From: Lewis Gaul <lewis.gaul@gmail.com>
Date: Thu, 21 Nov 2019 12:12:43 +0000
Subject: [PATCH 10/33] Improve naming in tests

---
 Lib/test/test__xxsubinterpreters.py | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/Lib/test/test__xxsubinterpreters.py b/Lib/test/test__xxsubinterpreters.py
index e5494212d4eada..d346f21aae976a 100644
--- a/Lib/test/test__xxsubinterpreters.py
+++ b/Lib/test/test__xxsubinterpreters.py
@@ -1203,23 +1203,23 @@ def test_ids_global(self):
 
         self.assertEqual(cid2, int(cid1) + 1)
         
-    def _assert_interpreters_returned_are(self, cid, send, recv):
-        ilist = set(interpreters.channel_list_interpreters(cid, send=True))
-        self.assertEqual(ilist, set(send))
-        ilist = set(interpreters.channel_list_interpreters(cid, recv=True))
-        self.assertEqual(ilist, set(recv))
+    def _assert_interpreters_returned(self, cid, send, recv):
+        actual = set(interpreters.channel_list_interpreters(cid, send=True))
+        self.assertEqual(actual, set(send))
+        actual = set(interpreters.channel_list_interpreters(cid, recv=True))
+        self.assertEqual(actual, set(recv))
         
     def test_channel_list_interpreters_empty(self):
         # Test for channel with no associated interpreters.
         cid = interpreters.channel_create()
-        self._assert_interpreters_returned_are(cid, [], [])
+        self._assert_interpreters_returned(cid, [], [])
         
     def test_channel_list_interpreters_mainline(self):
         interp0 = interpreters.get_main()
         cid = interpreters.channel_create()
         interpreters.channel_send(cid, "send")
         # Test for a channel that has one end associated to an interpreter.
-        self._assert_interpreters_returned_are(
+        self._assert_interpreters_returned(
             cid, [interp0], [])
         
         interp1 = interpreters.create()
@@ -1228,7 +1228,7 @@ def test_channel_list_interpreters_mainline(self):
             obj = _interpreters.channel_recv({cid})
             """))
         # Test for channel that has boths ends associated to an interpreter.
-        self._assert_interpreters_returned_are(
+        self._assert_interpreters_returned(
             cid, [interp0], [interp1])
         
     def test_channel_list_interpreters_multiple(self):
@@ -1252,7 +1252,7 @@ def test_channel_list_interpreters_multiple(self):
             import _interpreters
             obj = _interpreters.channel_recv({cid})
             """))
-        self._assert_interpreters_returned_are(
+        self._assert_interpreters_returned(
             cid, [interp0, interp1], [interp3, interp2])
 
     ####################

From 4d6ac6ed5362d0fbb61d355e5e2aefd67e5e6048 Mon Sep 17 00:00:00 2001
From: Lewis Gaul <lewis.gaul@gmail.com>
Date: Thu, 21 Nov 2019 13:36:30 +0000
Subject: [PATCH 11/33] Add macros for int64_t max etc

---
 Include/pyport.h                   | 24 ++++++++++++++++++++++++
 Modules/_xxsubinterpretersmodule.c | 11 ++++++++---
 2 files changed, 32 insertions(+), 3 deletions(-)

diff --git a/Include/pyport.h b/Include/pyport.h
index 64c73f012e7a71..14b3a2cd335615 100644
--- a/Include/pyport.h
+++ b/Include/pyport.h
@@ -72,10 +72,34 @@ Used in:  Py_SAFE_DOWNCAST
 
 #define PY_UINT32_T uint32_t
 #define PY_UINT64_T uint64_t
+#ifdef HAVE_STDINT_H
+#include <stdint.h>
+#define PY_UINT32_T_MIN UINT32_MIN
+#define PY_UINT32_T_MAX UINT32_MAX
+#define PY_UINT64_T_MIN UINT64_MIN
+#define PY_UINT64_T_MAX UINT64_MAX
+#else
+#define PY_UINT32_T_MIN 0
+#define PY_UINT32_T_MAX ((uint32_t)-1)
+#define PY_UINT64_T_MIN 0
+#define PY_UINT64_T_MAX ((uint64_t)-1)
+#endif
 
 /* Signed variants of the above */
 #define PY_INT32_T int32_t
 #define PY_INT64_T int64_t
+#ifdef HAVE_STDINT_H
+#include <stdint.h>
+#define PY_INT32_T_MIN INT32_MIN
+#define PY_INT32_T_MAX INT32_MAX
+#define PY_INT64_T_MIN INT64_MIN
+#define PY_INT64_T_MAX INT64_MAX
+#else
+#define PY_INT32_T_MAX ((int32_t)(((uint32_t)-1)>>1))
+#define PY_INT32_T_MIN (-PY_INT32_T_MAX-1)
+#define PY_INT64_T_MAX ((int64_t)(((uint64_t)-1)>>1))
+#define PY_INT64_T_MIN (-PY_INT64_T_MAX-1)
+#endif
 
 /* If PYLONG_BITS_IN_DIGIT is not defined then we'll use 30-bit digits if all
    the necessary integer types are available, and we're on a 64-bit platform
diff --git a/Modules/_xxsubinterpretersmodule.c b/Modules/_xxsubinterpretersmodule.c
index 0c28946c57ff98..0333f41d2f8ed7 100644
--- a/Modules/_xxsubinterpretersmodule.c
+++ b/Modules/_xxsubinterpretersmodule.c
@@ -631,8 +631,13 @@ static int64_t *
 _channelends_list_interpreters(_channelends *ends, int64_t *count, int send)
 {
     int64_t *ids = NULL;
-    int64_t numopen = send ? ends->numsendopen : ends->numrecvopen;
-    if (numopen >= PY_SSIZE_T_MAX) {
+    int64_t numopen;
+
+    assert(ends != NULL);
+    assert(count != NULL);
+    numopen = send ? ends->numsendopen : ends->numrecvopen;
+
+    if (numopen >= PY_INT64_T_MAX) {
         PyErr_SetString(PyExc_RuntimeError,
                         "too many interpreters using the channel");
         goto done;
@@ -1202,7 +1207,7 @@ _channels_list_all(_channels *channels, int64_t *count)
     int64_t *cids = NULL;
     PyThread_acquire_lock(channels->mutex, WAIT_LOCK);
     int64_t numopen = channels->numopen;
-    if (numopen >= PY_SSIZE_T_MAX) {
+    if (numopen >= PY_INT64_T_MAX) {
         PyErr_SetString(PyExc_RuntimeError, "too many channels open");
         goto done;
     }

From a44c2d271d4f526da0b4fd8c2e9611b5a9bc3218 Mon Sep 17 00:00:00 2001
From: Lewis Gaul <lewis.gaul@gmail.com>
Date: Thu, 21 Nov 2019 14:03:56 +0000
Subject: [PATCH 12/33] Fix error handling

---
 Modules/_xxsubinterpretersmodule.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/Modules/_xxsubinterpretersmodule.c b/Modules/_xxsubinterpretersmodule.c
index 0333f41d2f8ed7..320f9149d380a4 100644
--- a/Modules/_xxsubinterpretersmodule.c
+++ b/Modules/_xxsubinterpretersmodule.c
@@ -644,6 +644,7 @@ _channelends_list_interpreters(_channelends *ends, int64_t *count, int send)
     }
     ids = PyMem_NEW(int64_t, (Py_ssize_t)numopen);
     if (ids == NULL) {
+        PyErr_SetNone(PyExc_MemoryError);
         goto done;
     }
     _channelend *ref = send ? ends->send : ends->recv;
@@ -2383,6 +2384,9 @@ channel_list_interpreters(PyObject *self, PyObject *args, PyObject *kwds)
     }
 
     ids = _channelends_list_interpreters(chan->ends, &count, send);
+    if (ids == NULL) {
+        goto except;
+    }
     
     ret = PyList_New((Py_ssize_t)count);
     if (ret == NULL) {

From 215300b3161f9dcfaf331dfc5e49b0201770f35b Mon Sep 17 00:00:00 2001
From: Lewis Gaul <lewis.gaul@gmail.com>
Date: Thu, 21 Nov 2019 14:37:16 +0000
Subject: [PATCH 13/33] Expose _PyInterpreterState_LookUpID

