time.tzset() races on process-global timezone state

[djarfluka]

Bug report

Bug description:

import time, os, threading
N=8; ITERS=5000
barrier=threading.Barrier(N)
def setter():
    barrier.wait()
    for i in range(ITERS):
        os.environ['TZ']='UTC0' if i&1 else 'EST5EDT'
        time.tzset()
def reader():
    barrier.wait()
    for _ in range(ITERS):
        time.localtime(); 
        try: time.strftime('%Z')
        except Exception: pass
ts=[threading.Thread(target=setter) for _ in range(N//2)]+[threading.Thread(target=reader) for _ in range(N//2)]
[t.start() for t in ts]; [t.join() for t in ts]
print("done tzset")

WARNING: ThreadSanitizer: data race
  Read of size 4 by thread T3:
    #0 strncpy
    #1 get_zone       Modules/timemodule.c:1773
    #2 init_timezone  Modules/timemodule.c:1864
    #3 time_tzset     Modules/timemodule.c:1182
  Previous write of size 8 by thread T1 (malloc, in another time_tzset)
  Location is heap block of size 20 allocated by thread T1
(also: read at tmtotuple Modules/timemodule.c:490 — localtime/gmtime struct_time builder)

(truncated sanitizer output)

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux