Redundant process spawning when watching

[BHznJNs]

Description

I tried to use task to watch my FastAPI project.

The task script is like (since this project can not automatically kill the process, I used cleanup_port.py to do that):

  dev:
    deps: [install-dev]
    watch: true
    sources:
      - src/**
      - entry.py
    cmds:
      - uv run python scripts/cleanup_port.py 1460
      - uv run python entry.py

My entry.py points to my main.py, and this is how it look like:

# main.py
import argparse
import uvicorn
from loguru import logger
from .api import app

def main():
    parser = argparse.ArgumentParser()
    parser.add_argument("--port", type=int, default=1460)
    args = parser.parse_args()

    logger.info("Starting server on port {}", args.port)
    uvicorn.run(app, host="127.0.0.1", port=args.port) # no hot reload here

And here is the logs:

> task dev
task: Started watching for tasks: dev
task: Task "install-dev" is up to date
task: [dev] uv run python scripts/cleanup_port.py 1460
2026-02-02 23:54:38.913 | INFO     | __main__:kill_port_process:48 - Port 1460 is free, no need to clean
task: [dev] uv run python entry.py
INFO  [alembic.runtime.migration] Context impl SQLiteImpl.
INFO  [alembic.runtime.migration] Will assume non-transactional DDL.
2026-02-02 23:54:42.953 | INFO     | src.main:main:13 - Starting server on port 1460
INFO:     Started server process [19260]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://127.0.0.1:1460 (Press CTRL+C to quit) # I changed file here
task: Failed to run task "dev": exit status 1
task: Task "install-dev" is up to date
task: [dev] uv run python scripts/cleanup_port.py 1460
2026-02-02 23:55:02.488 | WARNING  | __main__:try_kill_process:11 - Killing process 19260 (python.exe)...
2026-02-02 23:55:02.514 | SUCCESS  | __main__:kill_port_process:43 - Process 19260 killed successfully
task: [dev] uv run python entry.py
task: Failed to run task "dev": exit status 1
task: Task "install-dev" is up to date
task: Task "install-dev" is up to date
task: [dev] uv run python scripts/cleanup_port.py 1460
task: [dev] uv run python scripts/cleanup_port.py 1460
2026-02-02 23:55:05.070 | INFO     | __main__:kill_port_process:48 - Port 1460 is free, no need to clean
2026-02-02 23:55:05.104 | INFO     | __main__:kill_port_process:48 - Port 1460 is free, no need to clean
task: [dev] uv run python entry.py
task: [dev] uv run python entry.py
INFO  [alembic.runtime.migration] Context impl SQLiteImpl.
INFO  [alembic.runtime.migration] Will assume non-transactional DDL.
2026-02-02 23:55:07.277 | INFO     | src.main:main:13 - Starting server on port 1460
INFO:     Started server process [16408]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://127.0.0.1:1460 (Press CTRL+C to quit)
INFO  [alembic.runtime.migration] Context impl SQLiteImpl.
INFO  [alembic.runtime.migration] Will assume non-transactional DDL.
2026-02-02 23:55:11.145 | INFO     | src.main:main:13 - Starting server on port 1460
INFO:     Started server process [20536]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
ERROR:    [Errno 10048] error while attempting to bind on address ('127.0.0.1', 1460): [winerror 10048] only one usage of each socket address (protocol/network address/port) is normally permitted
INFO:     Waiting for application shutdown.
INFO:     Application shutdown complete.
INFO  [alembic.runtime.migration] Context impl SQLiteImpl.
INFO  [alembic.runtime.migration] Will assume non-transactional DDL.
2026-02-02 23:55:11.688 | INFO     | src.main:main:13 - Starting server on port 1460
INFO:     Started server process [25608]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
ERROR:    [Errno 10048] error while attempting to bind on address ('127.0.0.1', 1460): [winerror 10048] only one usage of each socket address (protocol/network address/port) is normally permitted
INFO:     Waiting for application shutdown.
INFO:     Application shutdown complete.
task: Failed to run task "dev": exit status 1
task: Failed to run task "dev": exit status 1
task: Signal received: "interrupt"