---
 Include/cpython/pystate.h          |  2 ++
 Modules/_xxsubinterpretersmodule.c | 25 +++++++++++--------------
 2 files changed, 13 insertions(+), 14 deletions(-)

diff --git a/Include/cpython/pystate.h b/Include/cpython/pystate.h
index 6c8d2ae041ea54..9d647e957b5616 100644
--- a/Include/cpython/pystate.h
+++ b/Include/cpython/pystate.h
@@ -147,6 +147,8 @@ struct _ts {
    The caller must hold the GIL.*/
 PyAPI_FUNC(PyInterpreterState *) _PyInterpreterState_Get(void);
 
+PyAPI_FUNC(PyInterpreterState *) _PyInterpreterState_LookUpID(PY_INT64_T requested_id);
+
 PyAPI_FUNC(int) _PyState_AddModule(PyObject*, struct PyModuleDef*);
 PyAPI_FUNC(PyThreadState *) _PyThreadState_Prealloc(PyInterpreterState *);
 
diff --git a/Modules/_xxsubinterpretersmodule.c b/Modules/_xxsubinterpretersmodule.c
index 320f9149d380a4..21005448a6fb23 100644
--- a/Modules/_xxsubinterpretersmodule.c
+++ b/Modules/_xxsubinterpretersmodule.c
@@ -2394,20 +2394,17 @@ channel_list_interpreters(PyObject *self, PyObject *args, PyObject *kwds)
     }
 
     for (int64_t i=0; i < count; i++) {
-        PyInterpreterState *interp = PyInterpreterState_Head();
-        while (interp != NULL) {
-            PyObject *id_obj = _PyInterpreterState_GetIDObject(interp);
-            if (id_obj == NULL) {
-                Py_DECREF(id_obj);
-                goto except;
-            }
-            if (ids[i] == PyInterpreterState_GetID(interp)) {
-                PyList_SET_ITEM(ret, i, id_obj);
-                break;
-            } else {
-                Py_DECREF(id_obj);
-            }
-            interp = PyInterpreterState_Next(interp);
+        PyInterpreterState *interp = _PyInterpreterState_LookUpID(ids[i]);
+        PyObject *id_obj = _PyInterpreterState_GetIDObject(interp);
+        if (id_obj == NULL) {
+            Py_DECREF(id_obj);
+            goto except;
+        }
+        if (ids[i] == PyInterpreterState_GetID(interp)) {
+            PyList_SET_ITEM(ret, i, id_obj);
+            break;
+        } else {
+            Py_DECREF(id_obj);
         }
     }
 

From 9b124db988f86ed8c34777bec7d7179ca76a51f6 Mon Sep 17 00:00:00 2001
From: Lewis Gaul <lewis.gaul@gmail.com>
Date: Thu, 21 Nov 2019 15:29:01 +0000
Subject: [PATCH 14/33] Fix bad git rebase (use _xxsubinterpreters module name)

---
 Lib/test/test__xxsubinterpreters.py | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/Lib/test/test__xxsubinterpreters.py b/Lib/test/test__xxsubinterpreters.py
index d346f21aae976a..f57f565d60f7b1 100644
--- a/Lib/test/test__xxsubinterpreters.py
+++ b/Lib/test/test__xxsubinterpreters.py
@@ -1224,7 +1224,7 @@ def test_channel_list_interpreters_mainline(self):
         
         interp1 = interpreters.create()
         _run_output(interp1, dedent(f"""
-            import _interpreters
+            import _xxsubinterpreters as _interpreters
             obj = _interpreters.channel_recv({cid})
             """))
         # Test for channel that has boths ends associated to an interpreter.
@@ -1241,15 +1241,15 @@ def test_channel_list_interpreters_multiple(self):
         
         interpreters.channel_send(cid, "send")
         _run_output(interp1, dedent(f"""
-            import _interpreters
+            import _xxsubinterpreters as _interpreters
             obj = _interpreters.channel_send({cid}, "send")
             """))
         _run_output(interp2, dedent(f"""
-            import _interpreters
+            import _xxsubinterpreters as _interpreters
             obj = _interpreters.channel_recv({cid})
             """))
         _run_output(interp3, dedent(f"""
-            import _interpreters
+            import _xxsubinterpreters as _interpreters
             obj = _interpreters.channel_recv({cid})
             """))
         self._assert_interpreters_returned(

From 76621c714256bf2bfd4728a27a9b3335d95ee563 Mon Sep 17 00:00:00 2001
From: Lewis Gaul <lewis.gaul@gmail.com>
Date: Thu, 21 Nov 2019 15:36:05 +0000
Subject: [PATCH 15/33] Remove decref of NULL

---
 Modules/_xxsubinterpretersmodule.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/Modules/_xxsubinterpretersmodule.c b/Modules/_xxsubinterpretersmodule.c
index 21005448a6fb23..dc58621cca2460 100644
--- a/Modules/_xxsubinterpretersmodule.c
+++ b/Modules/_xxsubinterpretersmodule.c
@@ -2397,7 +2397,6 @@ channel_list_interpreters(PyObject *self, PyObject *args, PyObject *kwds)
         PyInterpreterState *interp = _PyInterpreterState_LookUpID(ids[i]);
         PyObject *id_obj = _PyInterpreterState_GetIDObject(interp);
         if (id_obj == NULL) {
-            Py_DECREF(id_obj);
             goto except;
         }
         if (ids[i] == PyInterpreterState_GetID(interp)) {

From d4c51bfbd3b8599e36cce32c4abe7ba48ea2873f Mon Sep 17 00:00:00 2001
From: Lewis Gaul <lewis.gaul@gmail.com>
Date: Thu, 21 Nov 2019 15:44:41 +0000
Subject: [PATCH 16/33] Fix leftover broken code from nested loop

---
 Modules/_xxsubinterpretersmodule.c | 7 +------
 1 file changed, 1 insertion(+), 6 deletions(-)

diff --git a/Modules/_xxsubinterpretersmodule.c b/Modules/_xxsubinterpretersmodule.c
index dc58621cca2460..949b1ceb29f969 100644
--- a/Modules/_xxsubinterpretersmodule.c
+++ b/Modules/_xxsubinterpretersmodule.c
@@ -2399,12 +2399,7 @@ channel_list_interpreters(PyObject *self, PyObject *args, PyObject *kwds)
         if (id_obj == NULL) {
             goto except;
         }
-        if (ids[i] == PyInterpreterState_GetID(interp)) {
-            PyList_SET_ITEM(ret, i, id_obj);
-            break;
-        } else {
-            Py_DECREF(id_obj);
-        }
+        PyList_SET_ITEM(ret, i, id_obj);
     }
 
     goto finally;

From f3c2b34bc718f642c86a6391f1d465031ccc601f Mon Sep 17 00:00:00 2001
From: Lewis Gaul <lewis.gaul@gmail.com>
Date: Thu, 21 Nov 2019 18:15:39 +0000
Subject: [PATCH 17/33] Remove whitespace from tests

