Fix channel notification for create_task async receive

event_loop_add_pending() now sends task_ready to worker, not just
pthread_cond_signal. This fixes async channel receive hanging when
using create_task (where Python is not waiting on the condition var).

Also fix Python 3.9 compat: use Optional[bytes] instead of bytes | None.

Author: benoitc

c_src/py_event_loop.c

@@ -3700,6 +3700,25 @@ bool event_loop_add_pending(erlang_event_loop_t *loop, event_type_t type,
     }
 
     pthread_mutex_unlock(&loop->mutex);
+
+    /*
+     * Also send task_ready to the worker if one exists.
+     * This is needed for create_task: Python is not waiting on the condition
+     * variable, so we need to notify the Erlang worker to call process_ready_tasks.
+     *
+     * Uses the same coalescing logic as submit_task to avoid message floods.
+     */
+    if (loop->has_worker) {
+        if (!atomic_exchange(&loop->task_wake_pending, true)) {
+            ErlNifEnv *msg_env = enif_alloc_env();
+            if (msg_env != NULL) {
+                ERL_NIF_TERM msg = enif_make_atom(msg_env, "task_ready");
+                enif_send(NULL, &loop->worker_pid, msg_env, msg);
+                enif_free_env(msg_env);
+            }
+        }
+    }
+
     return true;
 }
 

priv/_erlang_impl/_byte_channel.py

@@ -54,6 +54,8 @@ async def process_bytes_async(channel_ref):
     ch.send_bytes(b"HTTP/1.1 200 OK\\r\\n")
 """
 
+from typing import Optional
+
 __all__ = ['ByteChannel', 'ByteChannelClosed']
 
 
@@ -102,7 +104,7 @@ def send_bytes(self, data: bytes) -> bool:
         # Use _channel_send which sends raw bytes
         return erlang._channel_send(self._ref, bytes(data))
 
-    def try_receive_bytes(self) -> bytes | None:
+    def try_receive_bytes(self) -> Optional[bytes]:
         """Try to receive bytes without blocking.
 
         Returns:

test/py_byte_channel_SUITE.erl

@@ -41,7 +41,8 @@
     %% Close and drain tests
     close_drain_bytes_erlang_test/1,
     close_drain_bytes_python_sync_test/1,
-    close_drain_bytes_python_async_test/1
+    close_drain_bytes_python_async_test/1,
+    close_drain_bytes_create_task_async_test/1
 ]).
 
 all() -> [
@@ -72,7 +73,8 @@ all() -> [
     %% Close and drain tests
     close_drain_bytes_erlang_test,
     close_drain_bytes_python_sync_test,
-    close_drain_bytes_python_async_test
+    close_drain_bytes_python_async_test,
+    close_drain_bytes_create_task_async_test
 ].
 
 init_per_suite(Config) ->
@@ -530,3 +532,63 @@ async def async_drain_byte_channel(ch_ref):
         py:eval(Ctx, <<"erlang.run(async_drain_byte_channel(ch))">>, #{<<"ch">> => Ch}),
 
     ct:pal("Python async byte channel close+drain test passed").
+
+%% @doc Test async drain with create_task when data arrives after task starts
+%% This tests the notification callback path: task registers waiter, then data arrives
+close_drain_bytes_create_task_async_test(_Config) ->
+    {ok, Ch} = py_byte_channel:new(),
+
+    %% Define the async drain task
+    Ctx = py:context(1),
+    ok = py:exec(Ctx, <<"
+import erlang
+from erlang import ByteChannel
+
+async def drain_task(ch_ref, reply_pid):
+    '''Task that drains byte channel and sends results back.'''
+    try:
+        ch = ByteChannel(ch_ref)
+        chunks = []
+        async for chunk in ch:
+            chunks.append(chunk)
+        erlang.send(reply_pid, ('result', chunks))
+    except Exception as e:
+        erlang.send(reply_pid, ('error', str(e)))
+">>),
+
+    %% Create the task BEFORE sending any data
+    %% This forces the task to register a waiter and wait for notifications
+    TaskRef = py_event_loop:create_task(
+        "__main__", "drain_task", [Ch, self()]),
+
+    %% Give the task time to start and register waiter
+    timer:sleep(100),
+
+    %% Now send data - should trigger notification callback
+    ok = py_byte_channel:send(Ch, <<"chunk1">>),
+    ok = py_byte_channel:send(Ch, <<"chunk2">>),
+    ok = py_byte_channel:send(Ch, <<"chunk3">>),
+
+    %% Close the channel to signal end of stream
+    ok = py_byte_channel:close(Ch),
+
+    %% Wait for result from the task
+    receive
+        {<<"result">>, [<<"chunk1">>, <<"chunk2">>, <<"chunk3">>]} ->
+            ct:pal("create_task async drain with delayed data OK");
+        {<<"result">>, Other} ->
+            ct:pal("Unexpected result: ~p", [Other]),
+            ct:fail({unexpected_result, Other});
+        {<<"error">>, ErrMsg} ->
+            ct:pal("Task error: ~p", [ErrMsg]),
+            ct:fail({task_error, ErrMsg})
+    after 5000 ->
+        ct:fail("Timeout waiting for drain task result")
+    end,
+
+    %% Wait for task to complete
+    case py_event_loop:await(TaskRef, 5000) of
+        {ok, _} -> ok;
+        {error, AwaitErr} ->
+            ct:pal("Await error: ~p", [AwaitErr])
+    end.