Version

3.48.0

Operating system

Windows 11 25H2

Experiments Enabled

No response

Example Taskfile

version: '3'

tasks:
  install-dev:
    sources:
      - pyproject.toml
    generates:
      - .venv/**
    cmds:
      - uv sync --group dev

  dev:
    deps: [install-dev]
    watch: true
    sources:
      - src/**
      - entry.py
    cmds:
      - uv run python scripts/cleanup_port.py 1460
      - uv run python entry.py

[trulede]

It looks a bit odd. Perhaps add a sleep 5 as the first command, and a second between the commands, to see if that calms things down a bit. And make sure these commands really are executing in discrete steps, or perhaps interacting between restarts, which creates the odd behaviour.

[BHznJNs]

I added sleeps and removed deps like this:

  dev:
    watch: true
    sources:
      - src/**
      - entry.py
    cmds:
      - sleep 5
      - uv run python scripts/cleanup_port.py 1460
      - sleep 5
      - uv run python entry.py

And in the logs:

> task dev
task: Started watching for tasks: dev
task: [dev] sleep 5
task: [dev] uv run python scripts/cleanup_port.py 1460
2026-02-03 10:57:50.498 | INFO     | __main__:kill_port_process:48 - Port 1460 is free, no need to clean
task: [dev] sleep 5
task: [dev] uv run python entry.py
INFO  [alembic.runtime.migration] Context impl SQLiteImpl.
INFO  [alembic.runtime.migration] Will assume non-transactional DDL.
2026-02-03 10:57:59.595 | INFO     | src.main:main:13 - Starting server on port 1460
INFO:     Started server process [21520]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://127.0.0.1:1460 (Press CTRL+C to quit)
task: Failed to run task "dev": exit status 1
task: [dev] sleep 5
task: [dev] uv run python scripts/cleanup_port.py 1460
2026-02-03 10:58:14.308 | WARNING  | __main__:try_kill_process:11 - Killing process 21520 (python.exe)...
2026-02-03 10:58:14.331 | SUCCESS  | __main__:kill_port_process:43 - Process 21520 killed successfully
task: [dev] sleep 5
task: [dev] uv run python entry.py
task: Failed to run task "dev": exit status 1
task: [dev] sleep 5
task: [dev] sleep 5
INFO  [alembic.runtime.migration] Context impl SQLiteImpl.
INFO  [alembic.runtime.migration] Will assume non-transactional DDL.
2026-02-03 10:58:23.551 | INFO     | src.main:main:13 - Starting server on port 1460
INFO:     Started server process [24780]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://127.0.0.1:1460 (Press CTRL+C to quit)
task: [dev] uv run python scripts/cleanup_port.py 1460
task: [dev] uv run python scripts/cleanup_port.py 1460
2026-02-03 10:58:25.052 | WARNING  | __main__:try_kill_process:11 - Killing process 24780 (python.exe)...
2026-02-03 10:58:25.072 | INFO     | __main__:kill_port_process:48 - Port 1460 is free, no need to clean
2026-02-03 10:58:25.075 | SUCCESS  | __main__:kill_port_process:43 - Process 24780 killed successfully
task: [dev] sleep 5
task: [dev] sleep 5
task: [dev] uv run python entry.py
task: [dev] uv run python entry.py
INFO  [alembic.runtime.migration] Context impl SQLiteImpl.
INFO  [alembic.runtime.migration] Will assume non-transactional DDL.
2026-02-03 10:58:34.209 | INFO     | src.main:main:13 - Starting server on port 1460
INFO:     Started server process [13444]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://127.0.0.1:1460 (Press CTRL+C to quit)
INFO  [alembic.runtime.migration] Context impl SQLiteImpl.
INFO  [alembic.runtime.migration] Will assume non-transactional DDL.
2026-02-03 10:58:35.197 | INFO     | src.main:main:13 - Starting server on port 1460
INFO:     Started server process [26164]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
ERROR:    [Errno 10048] error while attempting to bind on address ('127.0.0.1', 1460): [winerror 10048] only one usage of each socket address (protocol/network address/port) is normally permitted
INFO:     Waiting for application shutdown.
INFO:     Application shutdown complete.
task: Failed to run task "dev": exit status 1

We can see that even the sleep command also be executed double.

