Ensure errors in sync periodic callbacks are logged.

panel/io/callbacks.py

@@ -86,10 +86,9 @@ def _exec_callback(self, post=False):
                     if self.count is not None and self.counter > self.count:
                         self.stop()
                 cb = self.callback() if self.running else None
-        except Exception:
-            cb = None
-        if post:
-            self._post_callback()
+        finally:
+            if post:
+                self._post_callback()
         return cb
 
     def _post_callback(self):
@@ -133,9 +132,6 @@ async def _periodic_callback(self):
                         await cb
                 else:
                     await cb
-        except Exception:
-            log.exception('Periodic callback failed.')
-            raise
         finally:
             self._post_callback()
 

panel/tests/test_server.py

@@ -341,6 +341,50 @@ def app():
     assert state.cache['count'] == count
 
 
+async def _async_erroring_cb():
+    raise ValueError("An erroring callback")
+
+def _sync_erroring_cb():
+    raise ValueError("An erroring callback")
+
+@pytest.mark.parametrize(
+    'threadpool, cb', [
+        (2, _async_erroring_cb),
+        (None, _async_erroring_cb),
+        (2, _sync_erroring_cb),
+        (None, _sync_erroring_cb)
+])
+def test_server_periodic_callback_error_logged(caplog, server_implementation, threadpool, cb):
+    """Ensure errors in periodic callbacks appear in the logs"""
+    loggers_to_check = [logging.getLogger(x) for x in ('panel','bokeh')]
+    orig_level_propagate = [(l.level,l.propagate) for l in loggers_to_check]
+    orig_threads = config.nthreads
+    repeats = 3
+
+    try:
+        config.nthreads = threadpool
+        for l in loggers_to_check:
+            l.propagate = True
+            l.setLevel(logging.WARNING)
+
+        def app():
+            state.add_periodic_callback(cb, 100, repeats)
+            def loaded():
+                state._schedule_on_load(state.curdoc, None)
+            state.execute(loaded, schedule=True)
+            return Row()
+
+        serve_and_request(app)
+        time.sleep(1)
+        num_errors_logged = caplog.text.count('ValueError: An erroring callback')
+        assert num_errors_logged == repeats
+    finally:
+        for l,level_prop in zip(loggers_to_check,orig_level_propagate):
+            l.setLevel(level_prop[0])
+            l.propagate = level_prop[1]
+        config.nthreads = orig_threads
+
+
 def test_server_schedule_repeat(server_implementation):
     state.cache['count'] = 0
     def periodic_cb():