fix copy_context in ws threadpool

CHANGELOG.md

@@ -8,6 +8,7 @@ This project adheres to [Semantic Versioning](https://semver.org/).
 - [#3826](https://github.com/plotly/dash/pull/3826) WebSocket callback dispatch no longer lets long-lived callbacks limit the number of concurrent users. Async callbacks (including session-persistent ones) run directly on the connection event loop instead of occupying a worker thread, and synchronous callbacks run on a shared `ThreadPoolExecutor` whose size is configurable via the new `websocket_max_workers` argument to `Dash` (default `4`). A synchronous persistent (no-output) callback now warns at registration since it would tie up a worker thread.
 
 ## Fixed
+- [#3861](https://github.com/plotly/dash/issues/3861) Fix synchronous WebSocket callbacks not inheriting `ContextVar` values bound by ASGI middleware. `copy_context()` is now captured on the event-loop thread before the callback is submitted to the thread pool, instead of inside the worker thread where only default values were visible.
 - [#3779](https://github.com/plotly/dash/pull/3779) Fix `dash.testing` `Browser.get_logs()` returning `None` on non-Chrome webdrivers, which broke assertions like `assert dash_duo.get_logs() == []`. It now returns `[]`, matching the Chrome code path.
 - [#3822](https://github.com/plotly/dash/pull/3822) Fix `UnboundLocalError` for `user_callback_output` in async background callbacks (Celery and Diskcache managers) when the callback raises `PreventUpdate` or another exception before the variable is assigned.
 - [#3819](https://github.com/plotly/dash/pull/3819) Fix `RuntimeError: No active request in context` when a non-Dash path falls through to the FastAPI catch-all route. Fixes [#3812](https://github.com/plotly/dash/issues/3812).

dash/backends/ws.py

@@ -550,14 +550,18 @@ def run_callback_in_executor(
         Future representing the pending callback execution
     """
 
+    # Snapshot the context on the calling thread (the event-loop thread) *before*
+    # submitting to the executor. ContextVars bound by ASGI middleware live on this
+    # thread; capturing inside execute() would run on a fresh worker thread and see
+    # only default values. This mirrors loop.run_in_executor's context propagation.
+    ctx = copy_context()
+
     def execute() -> dict:
         try:
             partial_func = _prepare_ws_partial(
                 dash_app, payload, ws_callback, response_adapter
             )
 
-            ctx = copy_context()
-
             # Run in new event loop (handles a callback that still returns a
             # coroutine, e.g. when reached outside the async dispatch path)
             def run_callback():

tests/unit/test_websocket_executor.py

@@ -8,9 +8,17 @@
 ``websocket_max_workers`` argument to ``Dash``.
 """
 
+import asyncio
+import threading
 from concurrent.futures import ThreadPoolExecutor
+from contextvars import ContextVar
+from typing import cast
 
-from dash import Dash
+import janus
+
+from dash import Dash, Input, Output
+from dash.backends.ws import DashWebsocketCallback, run_callback_in_executor
+from dash.types import CallbackExecutionBody
 
 
 def test_websocket_max_workers_default():
@@ -61,3 +69,60 @@ def test_shutdown_executor_allows_recreation():
         assert ex1 is not ex2
     finally:
         backend.shutdown_executor(wait=False)
+
+
+def test_run_callback_in_executor_propagates_contextvars():
+    """Sync WS callbacks inherit ContextVars bound on the calling thread.
+
+    Regression test for gh-3861: ``copy_context()`` must be captured in
+    ``run_callback_in_executor`` (on the event-loop thread, where ASGI middleware
+    binds per-request ContextVars) rather than inside the worker-thread ``execute``
+    closure, which would only ever see default values.
+    """
+    myvar: ContextVar = ContextVar("myvar", default="DEFAULT")
+
+    app = Dash(__name__)
+
+    @app.callback(Output("out", "children"), Input("in", "value"), websocket=True)
+    def cb(value):
+        return f"{myvar.get()}:{value}"
+
+    payload = cast(
+        CallbackExecutionBody,
+        {
+            "output": "out.children",
+            "outputs": {"id": "out", "property": "children"},
+            "inputs": [{"id": "in", "property": "value", "value": "hi"}],
+            "state": [],
+            "changedPropIds": ["in.value"],
+        },
+    )
+
+    executor = ThreadPoolExecutor(max_workers=2)
+
+    async def run():
+        # janus.Queue must be constructed with a running loop on Python < 3.10.
+        outbound_queue: janus.Queue = janus.Queue()
+        ws_cb = DashWebsocketCallback(
+            {}, "rid", outbound_queue, threading.Event(), None
+        )
+        try:
+            # Bind the ContextVar on this (calling/event-loop) thread, as
+            # middleware would; run_callback_in_executor must snapshot it here.
+            myvar.set("MIDDLEWARE_VALUE")
+            future = run_callback_in_executor(
+                executor, app, payload, ws_cb, app.backend.response_adapter()
+            )
+            return future.result(timeout=10)
+        finally:
+            outbound_queue.close()
+            await outbound_queue.wait_closed()
+
+    try:
+        result = asyncio.run(run())
+    finally:
+        executor.shutdown(wait=False)
+
+    assert result["status"] == "ok"
+    # The worker thread would see the default without the calling-thread snapshot.
+    assert result["data"]["response"]["out"]["children"] == "MIDDLEWARE_VALUE:hi"