OSError on reload when running Werkzeug server with vs code debugger

[sergehauri]

Type: Bug

Behaviour

Expected vs. Actual

The Werkzeug development server features a reloader, which checks the files of the wsgi application it's running and restarts the server if there are any changes. In Werkzeug 2.2.3, they fixed an issue with unclosed sockets the development server (issue: pallets/werkzeug#2421, fix: pallets/werkzeug#2517).

With that fix, running the werkzeug server with the vs code debugger causes the following exception when reloading the app:

Exception in thread Thread-6 (serve_forever):
Traceback (most recent call last):
  File "C:\Users\TAAHASE0\AppData\Local\Programs\Python\Python310\lib\threading.py", line 1009, in _bootstrap_inner
    self.run()
  File "C:\Users\TAAHASE0\AppData\Local\Programs\Python\Python310\lib\threading.py", line 946, in run
    self._target(*self._args, **self._kwargs)
  File "c:\Users\TAAHASE0\dev\projects\werkzeugtest\venv\lib\site-packages\werkzeug\serving.py", line 766, in serve_forever
    super().serve_forever(poll_interval=poll_interval)
  File "C:\Users\TAAHASE0\AppData\Local\Programs\Python\Python310\lib\socketserver.py", line 232, in serve_forever
    ready = selector.select(poll_interval)
  File "C:\Users\TAAHASE0\AppData\Local\Programs\Python\Python310\lib\selectors.py", line 324, in select
    r, w, _ = self._select(self._readers, self._writers, [], timeout)
  File "C:\Users\TAAHASE0\AppData\Local\Programs\Python\Python310\lib\selectors.py", line 315, in _select
    r, w, x = select.select(r, w, w, timeout)
OSError: [WinError 10038] An operation was attempted on something that is not a socket

The specific change causing this behaviour is the finally block in src/werkzeug/serving.py on lines 1066-1067.

        try:
            run_with_reloader(
                srv.serve_forever,
                extra_files=extra_files,
                exclude_patterns=exclude_patterns,
                interval=reloader_interval,
                reloader_type=reloader_type,
            )
        finally:
            srv.server_close()

Steps to reproduce:

1. Use a venv with flask and werkzeug 2.2.3 installed
2. app.py

from flask import Flask

app = Flask(__name__)

3. launch.json

{
    // Use IntelliSense to learn about possible attributes.
    // Hover to view descriptions of existing attributes.
    // For more information, visit: https://go.microsoft.com/fwlink/?linkid=830387
    "version": "0.2.0",
    "configurations": [
        {
            "name": "Python: Flask",
            "type": "python",
            "request": "launch",
            "module": "flask",
            "env": {
                "FLASK_APP": "app.py",
                "FLASK_DEBUG": "1"
            },
            "args": [
                "run",
                "--no-debugger"
            ],
            "jinja": true,
            "justMyCode": true
        }
    ]
}

4. "Start Debugging"
   vs code runs the following command:
   (venv) PS C:\Users\TAAHASE0\dev\projects\werkzeugtest> c:; cd 'c:\Users\TAAHASE0\dev\projects\werkzeugtest'; & 'c:\Users\TAAHASE0\dev\projects\werkzeugtest\venv\Scripts\python.exe' 'c:\Users\TAAHASE0\.vscode\extensions\ms-python.python-2023.6.0\pythonFiles\lib\python\debugpy\adapter/../..\debugpy\launcher' '50758' '--' '-m' 'flask' 'run' '--no-debugger'

5. Save the app.py file, which causes the werkzeug dev server to reload
   * Detected change in 'C:\\Users\\TAAHASE0\\dev\\projects\\werkzeugtest\\app.py', reloading

which leads to the before mentioned Exception.

