Observed Behavior: pyodbc.drivers() not thread safe

[ourcookiemonster]

pyodb.drivers() method can return an incomplete list of drivers if multiple threads call this method at the same time.

Based on the ODBC SQLDrivers method API, the c++ code must iterate through a list of drivers, specifying the start or the next available driver. If two threads have entered the for loop of mod_drivers() in pyodbcmodule.cpp one or both of the threads may miss a driver that the other thread has incremented through.

[ourcookiemonster]

It is likely mod_datasources() suffers the same behavior.

[ourcookiemonster]

import threading
import pyodbc

true_driver_count = len(pyodbc.drivers())
disparity_found = False

def test_drivers():
    global disparity_found
    while not disparity_found:
        thread_driver_count = len(pyodbc.drivers())
        if(thread_driver_count < true_driver_count):
            print('Driver Count Difference Detected')            
            disparity_found = True;

if __name__ == "__main__":
    thread_one = threading.Thread(target=test_drivers)
    thread_two = threading.Thread(target=test_drivers)

    thread_one.start()
    thread_two.start()

    thread_one.join()
    thread_two.join()

[v-makouz]

I can reproduce and will look into it

[v-makouz]

I made a PR that should fix it:
#722

I ran your repro and after several million iterations there were no mismatches

[keitherskine]

Fixed by #722.