deadlock.py

# This is pseudocode.  Adapted from https://en.wikipedia.org/wiki/Producer%E2%80%93consumer_problem
#
# Currrent design:
#
# A producer places items into a queue.
# A consumer removes items from the queue.
#
# 1. If the queue becomes empty, the consumer sleeps; until woken up later.
# 2. If the queue becomes one-more-than-empty, the producer wakes up the consumer.
# 3. If the queue becomes full, the producer sleeps; until woken up later.
# 4. If the queue becomes one-less-than-full, the consumer wakes up the producer.
#
# Can you spot the bug?

int itemCount = 0
BUFFER_SIZE = 10

def consumer()
    while (true):

        if (itemCount == 0):
            sleep() # waits for wakeup() call from producer below

        item = removeItemFromBuffer()
        itemCount = itemCount - 1

        if (itemCount == BUFFER_SIZE - 1):
            wakeup(producer)

        consumeItem(item)

def producer()
    while (True):
        item = produceItem()

        if (itemCount == BUFFER_SIZE):
            sleep()   # waits for wakeup() call from consumer above

        putItemIntoBuffer(item)
        itemCount = itemCount + 1

        if (itemCount == 1):
            wakeup(consumer)