Running the flask app directly from the terminal does not cause this behavior, it reloads the app without error, so it's not an issue of the werkzeug server itself:

(venv) PS C:\Users\TAAHASE0\dev\projects\werkzeugtest> flask run --debug --no-debugger
 * Debug mode: on
WARNING: This is a development server. Do not use it in a production deployment. Use a production WSGI server instead.
 * Running on http://127.0.0.1:5000
Press CTRL+C to quit
 * Restarting with stat
 * Detected change in 'C:\\Users\\TAAHASE0\\dev\\projects\\werkzeugtest\\app.py', reloading
 * Restarting with stat

Diagnostic data

• Python version (& distribution if applicable, e.g. Anaconda): 3.10.0
• Type of virtual environment used (e.g. conda, venv, virtualenv, etc.): Venv
• Value of the python.languageServer setting: Default

Output for Python in the Output panel (View→Output, change the drop-down the upper-right of the Output panel to Python)

DAP Server launched with command: c:\Users\TAAHASE0\dev\projects\werkzeugtest\venv\Scripts\python.exe c:\Users\TAAHASE0\.vscode\extensions\ms-python.python-2023.6.0\pythonFiles\lib\python\debugpy\adapter
Send text to terminal:  c:; cd 'c:\Users\TAAHASE0\dev\projects\werkzeugtest'; & 'c:\Users\TAAHASE0\dev\projects\werkzeugtest\venv\Scripts\python.exe' 'c:\Users\TAAHASE0\.vscode\extensions\ms-python.python-2023.6.0\pythonFiles\lib\python\debugpy\adapter/../..\debugpy\launcher' '53501' '--' '-m' 'flask' 'run' '--no-debugger' 
Connecting to DAP Server at:  127.0.0.1:53499
Connecting to DAP Server at:  127.0.0.1:53499

User Settings

languageServer: "Pylance"

linting
• flake8Enabled: true
• mypyEnabled: true

formatting
• provider: "black"

testing
• pytestEnabled: true

Extension version: 2023.6.0
VS Code version: Code 1.77.0 (7f329fe6c66b0f86ae1574c2911b681ad5a45d63, 2023-03-29T10:02:16.981Z)
OS version: Windows_NT x64 10.0.19042
Modes:
Sandboxed: No

System Info

Item	Value
CPUs	Intel(R) Core(TM) i5-8365U CPU @ 1.60GHz (8 x 1896)
GPU Status	2d_canvas: enabled canvas_oop_rasterization: disabled_off direct_rendering_display_compositor: disabled_off_ok gpu_compositing: enabled multiple_raster_threads: enabled_on opengl: enabled_on rasterization: enabled raw_draw: disabled_off_ok skia_renderer: enabled_on video_decode: enabled video_encode: enabled vulkan: disabled_off webgl: enabled webgl2: enabled webgpu: disabled_off
Load (avg)	undefined
Memory (System)	15.59GB (3.37GB free)
Process Argv	--crash-reporter-id 7fc99dfd-d84f-49f3-b924-c4f1ecf5ddd2
Screen Reader	no
VM	0%

A/B Experiments

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vscodeenable:30660116
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pythonms35:30701012

[paulacamargo25]

Hi @sergehauri can you try again with the latest version of the Python extension? I tried to replicate the error, but in my case it works fine. Could you confirm that the error persist? Thanks

[sergehauri]

Hi @paulacamargo25
I just tested on another computer with clean, new Python and VS Code installs and can reproduce the error.

Python version: 3.11.3
VS Code: 1.77.3
Python Extension: 2023.6.0, also tested with Prerelease v2023.7.11011538, same result
OS: Windows 11 Pro, Version 10.0.22621 Build 22621
Python packages in venv:

click==8.1.3
colorama==0.4.6
Flask==2.2.3
itsdangerous==2.1.2
Jinja2==3.1.2
MarkupSafe==2.1.2
Werkzeug==2.2.3

There's some additional Debugger warning messages in the output, though I don't think they're of relevance.

