Random hangs that can't be interrupted by Ctrl+C (not even in pdb)

gistfile1.py

#!/usr/bin/python

import sys
import csv
import Queue
import threading


max_threads = 30
# input CSV
infile=str(sys.argv[1])
# output CSV
outfile=str(sys.argv[2])
# amount of rows in input CSV
row_count = 0
# amount of rows processed until now
rows = 0

###############################################################################

def worker():
    """thread worker"""
    while True:       
        worker_data = queue.get()
        # empty queue?
        if worker_data is None:
            return
            
        writer = worker_data['writer']
        row = worker_data['row']
                       
                
        # process row
        # ...

        # build result
        result_row = ...

        # write immediately to not loose anything
        writer.writerows(result_row)
            
        # done
        queue.task_done()

###############################################################################


# initialize queue
queue = Queue.Queue()


with open(infile, 'r') as f_in, open(outfile, 'w') as f_out:
    # CSV writer
    writer = csv.writer(f_out, delimiter='|', quotechar='\'')
    # CSV reader
    csv_fields=['a', 'b', 'c', 'd', 'e']
    r = csv.DictReader(f_in, fieldnames=csv_fields, delimiter='|', quotechar='\'')
    
    # get amount of input rows
    row_count = sum(1 for row in r)
    # seek back to start
    f_in.seek(0)

    
    # launch thread pool
    for i in range(max_threads):
        t = threading.Thread(target=worker)
        t.daemon = True
        t.start()
        
    # walk all rows from input
    for row in r:
        # put row + writer in queue
        work_data = { 'writer' : writer, 'row' : row }
        queue.put(work_data)
        # don't bang the queue so we can still handle SIGINT
        time.sleep(0.1)

    queue.join()