---
 Lib/test/test__xxsubinterpreters.py | 28 ++++++++++++++--------------
 1 file changed, 14 insertions(+), 14 deletions(-)

diff --git a/Lib/test/test__xxsubinterpreters.py b/Lib/test/test__xxsubinterpreters.py
index f57f565d60f7b1..de69d19970f1af 100644
--- a/Lib/test/test__xxsubinterpreters.py
+++ b/Lib/test/test__xxsubinterpreters.py
@@ -1202,18 +1202,18 @@ def test_ids_global(self):
         cid2 = int(out.strip())
 
         self.assertEqual(cid2, int(cid1) + 1)
-        
+
     def _assert_interpreters_returned(self, cid, send, recv):
         actual = set(interpreters.channel_list_interpreters(cid, send=True))
         self.assertEqual(actual, set(send))
         actual = set(interpreters.channel_list_interpreters(cid, recv=True))
         self.assertEqual(actual, set(recv))
-        
+
     def test_channel_list_interpreters_empty(self):
         # Test for channel with no associated interpreters.
         cid = interpreters.channel_create()
         self._assert_interpreters_returned(cid, [], [])
-        
+
     def test_channel_list_interpreters_mainline(self):
         interp0 = interpreters.get_main()
         cid = interpreters.channel_create()
@@ -1221,7 +1221,7 @@ def test_channel_list_interpreters_mainline(self):
         # Test for a channel that has one end associated to an interpreter.
         self._assert_interpreters_returned(
             cid, [interp0], [])
-        
+
         interp1 = interpreters.create()
         _run_output(interp1, dedent(f"""
             import _xxsubinterpreters as _interpreters
@@ -1230,7 +1230,7 @@ def test_channel_list_interpreters_mainline(self):
         # Test for channel that has boths ends associated to an interpreter.
         self._assert_interpreters_returned(
             cid, [interp0], [interp1])
-        
+
     def test_channel_list_interpreters_multiple(self):
         # Test for channel with both ends associated to many interpreters.
         interp0 = interpreters.get_main()
@@ -1238,7 +1238,7 @@ def test_channel_list_interpreters_multiple(self):
         interp2 = interpreters.create()
         interp3 = interpreters.create()
         cid = interpreters.channel_create()
-        
+
         interpreters.channel_send(cid, "send")
         _run_output(interp1, dedent(f"""
             import _xxsubinterpreters as _interpreters
@@ -1566,28 +1566,28 @@ def test_close_used_multiple_times_by_single_user(self):
             interpreters.channel_send(cid, b'eggs')
         with self.assertRaises(interpreters.ChannelClosedError):
             interpreters.channel_recv(cid)
-            
+
     def test_channel_list_interpreters_invalid_channel(self):
-        cid = interpreters.channel_create()    
-        # Test for invalid channel ID.    
+        cid = interpreters.channel_create()
+        # Test for invalid channel ID.
         with self.assertRaises(interpreters.ChannelNotFoundError):
             interpreters.channel_list_interpreters(1000, send=True)
-            
+
         interpreters.channel_close(cid)
         # Test for a channel that has been closed.
         with self.assertRaises(interpreters.ChannelClosedError):
             interpreters.channel_list_interpreters(cid, send=True)
-            
+
     def test_channel_list_interpreters_invalid_args(self):
         # Tests for invalid arguments passed to the API.
         cid = interpreters.channel_create()
-        
+
         with self.assertRaises(ValueError):
             interpreters.channel_list_interpreters(cid)
-            
+
         with self.assertRaises(ValueError):
             interpreters.channel_list_interpreters(cid, send=True, recv=True)
-        
+
 
 class ChannelReleaseTests(TestBase):
 

From af10d1f3bf9c6317d8eb0546f230867ad8495e24 Mon Sep 17 00:00:00 2001
From: Lewis Gaul <lewis.gaul@gmail.com>
Date: Thu, 21 Nov 2019 18:19:00 +0000
Subject: [PATCH 18/33] Add to Misc/ACKS

---
 Misc/ACKS | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/Misc/ACKS b/Misc/ACKS
index 13c6676bace1fb..399a6724cd4990 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -444,6 +444,7 @@ Rodolpho Eckhardt
 Ulrich Eckhardt
 David Edelsohn
 John Edmonds
+Benjamin Edwards
 Grant Edwards
 Zvi Effron
 John Ehresman
@@ -557,6 +558,7 @@ Lars Marius Garshol
 Jake Garver
 Dan Gass
 Andrew Gaul
+Lewis Gaul
 Matthieu Gautier
 Stephen M. Gava
 Xavier de Gaye

From ac09b6cdeb0969e231f6e481bd7751894a1b28fc Mon Sep 17 00:00:00 2001
From: Lewis Gaul <lewis.gaul@gmail.com>
Date: Fri, 22 Nov 2019 14:09:16 +0000
Subject: [PATCH 19/33] Remove PY_INT64_T_MAX etc. and use stdint.h

---
 Include/pyport.h                   | 24 ------------------------
 Modules/_xxsubinterpretersmodule.c |  4 ++--
 2 files changed, 2 insertions(+), 26 deletions(-)

diff --git a/Include/pyport.h b/Include/pyport.h
index 14b3a2cd335615..64c73f012e7a71 100644
--- a/Include/pyport.h
+++ b/Include/pyport.h
@@ -72,34 +72,10 @@ Used in:  Py_SAFE_DOWNCAST
 
 #define PY_UINT32_T uint32_t
 #define PY_UINT64_T uint64_t
-#ifdef HAVE_STDINT_H
-#include <stdint.h>
-#define PY_UINT32_T_MIN UINT32_MIN
-#define PY_UINT32_T_MAX UINT32_MAX
-#define PY_UINT64_T_MIN UINT64_MIN
-#define PY_UINT64_T_MAX UINT64_MAX
-#else
-#define PY_UINT32_T_MIN 0
-#define PY_UINT32_T_MAX ((uint32_t)-1)
-#define PY_UINT64_T_MIN 0
-#define PY_UINT64_T_MAX ((uint64_t)-1)
-#endif
 
 /* Signed variants of the above */
 #define PY_INT32_T int32_t
 #define PY_INT64_T int64_t
-#ifdef HAVE_STDINT_H
-#include <stdint.h>
-#define PY_INT32_T_MIN INT32_MIN
-#define PY_INT32_T_MAX INT32_MAX
-#define PY_INT64_T_MIN INT64_MIN
-#define PY_INT64_T_MAX INT64_MAX
-#else
-#define PY_INT32_T_MAX ((int32_t)(((uint32_t)-1)>>1))
-#define PY_INT32_T_MIN (-PY_INT32_T_MAX-1)
-#define PY_INT64_T_MAX ((int64_t)(((uint64_t)-1)>>1))
-#define PY_INT64_T_MIN (-PY_INT64_T_MAX-1)
-#endif
 
 /* If PYLONG_BITS_IN_DIGIT is not defined then we'll use 30-bit digits if all
    the necessary integer types are available, and we're on a 64-bit platform
diff --git a/Modules/_xxsubinterpretersmodule.c b/Modules/_xxsubinterpretersmodule.c
index 949b1ceb29f969..9713d39742457b 100644
--- a/Modules/_xxsubinterpretersmodule.c
+++ b/Modules/_xxsubinterpretersmodule.c
@@ -637,7 +637,7 @@ _channelends_list_interpreters(_channelends *ends, int64_t *count, int send)
     assert(count != NULL);
     numopen = send ? ends->numsendopen : ends->numrecvopen;
 
-    if (numopen >= PY_INT64_T_MAX) {
+    if (numopen >= INT64_MAX) {
         PyErr_SetString(PyExc_RuntimeError,
                         "too many interpreters using the channel");
         goto done;
@@ -1208,7 +1208,7 @@ _channels_list_all(_channels *channels, int64_t *count)
     int64_t *cids = NULL;
     PyThread_acquire_lock(channels->mutex, WAIT_LOCK);
     int64_t numopen = channels->numopen;
-    if (numopen >= PY_INT64_T_MAX) {
+    if (numopen >= INT64_MAX) {
         PyErr_SetString(PyExc_RuntimeError, "too many channels open");
         goto done;
     }

From 8e698649558a862bbe423ca44db7983a32325ff4 Mon Sep 17 00:00:00 2001
From: Lewis Gaul <lewis.gaul@gmail.com>
Date: Fri, 22 Nov 2019 14:12:15 +0000
Subject: [PATCH 20/33] Remove whitespace