(venv) PS G:\My Drive\dev\projects\werkzeugtest>  & 'g:\My Drive\dev\projects\werkzeugtest\venv\Scripts\python.exe' 'c:\Users\serge\.vscode\extensions\ms-python.python-2023.7.11011538\pythonFiles\lib\python\debugpy\adapter/../..\debugpy\launcher' '50153' '--' '-m' 'flask' 'run' '--no-debugger' 
 * Serving Flask app 'app.py'
 * Debug mode: on
WARNING: This is a development server. Do not use it in a production deployment. Use a production WSGI server instead.
 * Running on http://127.0.0.1:5000
Press CTRL+C to quit
 * Restarting with stat
0.03s - Debugger warning: It seems that frozen modules are being used, which may
0.00s - make the debugger miss breakpoints. Please pass -Xfrozen_modules=off
0.00s - to python to disable frozen modules.
0.00s - Note: Debugging will proceed. Set PYDEVD_DISABLE_FILE_VALIDATION=1 to disable this validation.
 * Detected change in 'G:\\My Drive\\dev\\projects\\werkzeugtest\\app.py', reloading
Exception in thread Thread-6 (serve_forever):
Traceback (most recent call last):
  File "C:\Users\serge\AppData\Local\Programs\Python\Python311\Lib\threading.py", line 1038, in _bootstrap_inner
    self.run()
  File "C:\Users\serge\AppData\Local\Programs\Python\Python311\Lib\threading.py", line 975, in run
    self._target(*self._args, **self._kwargs)
  File "g:\My Drive\dev\projects\werkzeugtest\venv\Lib\site-packages\werkzeug\serving.py", line 766, in serve_forever
    super().serve_forever(poll_interval=poll_interval)
  File "C:\Users\serge\AppData\Local\Programs\Python\Python311\Lib\socketserver.py", line 233, in serve_forever
    ready = selector.select(poll_interval)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\serge\AppData\Local\Programs\Python\Python311\Lib\selectors.py", line 323, in select
    r, w, _ = self._select(self._readers, self._writers, [], timeout)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\serge\AppData\Local\Programs\Python\Python311\Lib\selectors.py", line 314, in _select
    r, w, x = select.select(r, w, w, timeout)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
OSError: [WinError 10038] An operation was attempted on something that is not a socket
 * Restarting with stat
0.02s - Debugger warning: It seems that frozen modules are being used, which may
0.00s - make the debugger miss breakpoints. Please pass -Xfrozen_modules=off
0.00s - to python to disable frozen modules.
0.00s - Note: Debugging will proceed. Set PYDEVD_DISABLE_FILE_VALIDATION=1 to disable this validation.

[nimicohgr]

I am experiencing the same issue with my Flask application in VS Code. I am running the same versions as @sergehauri in my app, but to uncomplicate things, I created a new project and followed their reproduction instructions and receive the same traceback when debugging following a reload.

Python version: 3.11.3
VS Code: 1.77.3
Python Extension: 2023.6.0, also tested with Prerelease v2023.7.11011538, same result
OS: Windows 11 Pro, Version 10.0.22621 Build 22621

[paulacamargo25]

Thanks @sergehauri, I will try to reproduce the bug.

[schlp]

facing the same issue, Python3.11.0 and 3.11.4, Flask-2.2.5 and Flask-2.3.2

[starball5]

Related Werkzeug issue ticket: pallets/werkzeug#2627

Related on Stack Overflow:

• Flask app throwing an error when I shut down the app
• VSCode debuging flask app creates Error [WinError 10038] on save/reload
• One from 2017: Running Flask raises "WinError 10038"

[anhhaibkhn]

May I ask if there are any updates on this issue, I have the same problem with Flask 2.3.3, werkzeug 2.3.7, Python 3.11.4

[rwmnau]

Just wanted to add that this is happening to me as well: Flask 3.0.0, Werkzeug 3.0.1, VS Code 1.83.1, and Python extensions 2023.20.0. Same behavior on Python 3.10.6, 3.11.4, and 3.12.0.

Running "flask run --debug" from the command line doesn't have this issue - I can change and save files and it detects the change and restarts with no issue, so it's only while debugging inside VS Code.

