【並列処理】ライブロック

状態に応じて処理を変えているが、これも結局、両方のスレッドが待ち状態になってしまい、ロックがかかってしまう。
一般的に、starvation(飢饉状態)という。

import time
from threading import Thread
from lock_with_name import LockWithName
dumplings = 20

class Philosopher(Thread):
    def __init__(self, name: str,
            left_chopstick: LockWithName,
            right_chopstick: LockWithName):
        super().__init__()
        self.name = name
        self.left_chopstick = left_chopstick
        self.right_chopstick = right_chopstick

    def run(self) -> None:
        global dumplings

        while dumplings > 0:
            self.left_chopstick.acquire()
            print(f"{self.left_chopstick.name} chopstick "
                f"grabbed by {self.name}")
            if self.right_chopstick.locked():
                print(f"{self.name} can not get the "
                    f"{self.right_chopstick.name} chopstick, "
                    f"politely concedes...")
            else:
                self.right_chopstick.acuire()
                print(f"{self.right_chopstick.name} chopstick "
                f"grabbed by {self.name}")
                dumplings -= 1
                print(f"{self.name} eats dumpling. Dumplings "
                    f"left: {dumplings}")
                time.sleep(1)
                self.right_chopstick.release()
            self.left_chopstick.release()

if __name__ == "__main__":
    chopstick_a = LockWithName("chopstick_a")
    chopstick_b = LockWithName("chopstick_b")

    philosopher_1 = Philosopher("Philosopher #1", chopstick_a, chopstick_b)
    philosopher_2 = Philosopher("Philosopher #2", chopstick_b, chopstick_a)

    philosopher_1.start()
    philosopher_2.start()

Philosopher #2 can not get the chopstick_a chopstick, politely concedes…
chopstick_b chopstick grabbed by Philosopher #2
Philosopher #2 can not get the chopstick_a chopstick, politely concedes…
chopstick_b chopstick grabbed by Philosopher #2
Philosopher #2 can not get the chopstick_a chopstick, politely concedes…
Philosopher #1 can not get the chopstick_b chopstick, politely concedes…
chopstick_a chopstick grabbed by Philosopher #1
Philosopher #1 can not get the chopstick_b chopstick, politely concedes…
chopstick_a chopstick grabbed by Philosopher #1
Philosopher #1 can not get the chopstick_b chopstick, politely concedes…
chopstick_a chopstick grabbed by Philosopher #1
Philosopher #1 can not get the chopstick_b chopstick, politely concedes…
chopstick_a chopsti^C
…