---
 Modules/_xxsubinterpretersmodule.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Modules/_xxsubinterpretersmodule.c b/Modules/_xxsubinterpretersmodule.c
index 9713d39742457b..f2991c494123f8 100644
--- a/Modules/_xxsubinterpretersmodule.c
+++ b/Modules/_xxsubinterpretersmodule.c
@@ -2387,7 +2387,7 @@ channel_list_interpreters(PyObject *self, PyObject *args, PyObject *kwds)
     if (ids == NULL) {
         goto except;
     }
-    
+
     ret = PyList_New((Py_ssize_t)count);
     if (ret == NULL) {
         goto except;

From 0546702c41a292565c3cca261945e8f11931ed5d Mon Sep 17 00:00:00 2001
From: "blurb-it[bot]" <43283697+blurb-it[bot]@users.noreply.github.com>
Date: Fri, 22 Nov 2019 14:34:48 +0000
Subject: [PATCH 21/33] =?UTF-8?q?=F0=9F=93=9C=F0=9F=A4=96=20Added=20by=20b?=
 =?UTF-8?q?lurb=5Fit.?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

---
 .../Core and Builtins/2019-11-22-14-34-47.bpo-38880.evcCPa.rst   | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 Misc/NEWS.d/next/Core and Builtins/2019-11-22-14-34-47.bpo-38880.evcCPa.rst

diff --git a/Misc/NEWS.d/next/Core and Builtins/2019-11-22-14-34-47.bpo-38880.evcCPa.rst b/Misc/NEWS.d/next/Core and Builtins/2019-11-22-14-34-47.bpo-38880.evcCPa.rst
new file mode 100644
index 00000000000000..6b3a356c8cfcf5
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2019-11-22-14-34-47.bpo-38880.evcCPa.rst	
@@ -0,0 +1 @@
+Added the ability to list interpreters associated with channel ends, as required for PEP 554.
\ No newline at end of file

From ad511ef6cf1f0882448407f8639cc05796fe43b3 Mon Sep 17 00:00:00 2001
From: Lewis G <Lewis@scphillips.com>
Date: Sat, 23 Nov 2019 22:18:53 +0000
Subject: [PATCH 22/33] Update news entry to remove mention of PEP 554

---
 .../Core and Builtins/2019-11-22-14-34-47.bpo-38880.evcCPa.rst  | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Misc/NEWS.d/next/Core and Builtins/2019-11-22-14-34-47.bpo-38880.evcCPa.rst b/Misc/NEWS.d/next/Core and Builtins/2019-11-22-14-34-47.bpo-38880.evcCPa.rst
index 6b3a356c8cfcf5..07a7f5ec22aa10 100644
--- a/Misc/NEWS.d/next/Core and Builtins/2019-11-22-14-34-47.bpo-38880.evcCPa.rst	
+++ b/Misc/NEWS.d/next/Core and Builtins/2019-11-22-14-34-47.bpo-38880.evcCPa.rst	
@@ -1 +1 @@
-Added the ability to list interpreters associated with channel ends, as required for PEP 554.
\ No newline at end of file
+Added the ability to list interpreters associated with channel ends in the internal subinterpreters module.

From a1b7c3a681146af342595d1ebfb81a69cebefbe0 Mon Sep 17 00:00:00 2001
From: Lewis Gaul <lewis.gaul@gmail.com>
Date: Sat, 23 Nov 2019 22:58:29 +0000
Subject: [PATCH 23/33] Remove unnecessary asserts

---
 Modules/_xxsubinterpretersmodule.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/Modules/_xxsubinterpretersmodule.c b/Modules/_xxsubinterpretersmodule.c
index f2991c494123f8..eab5e53a52420f 100644
--- a/Modules/_xxsubinterpretersmodule.c
+++ b/Modules/_xxsubinterpretersmodule.c
@@ -633,8 +633,6 @@ _channelends_list_interpreters(_channelends *ends, int64_t *count, int send)
     int64_t *ids = NULL;
     int64_t numopen;
 
-    assert(ends != NULL);
-    assert(count != NULL);
     numopen = send ? ends->numsendopen : ends->numrecvopen;
 
     if (numopen >= INT64_MAX) {

From d9b3e277d406509527e31f308dc04692ec675584 Mon Sep 17 00:00:00 2001
From: Lewis Gaul <lewis.gaul@gmail.com>
Date: Sat, 23 Nov 2019 23:01:09 +0000
Subject: [PATCH 24/33] Use single 'send' argument in
 channel_list_interpreters() API

---
 Lib/test/test__xxsubinterpreters.py |  8 ++------
 Modules/_xxsubinterpretersmodule.c  | 24 +++++++++---------------
 2 files changed, 11 insertions(+), 21 deletions(-)

diff --git a/Lib/test/test__xxsubinterpreters.py b/Lib/test/test__xxsubinterpreters.py
index de69d19970f1af..b4b4f7bc141155 100644
--- a/Lib/test/test__xxsubinterpreters.py
+++ b/Lib/test/test__xxsubinterpreters.py
@@ -1206,7 +1206,7 @@ def test_ids_global(self):
     def _assert_interpreters_returned(self, cid, send, recv):
         actual = set(interpreters.channel_list_interpreters(cid, send=True))
         self.assertEqual(actual, set(send))
-        actual = set(interpreters.channel_list_interpreters(cid, recv=True))
+        actual = set(interpreters.channel_list_interpreters(cid, send=False))
         self.assertEqual(actual, set(recv))
 
     def test_channel_list_interpreters_empty(self):
@@ -1581,13 +1581,9 @@ def test_channel_list_interpreters_invalid_channel(self):
     def test_channel_list_interpreters_invalid_args(self):
         # Tests for invalid arguments passed to the API.
         cid = interpreters.channel_create()
-
-        with self.assertRaises(ValueError):
+        with self.assertRaises(TypeError):
             interpreters.channel_list_interpreters(cid)
 
-        with self.assertRaises(ValueError):
-            interpreters.channel_list_interpreters(cid, send=True, recv=True)
-
 
 class ChannelReleaseTests(TestBase):
 
diff --git a/Modules/_xxsubinterpretersmodule.c b/Modules/_xxsubinterpretersmodule.c
index eab5e53a52420f..7b89113bda997e 100644
--- a/Modules/_xxsubinterpretersmodule.c
+++ b/Modules/_xxsubinterpretersmodule.c
@@ -2355,27 +2355,20 @@ Return the list of all IDs for active channels.");
 static PyObject *
 channel_list_interpreters(PyObject *self, PyObject *args, PyObject *kwds)
 {
-    static char *kwlist[] = {"cid", "send", "recv", NULL};
+    static char *kwlist[] = {"cid", "send", NULL};
     int64_t cid;
     _PyChannelState *chan;
-    int send = 0;           /* Send end? */
-    int recv = 0;           /* Receive end? */
+    int send = 0;           /* Send or receive end? */
     int64_t *ids = NULL;    /* Array of interpreter IDs */
     int64_t count = 0;      /* Number of interpreters to return */
     PyObject *ret = NULL;   /* Python list of interpreter IDs */
 
     if (!PyArg_ParseTupleAndKeywords(
-            args, kwds, "O&|$pp:channel_list_interpreters",
-            kwlist, channel_id_converter, &cid, &send, &recv)) {
+            args, kwds, "O&$p:channel_list_interpreters",
+            kwlist, channel_id_converter, &cid, &send)) {
         return NULL;
     }
 
-    if ((send && recv) || (!send && !recv)) {
-        PyErr_SetString(PyExc_ValueError,
-                        "Specify exactly one of send or recv");
-        goto except;
-    }
-
     chan = _channels_lookup(&_globals.channels, cid, NULL);
     if (chan == NULL) {
         goto except;
@@ -2412,11 +2405,12 @@ channel_list_interpreters(PyObject *self, PyObject *args, PyObject *kwds)
 }
 