[olidel001]

Hi,

Same here.
Exception in thread Thread-1 (serve_forever): Traceback (most recent call last): File "C:\Program Files\WindowsApps\PythonSoftwareFoundation.Python.3.11_3.11.1776.0_x64__qbz5n2kfra8p0\Lib\threading.py", line 1045, in _bootstrap_inner self.run() File "C:\Program Files\WindowsApps\PythonSoftwareFoundation.Python.3.11_3.11.1776.0_x64__qbz5n2kfra8p0\Lib\threading.py", line 982, in run self._target(*self._args, **self._kwargs) File "C:\Users\xxxxxxxxxxxxxxxx\AppData\Local\Packages\PythonSoftwareFoundation.Python.3.11_qbz5n2kfra8p0\LocalCache\local-packages\Python311\site-packages\werkzeug\serving.py", line 806, in serve_forever super().serve_forever(poll_interval=poll_interval) File "C:\Program Files\WindowsApps\PythonSoftwareFoundation.Python.3.11_3.11.1776.0_x64__qbz5n2kfra8p0\Lib\socketserver.py", line 233, in serve_forever ready = selector.select(poll_interval) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "C:\Program Files\WindowsApps\PythonSoftwareFoundation.Python.3.11_3.11.1776.0_x64__qbz5n2kfra8p0\Lib\selectors.py", line 323, in select r, w, _ = self._select(self._readers, self._writers, [], timeout) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "C:\Program Files\WindowsApps\PythonSoftwareFoundation.Python.3.11_3.11.1776.0_x64__qbz5n2kfra8p0\Lib\selectors.py", line 314, in _select r, w, x = select.select(r, w, w, timeout) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ OSError: [WinError 10038]

Thank you

[olidel001]

My python setup and module list are:

`
Flask 3.0.0
Werkzeug 3.0.1
Python 3.11.6

VSCODE:
Version: 1.84.1 (system setup)
Commit: 2b35e1e6d88f1ce073683991d1eff5284a32690f
Date: 2023-11-06T12:35:59.022Z
Electron: 25.9.2
ElectronBuildId: 24603566
Chromium: 114.0.5735.289
Node.js: 18.15.0
V8: 11.4.183.29-electron.0
OS: Windows_NT x64 10.0.19045

Python extension pack: v1.7.0
`

[rwmnau]

After a bit of digging, I think the real issue here is that VS Code Python debugger considers a System.Exit with any code other than zero to be an exception and there's no way (currently) to change this functionality (other than ignoring all exceptions, which isn't ideal). When it's running without a debugger, the child process executes the System.Exit(3) and it's caught by the outer process and triggers a restart of the server, all before the other exception (the socket issue) is exposed - but when the debugger is attached (either launched from inside VS Code or launched separately and then attached from VS Code), it stops on the Exit and also allows the other exception to be exposed instead of swallowed. I messed around a bit and can't get code that both swallows this exception and allows the server to restart on file changes as expected.

StackOverflow Post (with people checking in every year since 2018 with "Still not fixed"): https://stackoverflow.com/questions/52372810/visual-studio-code-python-debugging-exception-has-occurred-systemexit

Open DebugPy issue: #1306

[nour-s]

2024, I have the latest of everything from VSCode to Python 3.11 to Flask 3 and even Werkzeug 3.0.2 yet I have the exact issue.

Did all suggestions for Winsock2 fixes and nothing changed.

netsh winsock reset catalog
netsh int ipv4 reset reset.log
netsh int ipv6 reset reset.log

[danielniccoli]

@rwmnau @sergehauri @nimicohgr @olidel001

The behavior of debugpy is not in compliance with the specifications. Because the underlying issue is broader, I raised a separate ticket on the matter, which explains the problem and suggests a solution that should not only fix the issue you are having, but other issues as well. Unforatunately, it was closed and moved to a discussion. Please contribute, if you wish to see it fixed.

The issue: #1590
The discussion: #1591