test/py_channel_SUITE.erl

@@ -47,7 +47,8 @@
     %% Close and drain tests
     close_drain_erlang_test/1,
     close_drain_python_sync_test/1,
-    close_drain_python_async_test/1
+    close_drain_python_async_test/1,
+    close_drain_create_task_async_test/1
 ]).
 
 all() -> [
@@ -84,7 +85,8 @@ all() -> [
     %% Close and drain tests
     close_drain_erlang_test,
     close_drain_python_sync_test,
-    close_drain_python_async_test
+    close_drain_python_async_test,
+    close_drain_create_task_async_test
 ].
 
 init_per_suite(Config) ->
@@ -707,3 +709,63 @@ async def async_drain_channel(ch_ref):
         py:eval(Ctx, <<"erlang.run(async_drain_channel(ch))">>, #{<<"ch">> => Ch}),
 
     ct:pal("Python async close+drain test passed").
+
+%% @doc Test async drain with create_task when data arrives after task starts
+%% This tests the notification callback path: task registers waiter, then data arrives
+close_drain_create_task_async_test(_Config) ->
+    {ok, Ch} = py_channel:new(),
+
+    %% Define the async drain task
+    Ctx = py:context(1),
+    ok = py:exec(Ctx, <<"
+import erlang
+from erlang import Channel
+
+async def drain_task(ch_ref, reply_pid):
+    '''Task that drains channel and sends results back.'''
+    try:
+        ch = Channel(ch_ref)
+        messages = []
+        async for msg in ch:
+            messages.append(msg)
+        erlang.send(reply_pid, ('result', messages))
+    except Exception as e:
+        erlang.send(reply_pid, ('error', str(e)))
+">>),
+
+    %% Create the task BEFORE sending any data
+    %% This forces the task to register a waiter and wait for notifications
+    TaskRef = py_event_loop:create_task(
+        "__main__", "drain_task", [Ch, self()]),
+
+    %% Give the task time to start and register waiter
+    timer:sleep(100),
+
+    %% Now send data - should trigger notification callback
+    ok = py_channel:send(Ch, <<"msg1">>),
+    ok = py_channel:send(Ch, <<"msg2">>),
+    ok = py_channel:send(Ch, <<"msg3">>),
+
+    %% Close the channel to signal end of stream
+    ok = py_channel:close(Ch),
+
+    %% Wait for result from the task
+    receive
+        {<<"result">>, [<<"msg1">>, <<"msg2">>, <<"msg3">>]} ->
+            ct:pal("create_task async drain with delayed data OK");
+        {<<"result">>, Other} ->
+            ct:pal("Unexpected result: ~p", [Other]),
+            ct:fail({unexpected_result, Other});
+        {<<"error">>, ErrMsg} ->
+            ct:pal("Task error: ~p", [ErrMsg]),
+            ct:fail({task_error, ErrMsg})
+    after 5000 ->
+        ct:fail("Timeout waiting for drain task result")
+    end,
+
+    %% Wait for task to complete
+    case py_event_loop:await(TaskRef, 5000) of
+        {ok, _} -> ok;
+        {error, AwaitErr} ->
+            ct:pal("Await error: ~p", [AwaitErr])
+    end.