What's more wired, I tested another temp testing command:

  test_script:
    watch: true
    sources:
      - src/**
      - test.py
    cmds:
      - uv run python scripts/cleanup_port.py 1460
      - uv run python test.py

# test.py
print("test script")

And it just works fine:

> task test_script
task: Started watching for tasks: test_script
task: [test_script] uv run python scripts/cleanup_port.py 1460
2026-02-03 11:02:08.845 | INFO     | __main__:kill_port_process:48 - Port 1460 is free, no need to clean
task: [test_script] uv run python test.py
test script
task: task "test_script" finished running
task: Task "test_script" is up to date # saved file without modification
task: task "test_script" finished running
task: Task "test_script" is up to date
task: task "test_script" finished running
task: Task "test_script" is up to date
task: task "test_script" finished running
task: Task "test_script" is up to date
task: task "test_script" finished running
task: Task "test_script" is up to date
task: task "test_script" finished running
task: [test_script] uv run python scripts/cleanup_port.py 1460 # saved file with modification
2026-02-03 11:03:09.765 | INFO     | __main__:kill_port_process:48 - Port 1460 is free, no need to clean
task: [test_script] uv run python test.py
test script
task: task "test_script" finished running
task: [test_script] uv run python scripts/cleanup_port.py 1460
2026-02-03 11:03:14.462 | INFO     | __main__:kill_port_process:48 - Port 1460 is free, no need to clean
task: [test_script] uv run python test.py
test script
task: task "test_script" finished running

[trulede]

@BHznJNs Your test.py would print and then immediately exit. Also, not using any port. So there is nothing to kill.

You might try have your test.py sleep (for a long time), and see if it still gets kill cleanly (by task or your other script).

You see here, on the first restart, after your cleanup runs and a 5 second sleep, the entry.py does not even start:
task: [dev] sleep 5
task: [dev] uv run python entry.py
task: Failed to run task "dev": exit status 1

Very strange. See about making test.py sleep a bit, to see if somehow the cleanup is not working, and then I guess we have to wonder what that uvicorn might be doing.

[BHznJNs]

It's really strange. I created a minimal repo and try to reproduce, the problem does not appear again. I will try more later.

---

But there is still port occupation problem when using task with uvicorn.
With this python script:

import uvicorn
from fastapi import FastAPI

app = FastAPI()
uvicorn.run(app, host="127.0.0.1", port=1490)

And taskfile:

  uvicorn:
    watch: true
    sources:
      - ./*.py
    cmds:
      - sleep 5
      - uv run python uvicorn_test.py

And here is log:

task: Started watching for tasks: uvicorn
task: [uvicorn] sleep 5
task: Failed to run task "uvicorn": exit status 1
task: [uvicorn] sleep 5
task: [uvicorn] uv run python uvicorn_test.py
INFO:     Started server process [8520]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://127.0.0.1:1490 (Press CTRL+C to quit)
task: Failed to run task "uvicorn": exit status 1
task: [uvicorn] sleep 5
task: [uvicorn] uv run python uvicorn_test.py
INFO:     Started server process [20000]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
ERROR:    [Errno 10048] error while attempting to bind on address ('127.0.0.1', 1490): [winerror 10048] only one usage of each socket address (protocol/network address/port) is normally permitted
INFO:     Waiting for application shutdown.
INFO:     Application shutdown complete.
task: Failed to run task "uvicorn": exit status 1

---

By the way, the uvicorn port occupation problem might be caused by the uvicorn side, since I also tried watchdog to watch and restart, it still appear, and I have to add port detect and killing logic at the start of my project.

[trulede]

@BHznJNs do you have latest version of uvicorn installed (at least 0.34). It has had some similar problems in the past.

