칼과 도마: 데드락

import threading
import time

knife = threading.Lock()
board = threading.Lock()

def minho():
    with knife:              # 칼을 든다
        time.sleep(0.1)
        with board:          # 도마를 기다린다
            print("minho done")

def sua():
    with board:              # 도마를 든다
        time.sleep(0.1)
        with knife:          # 칼을 기다린다
            print("sua done")

a = threading.Thread(target=minho, daemon=True)
b = threading.Thread(target=sua, daemon=True)
a.start(); b.start()
a.join(timeout=1); b.join(timeout=1)
print("alive:", a.is_alive(), b.is_alive())
# alive: True True

knife = threading.Lock()   # 새 칼과 새 도마
board = threading.Lock()

def cook(name, first, second, patience):
    with first:
        time.sleep(0.1)
        if second.acquire(timeout=patience):   # patience초까지만 기다린다
            second.release()
            print(name, "done")
        else:
            print(name, f"gave up after {patience}s")

a = threading.Thread(target=cook, args=("minho", knife, board, 1.0))
b = threading.Thread(target=cook, args=("sua", board, knife, 0.5))
a.start(); b.start()
a.join(); b.join()
# sua gave up after 0.5s
# minho done

from contextlib import contextmanager

KNIFE = (1, threading.Lock())   # (번호, 잠금)
BOARD = (2, threading.Lock())

@contextmanager
def hold(*tools):
    ordered = sorted(tools, key=lambda t: t[0])   # 번호 작은 것부터
    for _, lock in ordered:
        lock.acquire()
    try:
        yield
    finally:
        for _, lock in reversed(ordered):        # 놓을 때는 거꾸로
            lock.release()

done = []

def minho():
    for _ in range(100):
        with hold(KNIFE, BOARD):
            time.sleep(0)
    done.append("minho")

def sua():
    for _ in range(100):
        with hold(BOARD, KNIFE):     # 적는 순서는 달라도 잡는 순서는 같다
            time.sleep(0)
    done.append("sua")

ts = [threading.Thread(target=f, daemon=True) for f in (minho, sua)]
for t in ts: t.start()
for t in ts: t.join(timeout=2)
print(sorted(done), [t.is_alive() for t in ts])
# ['minho', 'sua'] [False, False]

import random

tries = {}

def polite_cook(name, first, second):
    tries[name] = 0
    while True:
        tries[name] += 1
        with first:
            time.sleep(0.05)
            if second.acquire(timeout=0.1):     # 0.1초 안에 못 잡으면
                try:
                    return                       # 둘 다 잡았다: 요리 끝
                finally:
                    second.release()
        # with를 벗어나며 first를 내려놓았다
        time.sleep(random.uniform(0, 0.1))       # 무작위로 쉬었다가 다시

a = threading.Thread(target=polite_cook, args=("minho", knife, board))
b = threading.Thread(target=polite_cook, args=("sua", board, knife))
a.start(); b.start()
a.join(); b.join()
print(tries)
# 예: {'minho': 2, 'sua': 3}  (실행할 때마다 다르다)

N = 5

def service(ordered):
    knives = [threading.Lock() for _ in range(N)]   # 칼 다섯 자루
    meals = [0] * N

    def cook(i):
        left, right = i, (i + 1) % N
        if ordered:
            first, second = min(left, right), max(left, right)
        else:
            first, second = left, right
        for _ in range(3):
            with knives[first]:
                time.sleep(0.05)          # 첫 칼을 든 채 잠깐
                with knives[second]:
                    meals[i] += 1

    ts = [threading.Thread(target=cook, args=(i,), daemon=True) for i in range(N)]
    for t in ts: t.start()
    for t in ts: t.join(timeout=1)
    return sum(t.is_alive() for t in ts), meals

print("left first:", service(ordered=False))
print("small first:", service(ordered=True))
# left first: (5, [0, 0, 0, 0, 0])
# small first: (0, [3, 3, 3, 3, 3])

import faulthandler
import threading
import time

knife = threading.Lock()
board = threading.Lock()

def minho():
    with knife:
        time.sleep(0.1)
        with board:
            pass

def sua():
    with board:
        time.sleep(0.1)
        with knife:
            pass

faulthandler.dump_traceback_later(1)        # 1초 뒤에도 여기 있으면 모든 스레드를 출력
crew = [threading.Thread(target=f, name=f.__name__, daemon=True) for f in (minho, sua)]
for t in crew: t.start()
for t in crew: t.join(timeout=2)
faulthandler.cancel_dump_traceback_later()  # 제때 끝났다면 예약을 지운다

Timeout (0:00:01)!
Thread 0x00009560 [sua] (most recent call first):
  File "stuck.py", line 17 in sua
  ...
Thread 0x0000a670 [minho] (most recent call first):
  File "stuck.py", line 11 in minho
  ...

PAN = (3, threading.Lock())
meals = {}

def cook(name, *tools):
    for _ in range(100):
        with hold(*tools):            # 언제나 번호 작은 도구부터
            time.sleep(0)
            meals[name] = meals.get(name, 0) + 1

crew = [threading.Thread(target=cook, args=a, daemon=True) for a in [
    ("minho", KNIFE, BOARD), ("sua", BOARD, KNIFE), ("tak", PAN, BOARD, KNIFE)]]
for t in crew: t.start()
for t in crew: t.join(timeout=3)
print([meals[n] for n in ("minho", "sua", "tak")], sum(t.is_alive() for t in crew))
# [100, 100, 100] 0