Surface node scheduling errors instead of crashing the worker

A node whose FUNCTION points at a method that does not exist (e.g. a typo
in a custom node) raised an AttributeError inside the scheduling heuristic
(ux_friendly_pick_node -> is_async). That exception escaped
stage_node_execution() and the prompt worker's error handling, silently
killing the worker thread with nothing reported to the client.

- is_async() now treats a node whose FUNCTION does not resolve to a method
  as non-async, so scheduling proceeds and the missing-method error is
  raised and reported through the normal execution path.
- stage_node_execution() wraps node picking so any unexpected scheduling
  error is returned as an execution error (attributed to an available
  node) rather than propagating and killing the worker thread.

Add regression tests covering both paths.

comfy_execution/graph.py

@@ -3,6 +3,7 @@ from typing import Type, Literal
 import nodes
 import asyncio
 import inspect
+import traceback
 from comfy_execution.graph_utils import is_link, ExecutionBlocker
 from comfy.comfy_types.node_typing import ComfyNodeABC, InputTypeDict, InputTypeOptions
 
@@ -263,7 +264,25 @@ class ExecutionList(TopologicalSort):
             }
             return None, error_details, ex
 
-        self.staged_node_id = self.ux_friendly_pick_node(available)
+        try:
+            self.staged_node_id = self.ux_friendly_pick_node(available)
+        except Exception as ex:
+            # Picking a node is a scheduling heuristic that inspects node
+            # definitions; a malformed custom node must not crash the prompt
+            # worker thread silently. Blame an available node and surface the
+            # error to the frontend like any other execution error.
+            blamed_node = self.dynprompt.get_display_node_id(available[0])
+            exception_type = type(ex).__qualname__
+            if type(ex).__module__ != "builtins":
+                exception_type = type(ex).__module__ + "." + exception_type
+            error_details = {
+                "node_id": blamed_node,
+                "exception_message": str(ex),
+                "exception_type": exception_type,
+                "traceback": traceback.format_tb(ex.__traceback__),
+                "current_inputs": []
+            }
+            return None, error_details, ex
         return self.staged_node_id, None, None
 
     def ux_friendly_pick_node(self, node_list):
@@ -283,7 +302,17 @@ class ExecutionList(TopologicalSort):
         def is_async(node_id):
             class_type = self.dynprompt.get_node(node_id)["class_type"]
             class_def = nodes.NODE_CLASS_MAPPINGS[class_type]
-            return inspect.iscoroutinefunction(getattr(class_def, class_def.FUNCTION))
+            # A malformed node (e.g. FUNCTION pointing at a method that does not
+            # exist because of a typo) must not crash scheduling here. Treat it as
+            # non-async so it proceeds to normal execution, where the missing
+            # method raises an error that is caught and reported to the frontend.
+            function_name = getattr(class_def, "FUNCTION", None)
+            if function_name is None:
+                return False
+            func = getattr(class_def, function_name, None)
+            if func is None:
+                return False
+            return inspect.iscoroutinefunction(func)
 
         for node_id in node_list:
             if is_output(node_id) or is_async(node_id):

tests-unit/execution_test/scheduler_malformed_node_test.py

@@ -0,0 +1,68 @@
+"""Regression tests for scheduler resilience to malformed nodes.
+
+A node whose FUNCTION points at a method that does not exist (e.g. a typo in a
+custom node) used to raise inside the scheduling heuristic, escaping the prompt
+worker's error handling and silently killing the worker thread. Scheduling must
+instead either proceed (so the error surfaces through normal execution) or report
+the failure as an execution error.
+"""
+import asyncio
+
+import nodes
+from comfy_execution.graph import DynamicPrompt, ExecutionList
+
+
+class _MalformedV1Node:
+    @classmethod
+    def INPUT_TYPES(cls):
+        return {"required": {}}
+
+    RETURN_TYPES = ("IMAGE",)
+    FUNCTION = "invert"  # the actual method below is misspelled
+    OUTPUT_NODE = True
+    CATEGORY = "Test"
+
+    def invvert(self):
+        return (None,)
+
+
+class _FakeOutputCache:
+    def all_node_ids(self):
+        return set()
+
+    async def get(self, node_id):
+        return None
+
+
+def _make_execution_list(class_type, class_def):
+    nodes.NODE_CLASS_MAPPINGS[class_type] = class_def
+    prompt = {"1": {"class_type": class_type, "inputs": {}}}
+    execution_list = ExecutionList(DynamicPrompt(prompt), _FakeOutputCache())
+    execution_list.add_node("1")
+    return execution_list
+
+
+def test_malformed_function_does_not_crash_scheduler():
+    """A FUNCTION-typo node schedules without raising; the error surfaces later."""
+    execution_list = _make_execution_list("MalformedV1Node", _MalformedV1Node)
+    node_id, error, ex = asyncio.run(execution_list.stage_node_execution())
+    assert ex is None
+    assert error is None
+    assert node_id == "1"
+
+
+def test_pick_node_failure_is_reported_not_raised():
+    """An unexpected scheduling error is returned as an error, not raised."""
+    execution_list = _make_execution_list("MalformedV1Node", _MalformedV1Node)
+
+    def raise_on_pick(_available):
+        raise RuntimeError("boom")
+
+    execution_list.ux_friendly_pick_node = raise_on_pick
+    node_id, error, ex = asyncio.run(execution_list.stage_node_execution())
+    assert node_id is None
+    assert isinstance(ex, RuntimeError)
+    assert error["node_id"] == "1"
+    assert error["exception_type"] == "RuntimeError"
+    assert error["exception_message"] == "boom"
+    assert error["traceback"]