Kludex/uvicorn#2281
Kludex/uvicorn#2547
mxcube/video-streamer@f59b44f

And perhaps see the answer here:
https://stackoverflow.com/questions/60424390/is-there-a-way-to-kill-uvicorn-cleanly

[BHznJNs]

Very thanks for the references you provided! I uses the latest 0.40.0 of uvicorn in my project.

---

I retried to use the watch function of taskfile like this:

  dev:
    deps: [install-dev]
    watch: true
    sources:
      - src/**/*.py
      - pyproject.toml
    cmds:
      - uv run python entry.py

And it strangely worked! Here is my main.py now:

import argparse
import uvicorn
from loguru import logger
from . import IS_DEV
from .api import app
from .db import migrate_db

def prevent_port_occupancy(port: int):
    import time
    import psutil

    def try_kill(proc: psutil.Process):
        if proc.pid <= 4:
            # system critical process, skip
            return

        logger.info(f"Port {port} is occupied by process {proc.info["pid"]}, try killing...")
        try:
            proc.terminate()
            _, alive = psutil.wait_procs([proc], timeout=3)
            if alive: proc.kill()
        except (psutil.NoSuchProcess, psutil.AccessDenied, psutil.ZombieProcess):
            pass
        except Exception as e:
            logger.error(f"Error while killing process: {e}")

    for proc in psutil.process_iter(["pid", "name"]):
        try:
            for conn in proc.net_connections(kind="inet"):
                if conn.laddr.port == port:
                    try_kill(proc)
        except (psutil.NoSuchProcess, psutil.AccessDenied):
            continue
    time.sleep(1)

def main():
    parser = argparse.ArgumentParser()
    parser.add_argument("--port", type=int, default=1460)
    args = parser.parse_args()
    migrate_db()

    if IS_DEV:
        prevent_port_occupancy(args.port)
    uvicorn.run(app, host="127.0.0.1", port=args.port)

I will re-report about the redundant problem when I meet it again.

---

But I found two extra small issue:
The one is the parameter run: always seems not working while use with watch: true, with the taskfile I provided above, it always wait the file change and then start the server. But since #1763, it still completely work in my project.
The another one is the task exit machanism sometimes detects wrong task exit, like the logs below (this can not be stably reproduced and I dont know why, sorry):

> task dev
task: Started watching for tasks: dev
task: Task "install-dev" is up to date
task: [dev] uv run python entry.py
task: Failed to run task "dev": exit status 1 # <-- see this line !!!
INFO  [alembic.runtime.migration] Context impl SQLiteImpl.
INFO  [alembic.runtime.migration] Will assume non-transactional DDL.
INFO:     Started server process [28340]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://127.0.0.1:1460 (Press CTRL+C to quit)
task: Task "install-dev" is up to date
task: [dev] uv run python entry.py
task: Failed to run task "dev": exit status 1
INFO  [alembic.runtime.migration] Context impl SQLiteImpl.
INFO  [alembic.runtime.migration] Will assume non-transactional DDL.
2026-02-07 23:24:51.226 | INFO     | src.main:try_kill:17 - Port 1460 is occupied by process 28340, try killing...
INFO:     Started server process [27752]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://127.0.0.1:1460 (Press CTRL+C to quit)
task: Task "install-dev" is up to date
task: [dev] uv run python entry.py
task: Failed to run task "dev": exit status 1
INFO  [alembic.runtime.migration] Context impl SQLiteImpl.
INFO  [alembic.runtime.migration] Will assume non-transactional DDL.
2026-02-07 23:25:01.898 | INFO     | src.main:try_kill:17 - Port 1460 is occupied by process 27752, try killing...
INFO:     Started server process [19068]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://127.0.0.1:1460 (Press CTRL+C to quit)
task: Signal received: "interrupt"

[trulede]

Task just informs you of the error code that your application is returning. There is no magic there (on the side of Task) so unless to can point to something specific?

I just opened a PR for the other issue. It mentions a workaround ... there are lots of PRs open at the moment so I don't know when it gets reviewed.

You might like to test it.