#!/usr/bin/env python3

import sqlite3
import time
import threading

DB_FILE = "foo.db"

# There are multiple ways to fix this bug
# set either or both of the following constants to fix
FIX_0 = False
FIX_1 = False
FIX_2 = True

conn = sqlite3.connect(DB_FILE, timeout=60)
cursor = conn.cursor()

# Init table and add some sample values
cursor.execute("create table if not exists t (k int primary key, v int)")
with conn:
    cursor.execute("insert or ignore into t (k, v) values (1, 1)")

def fn():
    # NB: we use a large timeout but it does *nothing(
    conn = sqlite3.connect(DB_FILE, timeout=60)
    with conn:
        cursor = conn.cursor()
        cursor2 = conn.cursor()
        if FIX_0:
            # This line fixes the code because an DML (like update)
            # causes the sqlite module to start a transaction via "BEGIN"
            cursor2.execute("update t set v = 3 where k = ?", (2,))
        # This statement will cause SQLite to acquire a SHARED lock
        cursor.execute("select k from t")
        if FIX_1:
            # If FIX_0 was not enabled and we aren't currently in a transaction
            # then this will release the SHARED lock on the database by completely
            # stepping the SQLite select statement, thus allowing the following
            # write transaction to acquire a lock
            cursor.fetchall()
        if FIX_2:
            cursor.close()
        cursor2.execute("update t set v = 3 where k = ?", (2,))


threads = []
for _ in range(10):
    threads.append(threading.Thread(target=fn))
    threads[-1].start()

for thr in threads:
    thr.join()