 PyDoc_STRVAR(channel_list_interpreters_doc,
-"channel_list_interpreters(cid, *, send=False, recv=False) -> [id]\n\
+"channel_list_interpreters(cid, *, send) -> [id]\n\
+\n\
+Return the list of all interpreter IDs associated with an end of the channel.\n\
 \n\
-Return the list of all interpreter IDs associated with the channel\n\
-Exactly one of 'send' or 'recv' should be True, corresponding to the end of\n\
-the channel to list interpreters for.");
+The 'send' argument should be a boolean indicating whether to use the send or\n\
+receive end.");
 
 
 static PyObject *

From f4990b01a541a222a7d8d5dc32cd34240d16e9af Mon Sep 17 00:00:00 2001
From: Lewis Gaul <lewis.gaul@gmail.com>
Date: Sat, 23 Nov 2019 23:07:21 +0000
Subject: [PATCH 25/33] Tidy up _channelends_list_interpreters() function

---
 Modules/_xxsubinterpretersmodule.c | 19 ++++++-------------
 1 file changed, 6 insertions(+), 13 deletions(-)

diff --git a/Modules/_xxsubinterpretersmodule.c b/Modules/_xxsubinterpretersmodule.c
index 7b89113bda997e..8d6a45fe5d13f3 100644
--- a/Modules/_xxsubinterpretersmodule.c
+++ b/Modules/_xxsubinterpretersmodule.c
@@ -630,28 +630,21 @@ _channelends_associate(_channelends *ends, int64_t interp, int send)
 static int64_t *
 _channelends_list_interpreters(_channelends *ends, int64_t *count, int send)
 {
-    int64_t *ids = NULL;
-    int64_t numopen;
-
-    numopen = send ? ends->numsendopen : ends->numrecvopen;
+    int64_t numopen = send ? ends->numsendopen : ends->numrecvopen;
 
-    if (numopen >= INT64_MAX) {
-        PyErr_SetString(PyExc_RuntimeError,
-                        "too many interpreters using the channel");
-        goto done;
-    }
-    ids = PyMem_NEW(int64_t, (Py_ssize_t)numopen);
+    int64_t *ids = PyMem_NEW(int64_t, (Py_ssize_t)numopen);
     if (ids == NULL) {
-        PyErr_SetNone(PyExc_MemoryError);
-        goto done;
+        PyErr_NoMemory();
+        return NULL;
     }
+
     _channelend *ref = send ? ends->send : ends->recv;
     for (int64_t i=0; ref != NULL; ref = ref->next, i++) {
         ids[i] = ref->interp;
     }
+
     *count = numopen;
 
-done:
     return ids;
 }
 

From c7dbb04af0fafe3044039bfb73ea47d03ae84e6c Mon Sep 17 00:00:00 2001
From: Lewis Gaul <lewis.gaul@gmail.com>
Date: Sat, 23 Nov 2019 23:30:05 +0000
Subject: [PATCH 26/33] Move variable declarations inline

---
 Modules/_xxsubinterpretersmodule.c | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/Modules/_xxsubinterpretersmodule.c b/Modules/_xxsubinterpretersmodule.c
index 8d6a45fe5d13f3..739fd49eb99c7e 100644
--- a/Modules/_xxsubinterpretersmodule.c
+++ b/Modules/_xxsubinterpretersmodule.c
@@ -2349,12 +2349,9 @@ static PyObject *
 channel_list_interpreters(PyObject *self, PyObject *args, PyObject *kwds)
 {
     static char *kwlist[] = {"cid", "send", NULL};
-    int64_t cid;
-    _PyChannelState *chan;
+    int64_t cid;            /* Channel ID */
     int send = 0;           /* Send or receive end? */
-    int64_t *ids = NULL;    /* Array of interpreter IDs */
-    int64_t count = 0;      /* Number of interpreters to return */
-    PyObject *ret = NULL;   /* Python list of interpreter IDs */
+    PyObject *ret = NULL;
 
     if (!PyArg_ParseTupleAndKeywords(
             args, kwds, "O&$p:channel_list_interpreters",
@@ -2362,12 +2359,13 @@ channel_list_interpreters(PyObject *self, PyObject *args, PyObject *kwds)
         return NULL;
     }
 
-    chan = _channels_lookup(&_globals.channels, cid, NULL);
+    _PyChannelState *chan = _channels_lookup(&_globals.channels, cid, NULL);
     if (chan == NULL) {
-        goto except;
+        return NULL;
     }
 
-    ids = _channelends_list_interpreters(chan->ends, &count, send);
+    int64_t count = 0;  /* Number of interpreters */
+    int64_t *ids = _channelends_list_interpreters(chan->ends, &count, send);
     if (ids == NULL) {
         goto except;
     }

From 6202ecd358124f46ed6fad68c778b1cf6dfd1b49 Mon Sep 17 00:00:00 2001
From: Lewis Gaul <lewis.gaul@gmail.com>
Date: Sat, 23 Nov 2019 23:40:24 +0000
Subject: [PATCH 27/33] Use _PyInterpreterID_New() instead of getting existing
 objects

---
 Include/cpython/pystate.h          | 2 --
 Modules/_xxsubinterpretersmodule.c | 3 +--
 2 files changed, 1 insertion(+), 4 deletions(-)

diff --git a/Include/cpython/pystate.h b/Include/cpython/pystate.h
index 9d647e957b5616..6c8d2ae041ea54 100644
--- a/Include/cpython/pystate.h
+++ b/Include/cpython/pystate.h
@@ -147,8 +147,6 @@ struct _ts {
    The caller must hold the GIL.*/
 PyAPI_FUNC(PyInterpreterState *) _PyInterpreterState_Get(void);
 
-PyAPI_FUNC(PyInterpreterState *) _PyInterpreterState_LookUpID(PY_INT64_T requested_id);
-
 PyAPI_FUNC(int) _PyState_AddModule(PyObject*, struct PyModuleDef*);
 PyAPI_FUNC(PyThreadState *) _PyThreadState_Prealloc(PyInterpreterState *);
 
diff --git a/Modules/_xxsubinterpretersmodule.c b/Modules/_xxsubinterpretersmodule.c
index 739fd49eb99c7e..4457f07c64b1ae 100644
--- a/Modules/_xxsubinterpretersmodule.c
+++ b/Modules/_xxsubinterpretersmodule.c
@@ -2376,8 +2376,7 @@ channel_list_interpreters(PyObject *self, PyObject *args, PyObject *kwds)
     }
 
     for (int64_t i=0; i < count; i++) {
-        PyInterpreterState *interp = _PyInterpreterState_LookUpID(ids[i]);
-        PyObject *id_obj = _PyInterpreterState_GetIDObject(interp);
+        PyObject *id_obj = _PyInterpreterID_New(ids[i]);
         if (id_obj == NULL) {
             goto except;
         }

From 357101fd1551eb367d7df201d0c1ad6e3a217269 Mon Sep 17 00:00:00 2001
From: Lewis Gaul <lewis.gaul@gmail.com>
Date: Sat, 14 Dec 2019 17:54:53 +0000
Subject: [PATCH 28/33] Markups - remove check for number of open channels and
 improve test structure

---
 Lib/test/test__xxsubinterpreters.py | 37 ++++++++++++++++-------------
 Modules/_xxsubinterpretersmodule.c  |  5 ----
 2 files changed, 21 insertions(+), 21 deletions(-)

diff --git a/Lib/test/test__xxsubinterpreters.py b/Lib/test/test__xxsubinterpreters.py
index b4b4f7bc141155..2b27aafb3a81bf 100644
--- a/Lib/test/test__xxsubinterpreters.py
+++ b/Lib/test/test__xxsubinterpreters.py
@@ -1203,24 +1203,25 @@ def test_ids_global(self):
 
         self.assertEqual(cid2, int(cid1) + 1)
 
-    def _assert_interpreters_returned(self, cid, send, recv):
-        actual = set(interpreters.channel_list_interpreters(cid, send=True))
-        self.assertEqual(actual, set(send))
-        actual = set(interpreters.channel_list_interpreters(cid, send=False))
-        self.assertEqual(actual, set(recv))
-
-    def test_channel_list_interpreters_empty(self):
+    def test_channel_list_interpreters_none(self):
+        """Test listing interpreters for a channel with no associations."""
         # Test for channel with no associated interpreters.
         cid = interpreters.channel_create()
-        self._assert_interpreters_returned(cid, [], [])
+        send_interps = interpreters.channel_list_interpreters(cid, send=True)
+        recv_interps = interpreters.channel_list_interpreters(cid, send=False)
+        self.assertEqual(send_interps, [])
+        self.assertEqual(recv_interps, [])
 
