From de30ecc359f3cc8389d1bb24ec7db2533f05dbf4 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Iv=C3=A1n=20Ovejero?= <ivov.src@gmail.com>
Date: Tue, 2 Sep 2025 13:12:48 +0200
Subject: [PATCH] fix(core): Resolve Python multiprocessing queue deadlock
 (#19084)

---
 .../task-runner-python/src/task_executor.py   | 53 ++++++++++++++++---
 1 file changed, 47 insertions(+), 6 deletions(-)

diff --git a/packages/@n8n/task-runner-python/src/task_executor.py b/packages/@n8n/task-runner-python/src/task_executor.py
index a07afc1fc0..d5977fcc4e 100644
--- a/packages/@n8n/task-runner-python/src/task_executor.py
+++ b/packages/@n8n/task-runner-python/src/task_executor.py
@@ -5,6 +5,8 @@ import textwrap
 import json
 import os
 import sys
+import tempfile
+import pickle
 
 from src.errors import (
     TaskResultMissingError,
@@ -25,6 +27,7 @@ from typing import Any, Set
 from multiprocessing.context import SpawnProcess
 
 MULTIPROCESSING_CONTEXT = multiprocessing.get_context("spawn")
+MAX_PRINT_STATEMENTS_ALLOWED = 100
 
 PrintArgs = list[list[Any]]  # Args to all `print()` calls in a Python code task
 
@@ -97,7 +100,16 @@ class TaskExecutor:
             if "error" in returned:
                 raise TaskRuntimeError(returned["error"])
 
-            result = returned.get("result", [])
+            if "result_file" not in returned:
+                raise TaskResultMissingError()
+
+            result_file = returned["result_file"]
+            try:
+                with open(result_file, "rb") as f:
+                    result = pickle.load(f)
+            finally:
+                os.unlink(result_file)
+
             print_args = returned.get("print_args", [])
 
             return result, print_args
@@ -152,9 +164,8 @@ class TaskExecutor:
 
             exec(compiled_code, globals)
 
-            queue.put(
-                {"result": globals[EXECUTOR_USER_OUTPUT_KEY], "print_args": print_args}
-            )
+            result = globals[EXECUTOR_USER_OUTPUT_KEY]
+            TaskExecutor._put_result(queue, result, print_args)
 
         except Exception as e:
             TaskExecutor._put_error(queue, e, print_args)
@@ -201,7 +212,7 @@ class TaskExecutor:
                 user_output["pairedItem"] = {"item": index}
                 result.append(user_output)
 
-            queue.put({"result": result, "print_args": print_args})
+            TaskExecutor._put_result(queue, result, print_args)
 
         except Exception as e:
             TaskExecutor._put_error(queue, e, print_args)
@@ -211,12 +222,26 @@ class TaskExecutor:
         indented_code = textwrap.indent(raw_code, "    ")
         return f"def _user_function():\n{indented_code}\n\n{EXECUTOR_USER_OUTPUT_KEY} = _user_function()"
 
+    @staticmethod
+    def _put_result(
+        queue: multiprocessing.Queue, result: list[Any], print_args: PrintArgs
+    ):
+        with tempfile.NamedTemporaryFile(
+            mode="wb", delete=False, prefix="n8n_result_"
+        ) as f:
+            pickle.dump(result, f)
+            result_file = f.name
+
+        print_args_to_send = TaskExecutor._truncate_print_args(print_args)
+        queue.put({"result_file": result_file, "print_args": print_args_to_send})
+
     @staticmethod
     def _put_error(queue: multiprocessing.Queue, e: Exception, print_args: PrintArgs):
+        print_args_to_send = TaskExecutor._truncate_print_args(print_args)
         queue.put(
             {
                 "error": {"message": str(e), "stack": traceback.format_exc()},
-                "print_args": print_args,
+                "print_args": print_args_to_send,
             }
         )
 
@@ -273,6 +298,22 @@ class TaskExecutor:
 
         return formatted
 
+    @staticmethod
+    def _truncate_print_args(print_args: PrintArgs) -> PrintArgs:
+        """Truncate print_args to prevent pipe buffer overflow."""
+
+        if not print_args or len(print_args) <= MAX_PRINT_STATEMENTS_ALLOWED:
+            return print_args
+
+        truncated = print_args[:MAX_PRINT_STATEMENTS_ALLOWED]
+        truncated.append(
+            [
+                f"[Output truncated - {len(print_args) - MAX_PRINT_STATEMENTS_ALLOWED} more print statements]"
+            ]
+        )
+
+        return truncated
+
     # ========== security ==========
 
     @staticmethod