Breakpoints don't work in threads created after `debugpy.listen()` is called

[samfrances]

Environment data

• debugpy version: 1.8.14 / 1.8.17
• OS and version: Mac OS 15.6.1
• Python version (& distribution if applicable, e.g. Anaconda): 3.10.12
• Using VS Code or Visual Studio: Visual Studio Code Version: 1.104.1

Actual behavior

In threads which are started (via threading.Thread.start()) after the call to debugpy.listen(), breakpoints will not work.

Expected behavior

Breakpoints should work in all threads, regardless of when they are started.

Steps to reproduce:

1. Use the following script:

# multithread_debugging.py

import sys
import threading
from queue import Queue

import debugpy

threads: list[threading.Thread] = []
queues: list[Queue] = []

N_THREADS = 10
POISON_PILL = -1


def handler(q: Queue):
    while True:
        item = q.get()
        print(threading.current_thread(), item)
        if item == POISON_PILL:
            print(threading.current_thread(), "exiting")
            return


def send(i: int, item):
    queues[i].put(item)


def end(i: int, *, wait=False):
    send(i, POISON_PILL)
    if wait:
        threads[i].join()


def end_all():
    for i in range(N_THREADS):
        end(i)
    for t in threads:
        t.join()
    print("all done")


if __name__ == "__main__":
    start_listener_at_index = 0
    if len(sys.argv) >= 2:
        start_listener_at_index = int(sys.argv[1])

    for i in range(10):
        q = Queue()
        t = threading.Thread(target=handler, args=[q])
        threads.append(t)
        queues.append(q)

    for i, t in enumerate(threads):
        if i == start_listener_at_index:
            print(f"starting debug listener before thread {i} starts")
            debugpy.listen(("0.0.0.0", 5680))
        t.start()

2. Add the following launch configuration:

    {
      "name": "Attach to multithreading example",
      "type": "debugpy",
      "request": "attach",
      "connect": {
        "host": "localhost",
        "port": 5680
      }
    },

3. Run python -i multithread_debugging.py 5
4. Add a breakpoint on the print statement in the handler() method.
5. Run the "Attach to multithreading example" debug configuration.
6. In the REPL, run: send(0, "foo"). Note that the breakpoint is not triggered.
7. In the REPL, run: send(4, "foo"). Note that the breakpoint is not triggered.
8. In the REPL, run send(5, "foo"). Note that the breakpoint IS triggered (because thread 5 is started after the debugpy listen call).

Further notes

If you set the breakpoint using the breakpoint() function, it works in all threads. However, if you set it using the vscode GUI, you get the buggy behaviour described above.

[rchiodo]

This works for me. What's your version of debugpy in your environment?

[rchiodo]

Might also depend on version of Python too. I'm using 3.13 to test this.

[rchiodo]

Works with 3.11 for me too. I'm using debugpy in the env of 1.8.17

[samfrances]

Ok, maybe a Python 3.10 issue then. I tried this with debugpy 1.8.14 and 1.8.17.