-    def test_channel_list_interpreters_mainline(self):
+    def test_channel_list_interpreters_basic(self):
+        """Test basic listing channel interpreters."""
         interp0 = interpreters.get_main()
         cid = interpreters.channel_create()
         interpreters.channel_send(cid, "send")
         # Test for a channel that has one end associated to an interpreter.
-        self._assert_interpreters_returned(
-            cid, [interp0], [])
+        send_interps = interpreters.channel_list_interpreters(cid, send=True)
+        recv_interps = interpreters.channel_list_interpreters(cid, send=False)
+        self.assertEqual(send_interps, [interp0])
+        self.assertEqual(recv_interps, [])
 
         interp1 = interpreters.create()
         _run_output(interp1, dedent(f"""
@@ -1228,11 +1229,13 @@ def test_channel_list_interpreters_mainline(self):
             obj = _interpreters.channel_recv({cid})
             """))
         # Test for channel that has boths ends associated to an interpreter.
-        self._assert_interpreters_returned(
-            cid, [interp0], [interp1])
+        send_interps = interpreters.channel_list_interpreters(cid, send=True)
+        recv_interps = interpreters.channel_list_interpreters(cid, send=False)
+        self.assertEqual(send_interps, [interp0])
+        self.assertEqual(recv_interps, [interp1])
 
     def test_channel_list_interpreters_multiple(self):
-        # Test for channel with both ends associated to many interpreters.
+        """Test listing interpreters for a channel with many associations."""
         interp0 = interpreters.get_main()
         interp1 = interpreters.create()
         interp2 = interpreters.create()
@@ -1252,8 +1255,10 @@ def test_channel_list_interpreters_multiple(self):
             import _xxsubinterpreters as _interpreters
             obj = _interpreters.channel_recv({cid})
             """))
-        self._assert_interpreters_returned(
-            cid, [interp0, interp1], [interp3, interp2])
+        send_interps = interpreters.channel_list_interpreters(cid, send=True)
+        recv_interps = interpreters.channel_list_interpreters(cid, send=False)
+        self.assertEqual(set(send_interps), {interp0, interp1})
+        self.assertEqual(set(recv_interps), {interp2, interp3})
 
     ####################
 
diff --git a/Modules/_xxsubinterpretersmodule.c b/Modules/_xxsubinterpretersmodule.c
index 4457f07c64b1ae..95602d8669f076 100644
--- a/Modules/_xxsubinterpretersmodule.c
+++ b/Modules/_xxsubinterpretersmodule.c
@@ -1198,11 +1198,6 @@ _channels_list_all(_channels *channels, int64_t *count)
 {
     int64_t *cids = NULL;
     PyThread_acquire_lock(channels->mutex, WAIT_LOCK);
-    int64_t numopen = channels->numopen;
-    if (numopen >= INT64_MAX) {
-        PyErr_SetString(PyExc_RuntimeError, "too many channels open");
-        goto done;
-    }
     int64_t *ids = PyMem_NEW(int64_t, (Py_ssize_t)(channels->numopen));
     if (ids == NULL) {
         goto done;

From faca1dfba7b1a596a052796fbfc79c6406f3d5f7 Mon Sep 17 00:00:00 2001
From: Lewis Gaul <lewis.gaul@gmail.com>
Date: Tue, 21 Jan 2020 22:26:23 +0000
Subject: [PATCH 29/33] Add more listing subinterpreter tests

---
 Lib/test/test__xxsubinterpreters.py | 97 +++++++++++++++++++++++++++++
 1 file changed, 97 insertions(+)

diff --git a/Lib/test/test__xxsubinterpreters.py b/Lib/test/test__xxsubinterpreters.py
index 2b27aafb3a81bf..3d7e97b0173551 100644
--- a/Lib/test/test__xxsubinterpreters.py
+++ b/Lib/test/test__xxsubinterpreters.py
@@ -1260,6 +1260,103 @@ def test_channel_list_interpreters_multiple(self):
         self.assertEqual(set(send_interps), {interp0, interp1})
         self.assertEqual(set(recv_interps), {interp2, interp3})
 
+    @unittest.skip("Failing due to handling of destroyed interpreters")
+    def test_channel_list_interpreters_destroyed(self):
+        """Test listing channel interpreters with a destroyed interpreter."""
+        interp0 = interpreters.get_main()
+        interp1 = interpreters.create()
+        cid = interpreters.channel_create()
+        interpreters.channel_send(cid, "send")
+        _run_output(interp1, dedent(f"""
+            import _xxsubinterpreters as _interpreters
+            obj = _interpreters.channel_recv({cid})
+            """))
+        # Should be one interpreter associated with each end.
+        send_interps = interpreters.channel_list_interpreters(cid, send=True)
+        recv_interps = interpreters.channel_list_interpreters(cid, send=False)
+        self.assertEqual(send_interps, [interp0])
+        self.assertEqual(recv_interps, [interp1])
+
+        interpreters.destroy(interp1)
+        # Destroyed interpreter should not be listed.
+        send_interps = interpreters.channel_list_interpreters(cid, send=True)
+        recv_interps = interpreters.channel_list_interpreters(cid, send=False)
+        self.assertEqual(send_interps, [interp0])
+        self.assertEqual(recv_interps, [])
+
+    def test_channel_list_interpreters_released(self):
+        """Test listing channel interpreters with a released channel."""
+        # Set up one channel with main interpreter on the send end and two
+        # subinterpreters on the receive end.
+        interp0 = interpreters.get_main()
+        interp1 = interpreters.create()
+        interp2 = interpreters.create()
+        cid = interpreters.channel_create()
+        interpreters.channel_send(cid, "data")
+        _run_output(interp1, dedent(f"""
+            import _xxsubinterpreters as _interpreters
+            obj = _interpreters.channel_recv({cid})
+            """))
+        interpreters.channel_send(cid, "data")
+        _run_output(interp2, dedent(f"""
+            import _xxsubinterpreters as _interpreters
+            obj = _interpreters.channel_recv({cid})
+            """))
+        # Check the setup.
+        send_interps = interpreters.channel_list_interpreters(cid, send=True)
+        recv_interps = interpreters.channel_list_interpreters(cid, send=False)
+        self.assertEqual(len(send_interps), 1)
+        self.assertEqual(len(recv_interps), 2)
+
+        # Release the main interpreter from the send end.
+        interpreters.channel_release(cid, send=True)
+        # Send end should raise an error for the main interpreter.
+        with self.assertRaises(interpreters.ChannelClosedError):
+            send_interps = interpreters.channel_list_interpreters(cid, send=True)
+        recv_interps = interpreters.channel_list_interpreters(cid, send=False)
+        self.assertEqual(len(recv_interps), 2)
+
+        # Release one of the subinterpreters from the receive end.
+        _run_output(interp2, dedent(f"""
+            import _xxsubinterpreters as _interpreters
+            obj = _interpreters.channel_release({cid})
+            """))
+        # Receive end should have the released interpreter removed.
+        send_interps = interpreters.channel_list_interpreters(cid, send=True)
+        recv_interps = interpreters.channel_list_interpreters(cid, send=False)
+        self.assertEqual(len(send_interps), 1)
+        self.assertEqual(recv_interps, [interp1])
+
+    def test_channel_list_interpreters_closed(self):
+        """Test listing channel interpreters with a closed channel."""
+        interp0 = interpreters.get_main()
+        interp1 = interpreters.create()
+        cid = interpreters.channel_create()
+        # Should be no interpreters associated with either end.
+        send_interps = interpreters.channel_list_interpreters(cid, send=True)
+        recv_interps = interpreters.channel_list_interpreters(cid, send=False)
+        self.assertEqual(len(send_interps), 0)
+        self.assertEqual(len(recv_interps), 0)
+
+        # Close the send end of the channel.
+        interpreters.channel_close(cid, send=True)
+        # Send end should raise an error.
+        with self.assertRaises(interpreters.ChannelClosedError):
+            send_interps = interpreters.channel_list_interpreters(cid, send=True)
+        recv_interps = interpreters.channel_list_interpreters(cid, send=False)
+        self.assertEqual(len(recv_interps), 0)
+
+        # Close the receive end of the channel from a subinterpreter.
+        _run_output(interp3, dedent(f"""
+            import _xxsubinterpreters as _interpreters
+            obj = _interpreters.channel_close({cid})
+            """))
+        # Both ends should raise an error.
+        with self.assertRaises(interpreters.ChannelClosedError):
+            send_interps = interpreters.channel_list_interpreters(cid, send=True)
+        with self.assertRaises(interpreters.ChannelClosedError):
+            recv_interps = interpreters.channel_list_interpreters(cid, send=False)
+
     ####################
 
     def test_send_recv_main(self):

