Claim the Document thread before running app code

panel/io/application.py

@@ -6,6 +6,7 @@
 import json
 import logging
 import os
+import threading
 import typing as t
 
 from functools import partial
@@ -165,6 +166,10 @@ def _set_session_prefix(self, doc):
 
     def initialize_document(self, doc):
         logger.info(LOG_SESSION_LAUNCHING, id(doc))
+        # Claim the Document for the current thread before user code runs,
+        # so APIs that behave differently when invoked off the Document's
+        # thread (e.g. hold) can tell the two cases apart during the build.
+        state._thread_id_[doc] = threading.get_ident()
         self._set_session_prefix(doc)
         super().initialize_document(doc)
         if doc in state._templates and doc not in state._templates[doc]._documents:

panel/tests/io/test_document.py

@@ -78,6 +78,27 @@ def update_in_hold(i):
     wait_until(lambda: not doc.callbacks.hold_value, timeout=5000)
 
 
+@pytest.mark.xdist_group(name="server")
+def test_hold_in_app_callable_does_not_leak_hold():
+    build = {}
+
+    def app():
+        with hold():
+            pass
+        # A hold leaked here defers the unhold past ServerSession
+        # construction, which registers this callback a second time
+        # when the queued SessionCallbackAdded event is replayed.
+        pn.state.add_periodic_callback(lambda: None, period=10000)
+        build['thread_id'] = state._thread_id
+        build['hold'] = state.curdoc.callbacks.hold_value
+        return IntSlider()
+
+    serve_and_request(app)
+
+    assert build['thread_id'] is not None
+    assert build['hold'] is None
+
+
 class _FakeProtocol:
     def create(self, msgtype, events):
         return object()