concurrent.interpreters Queue truncates fractional timeouts with int() (and accepts negative fractional timeouts) #160949

Bug report

Bug description

The cross-interpreter Queue returned by
concurrent.interpreters.create_queue() documents queue.Queue semantics
for its put()/get() timeouts — the docs say that if timeout is a
positive number "it blocks at most timeout seconds"
(Doc/library/concurrent.interpreters.rst).
However, both methods convert the timeout with int(timeout) before
computing the deadline (Lib/concurrent/interpreters/_queues.py), which
truncates fractional timeouts and defeats the negative-timeout check:

import time
from concurrent import interpreters

q = interpreters.create_queue()

def timed(fn, label):
    start = time.monotonic()
    try:
        fn()
    except Exception as e:
        print(f"{label}: raised {type(e).__name__} "
              f"after {time.monotonic() - start:.2f}s")
    else:
        print(f"{label}: returned after {time.monotonic() - start:.2f}s")

timed(lambda: q.get(timeout=0.5),  "get(timeout=0.5)")
timed(lambda: q.get(timeout=1.9),  "get(timeout=1.9)")
timed(lambda: q.get(timeout=-0.5), "get(timeout=-0.5)")

Output on 3.16.0a1+ (main @ 83fe630):

get(timeout=0.5):  raised QueueEmpty after 0.00s   # expected: ~0.50s
get(timeout=1.9):  raised QueueEmpty after 1.01s   # expected: ~1.90s
get(timeout=-0.5): raised QueueEmpty after 0.00s   # expected: ValueError

put() on a full queue behaves the same way. For comparison,
queue.Queue.get(timeout=0.5) waits the full 0.50s and
queue.Queue.get(timeout=-0.5) raises
ValueError: 'timeout' must be a non-negative number immediately.

The truncation also explains why the existing tests did not catch this:
they use timeout=0.1, which truncates to 0 and still raises, just
0.1s too early.

Two knock-on effects of the int() conversion: timeout='1' is silently
accepted (stdlib raises TypeError from the < 0 comparison), and
timeout=float('inf') / float('nan') leak int()'s
OverflowError / ValueError with messages about integer conversion
rather than anything timeout-related.

Expected behaviour

Fractional timeouts are honoured like queue.Queue: the call blocks for
approximately the requested duration; any negative timeout (including
fractional ones such as -0.5) raises ValueError before waiting;
timeout=0 remains a non-blocking poll.

I have a fix and regression tests ready and will open a PR.

CPython versions tested on

CPython main (3.16.0a1+dev, 83fe630), built locally on Linux x86-64.

CPython versions tested on:

3.16

Operating systems tested on:

No response

Linked PRs

• gh-160949: Fix concurrent.interpreters Queue truncating fractional timeouts #160951