From 75e791f66e3c83f53084d0a0d0463b68d9d929f8 Mon Sep 17 00:00:00 2001
From: Lewis Gaul <lewis.gaul@gmail.com>
Date: Sat, 18 Apr 2020 21:37:47 +0100
Subject: [PATCH 30/33] Implementation rewrite upon Eric's suggestion. Just one
 testcase now failing

---
 Lib/test/test__xxsubinterpreters.py |  9 ++--
 Modules/_xxsubinterpretersmodule.c  | 84 ++++++++++++++---------------
 2 files changed, 44 insertions(+), 49 deletions(-)

diff --git a/Lib/test/test__xxsubinterpreters.py b/Lib/test/test__xxsubinterpreters.py
index 3d7e97b0173551..77e9f05296601c 100644
--- a/Lib/test/test__xxsubinterpreters.py
+++ b/Lib/test/test__xxsubinterpreters.py
@@ -1260,7 +1260,6 @@ def test_channel_list_interpreters_multiple(self):
         self.assertEqual(set(send_interps), {interp0, interp1})
         self.assertEqual(set(recv_interps), {interp2, interp3})
 
-    @unittest.skip("Failing due to handling of destroyed interpreters")
     def test_channel_list_interpreters_destroyed(self):
         """Test listing channel interpreters with a destroyed interpreter."""
         interp0 = interpreters.get_main()
@@ -1310,10 +1309,10 @@ def test_channel_list_interpreters_released(self):
 
         # Release the main interpreter from the send end.
         interpreters.channel_release(cid, send=True)
-        # Send end should raise an error for the main interpreter.
-        with self.assertRaises(interpreters.ChannelClosedError):
-            send_interps = interpreters.channel_list_interpreters(cid, send=True)
+        # Send end should have no associated interpreters.
+        send_interps = interpreters.channel_list_interpreters(cid, send=True)
         recv_interps = interpreters.channel_list_interpreters(cid, send=False)
+        self.assertEqual(len(send_interps), 0)
         self.assertEqual(len(recv_interps), 2)
 
         # Release one of the subinterpreters from the receive end.
@@ -1324,7 +1323,7 @@ def test_channel_list_interpreters_released(self):
         # Receive end should have the released interpreter removed.
         send_interps = interpreters.channel_list_interpreters(cid, send=True)
         recv_interps = interpreters.channel_list_interpreters(cid, send=False)
-        self.assertEqual(len(send_interps), 1)
+        self.assertEqual(len(send_interps), 0)
         self.assertEqual(recv_interps, [interp1])
 
     def test_channel_list_interpreters_closed(self):
diff --git a/Modules/_xxsubinterpretersmodule.c b/Modules/_xxsubinterpretersmodule.c
index 95602d8669f076..6e8d8d2b502dc4 100644
--- a/Modules/_xxsubinterpretersmodule.c
+++ b/Modules/_xxsubinterpretersmodule.c
@@ -536,7 +536,7 @@ _channelend_find(_channelend *first, int64_t interp, _channelend **pprev)
 
 typedef struct _channelassociations {
     // Note that the list entries are never removed for interpreter
-    // for which the channel is closed.  This should be a problem in
+    // for which the channel is closed.  This should not be a problem in
     // practice.  Also, a channel isn't automatically closed when an
     // interpreter is destroyed.
     int64_t numsendopen;
@@ -627,27 +627,6 @@ _channelends_associate(_channelends *ends, int64_t interp, int send)
     return 0;
 }
 
-static int64_t *
-_channelends_list_interpreters(_channelends *ends, int64_t *count, int send)
-{
-    int64_t numopen = send ? ends->numsendopen : ends->numrecvopen;
-
-    int64_t *ids = PyMem_NEW(int64_t, (Py_ssize_t)numopen);
-    if (ids == NULL) {
-        PyErr_NoMemory();
-        return NULL;
-    }
-
-    _channelend *ref = send ? ends->send : ends->recv;
-    for (int64_t i=0; ref != NULL; ref = ref->next, i++) {
-        ids[i] = ref->interp;
-    }
-
-    *count = numopen;
-
-    return ids;
-}
-
 static int
 _channelends_is_open(_channelends *ends)
 {
@@ -1409,6 +1388,21 @@ _channel_close(_channels *channels, int64_t id, int end, int force)
     return _channels_close(channels, id, NULL, end, force);
 }
 
+static int
+_channel_is_associated(_channels *channels, int64_t cid, int64_t interp,
+                       int send)
+{
+    _PyChannelState *chan = _channels_lookup(channels, cid, NULL);
+    if (chan == NULL) {
+        return -1;
+    }
+
+    _channelend *end = _channelend_find(send ? chan->ends->send : chan->ends->recv,
+                                        interp, NULL);
+
+    return (end != NULL && end->open);
+}
+
 /* ChannelID class */
 
 static PyTypeObject ChannelIDtype;
@@ -2339,14 +2333,15 @@ PyDoc_STRVAR(channel_list_all_doc,
 \n\
 Return the list of all IDs for active channels.");
 
-
 static PyObject *
 channel_list_interpreters(PyObject *self, PyObject *args, PyObject *kwds)
 {
     static char *kwlist[] = {"cid", "send", NULL};
     int64_t cid;            /* Channel ID */
     int send = 0;           /* Send or receive end? */
-    PyObject *ret = NULL;
+    int64_t id;
+    PyObject *ids, *id_obj;
+    PyInterpreterState *interp;
 
     if (!PyArg_ParseTupleAndKeywords(
             args, kwds, "O&$p:channel_list_interpreters",
@@ -2354,39 +2349,40 @@ channel_list_interpreters(PyObject *self, PyObject *args, PyObject *kwds)
         return NULL;
     }
 
-    _PyChannelState *chan = _channels_lookup(&_globals.channels, cid, NULL);
-    if (chan == NULL) {
-        return NULL;
-    }
-
-    int64_t count = 0;  /* Number of interpreters */
-    int64_t *ids = _channelends_list_interpreters(chan->ends, &count, send);
+    ids = PyList_New(0);
     if (ids == NULL) {
         goto except;
     }
 
-    ret = PyList_New((Py_ssize_t)count);
-    if (ret == NULL) {
-        goto except;
-    }
-
-    for (int64_t i=0; i < count; i++) {
-        PyObject *id_obj = _PyInterpreterID_New(ids[i]);
-        if (id_obj == NULL) {
+    interp = PyInterpreterState_Head();
+    while (interp != NULL) {
+        id = PyInterpreterState_GetID(interp);
+        assert(id >= 0);
+        int res = _channel_is_associated(&_globals.channels, cid, id, send);
+        if (res < 0) {
             goto except;
         }
-        PyList_SET_ITEM(ret, i, id_obj);
+        if (res) {
+            id_obj = _PyInterpreterState_GetIDObject(interp);
+            if (id_obj == NULL) {
+                goto except;
+            }
+            res = PyList_Insert(ids, 0, id_obj);
+            if (res < 0) {
+                goto except;
+            }
+        }
+        interp = PyInterpreterState_Next(interp);
     }
 
     goto finally;
 
 except:
-    Py_XDECREF(ret);
-    ret = NULL;
+    Py_XDECREF(ids);
+    ids = NULL;
 
 finally:
-    PyMem_Free(ids);
-    return ret;
+    return ids;
 }
 
 PyDoc_STRVAR(channel_list_interpreters_doc,

From 79f5d35a8fee381527fe44c790eb79673da40666 Mon Sep 17 00:00:00 2001
From: Lewis Gaul <lewis.gaul@gmail.com>
Date: Tue, 28 Apr 2020 22:24:14 +0100
Subject: [PATCH 31/33] Fix issue with ChannelClosedError not being raised when
 'send' end of channel is closed

---
 Lib/test/test__xxsubinterpreters.py | 22 +++++++++++++---------
 Modules/_xxsubinterpretersmodule.c  |  3 +++
 2 files changed, 16 insertions(+), 9 deletions(-)

diff --git a/Lib/test/test__xxsubinterpreters.py b/Lib/test/test__xxsubinterpreters.py
index 77e9f05296601c..633cff6e7e2ab2 100644
--- a/Lib/test/test__xxsubinterpreters.py
+++ b/Lib/test/test__xxsubinterpreters.py
@@ -1245,7 +1245,7 @@ def test_channel_list_interpreters_multiple(self):
         interpreters.channel_send(cid, "send")
         _run_output(interp1, dedent(f"""
             import _xxsubinterpreters as _interpreters
-            obj = _interpreters.channel_send({cid}, "send")
+            _interpreters.channel_send({cid}, "send")
             """))
         _run_output(interp2, dedent(f"""
             import _xxsubinterpreters as _interpreters
@@ -1318,7 +1318,7 @@ def test_channel_list_interpreters_released(self):
         # Release one of the subinterpreters from the receive end.
         _run_output(interp2, dedent(f"""
             import _xxsubinterpreters as _interpreters
-            obj = _interpreters.channel_release({cid})
+            _interpreters.channel_release({cid})
             """))
         # Receive end should have the released interpreter removed.
         send_interps = interpreters.channel_list_interpreters(cid, send=True)
@@ -1331,30 +1331,34 @@ def test_channel_list_interpreters_closed(self):
         interp0 = interpreters.get_main()
         interp1 = interpreters.create()
         cid = interpreters.channel_create()
-        # Should be no interpreters associated with either end.
+        # Put something in the channel so that it's not empty.
+        interpreters.channel_send(cid, "send")
+
+        # Check initial state.
         send_interps = interpreters.channel_list_interpreters(cid, send=True)
         recv_interps = interpreters.channel_list_interpreters(cid, send=False)
-        self.assertEqual(len(send_interps), 0)
+        self.assertEqual(len(send_interps), 1)
         self.assertEqual(len(recv_interps), 0)
 
         # Close the send end of the channel.
         interpreters.channel_close(cid, send=True)
         # Send end should raise an error.
         with self.assertRaises(interpreters.ChannelClosedError):
-            send_interps = interpreters.channel_list_interpreters(cid, send=True)
+            interpreters.channel_list_interpreters(cid, send=True)
+        # Receive end should not be closed (since channel is not empty).
         recv_interps = interpreters.channel_list_interpreters(cid, send=False)
         self.assertEqual(len(recv_interps), 0)
 
         # Close the receive end of the channel from a subinterpreter.
-        _run_output(interp3, dedent(f"""
+        _run_output(interp1, dedent(f"""
             import _xxsubinterpreters as _interpreters
-            obj = _interpreters.channel_close({cid})
+            _interpreters.channel_close({cid}, force=True)
             """))
         # Both ends should raise an error.
         with self.assertRaises(interpreters.ChannelClosedError):
-            send_interps = interpreters.channel_list_interpreters(cid, send=True)
+            interpreters.channel_list_interpreters(cid, send=True)
         with self.assertRaises(interpreters.ChannelClosedError):
-            recv_interps = interpreters.channel_list_interpreters(cid, send=False)
+            interpreters.channel_list_interpreters(cid, send=False)
 
     ####################
 
diff --git a/Modules/_xxsubinterpretersmodule.c b/Modules/_xxsubinterpretersmodule.c
index 6e8d8d2b502dc4..d460eeca1b8dc5 100644
--- a/Modules/_xxsubinterpretersmodule.c
+++ b/Modules/_xxsubinterpretersmodule.c
@@ -1395,6 +1395,9 @@ _channel_is_associated(_channels *channels, int64_t cid, int64_t interp,
     _PyChannelState *chan = _channels_lookup(channels, cid, NULL);
     if (chan == NULL) {
         return -1;
+    } else if (send && chan->closing != NULL) {
+        PyErr_Format(ChannelClosedError, "channel %" PRId64 " closed", cid);
+        return -1;
     }
 
     _channelend *end = _channelend_find(send ? chan->ends->send : chan->ends->recv,

From 5481e82e070d2472affa0313e0aa87e4936178d1 Mon Sep 17 00:00:00 2001
From: Lewis Gaul <lewis.gaul@gmail.com>
Date: Tue, 28 Apr 2020 22:28:47 +0100
Subject: [PATCH 32/33] Add testcase for listing associated interpreters with
 basic closed channel

---
 Lib/test/test__xxsubinterpreters.py | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/Lib/test/test__xxsubinterpreters.py b/Lib/test/test__xxsubinterpreters.py
index 633cff6e7e2ab2..a98a8496ab8626 100644
--- a/Lib/test/test__xxsubinterpreters.py
+++ b/Lib/test/test__xxsubinterpreters.py
@@ -1340,6 +1340,28 @@ def test_channel_list_interpreters_closed(self):
         self.assertEqual(len(send_interps), 1)
         self.assertEqual(len(recv_interps), 0)
 
+        # Force close the channel.
+        interpreters.channel_close(cid, force=True)
+        # Both ends should raise an error.
+        with self.assertRaises(interpreters.ChannelClosedError):
+            interpreters.channel_list_interpreters(cid, send=True)
+        with self.assertRaises(interpreters.ChannelClosedError):
+            interpreters.channel_list_interpreters(cid, send=False)
+
+    def test_channel_list_interpreters_closed_send_end(self):
+        """Test listing channel interpreters with a channel's send end closed."""
+        interp0 = interpreters.get_main()
+        interp1 = interpreters.create()
+        cid = interpreters.channel_create()
+        # Put something in the channel so that it's not empty.
+        interpreters.channel_send(cid, "send")
+
+        # Check initial state.
+        send_interps = interpreters.channel_list_interpreters(cid, send=True)
+        recv_interps = interpreters.channel_list_interpreters(cid, send=False)
+        self.assertEqual(len(send_interps), 1)
+        self.assertEqual(len(recv_interps), 0)
+
         # Close the send end of the channel.
         interpreters.channel_close(cid, send=True)
         # Send end should raise an error.

From 79be8a0d8bdd1b2f99d3495e7fcbd078dc1bcb47 Mon Sep 17 00:00:00 2001
From: Eric Snow <ericsnowcurrently@gmail.com>
Date: Tue, 28 Apr 2020 17:15:59 -0600
Subject: [PATCH 33/33] Fix a refleak.

---
 Modules/_xxsubinterpretersmodule.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/Modules/_xxsubinterpretersmodule.c b/Modules/_xxsubinterpretersmodule.c
index d460eeca1b8dc5..e9ed08206aa3e0 100644
--- a/Modules/_xxsubinterpretersmodule.c
+++ b/Modules/_xxsubinterpretersmodule.c
@@ -2371,6 +2371,7 @@ channel_list_interpreters(PyObject *self, PyObject *args, PyObject *kwds)
                 goto except;
             }
             res = PyList_Insert(ids, 0, id_obj);
+            Py_DECREF(id_obj);
             if (res < 0) {
                 goto except;
             }