gh-154836: Fix Popen.wait() with very large timeouts on the pidfd/kqueue wait paths

[calvinrp]

Body

Fixes the 3.15 regression where subprocess.Popen.wait(timeout=...) (and
subprocess.run(..., timeout=...)) fails for very large timeout values such
as float("inf"), sys.maxsize or 1e10 — all accepted on 3.14 and
earlier. The event-driven wait introduced by gh-83069 (#144047) passes the
caller's timeout unclamped to poll() / kqueue.control(); values that do
not fit the C timestamp conversion raise OverflowError on Linux and, on
macOS/BSD, a misleading TypeError: timeout must be a real number or None, not float.

Changes

• Lib/subprocess.py — both _wait_pidfd() and _wait_kqueue() now
  clamp each OS-level wait to a new module constant
  _MAXIMUM_WAIT_TIMEOUT = 24 * 3600 (following asyncio's
  MAXIMUM_SELECT_TIMEOUT precedent, Lib/asyncio/base_events.py) and loop
  until the caller's real deadline, so TimeoutExpired is still raised at
  the right time and huge-but-legal timeouts never reach the C conversion.
  In _wait_kqueue() the kevent changelist is only submitted on the first
  control() call (the KQ_EV_ADD | KQ_EV_ONESHOT registration persists
  across timed-out waits).
• Modules/selectmodule.c — the kqueue control() timeout-conversion
  failure is now only rewritten into the "timeout must be a real number or
  None" TypeError when the original exception is a TypeError, guarded
  by PyErr_ExceptionMatches(PyExc_TypeError) exactly like the sibling
  select(), poll(), devpoll() and epoll() sites already do. Huge
  timeouts now surface the true OverflowError instead of a false claim
  that a float is not a real number.

Tests

• Lib/test/test_subprocess.py (FastWaitTestCase):
  test_wait_huge_timeout (10**10, sys.maxsize, float('inf')),
  test_run_huge_timeout (the run()/communicate() path), and
  test_wait_slices_do_not_expire_early (with the slice limit patched down
  to 10 ms, a process outliving many slices is still waited for — guards
  against a clamp that fires TimeoutExpired at the slice boundary). All
  POSIX-gated: the Windows _wait has a separate, pre-existing
  int(timeout * 1000) overflow for float("inf") that predates 3.15 and
  is deliberately out of scope here.
• Lib/test/test_kqueue.py: test_control_overflowing_timeout — an
  out-of-range timeout raises OverflowError, not TypeError; non-numbers
  still raise TypeError.

How tested

On macOS (arm64), against a 3.15.0b4 build with the patched
subprocess.py overlaid: the three new test_subprocess tests pass, the
entire pre-existing FastWaitTestCase and the POSIX wait-related suite
stay green, and wait(timeout=float("inf")) / run(..., timeout=1e10)
succeed. The Python-level clamp alone already fixes the user-visible
wait()/run() breakage (the overflowing value never reaches
kq.control()). The test_kqueue addition and the OverflowError
surfacing require the rebuilt select module (verified expected-fail
against the unpatched 3.15.0b4 C module); the Linux _wait_pidfd loop is
exercised by the new tests on Linux CI.

• Issue: 3.15 regression: Popen.wait(timeout=float("inf")) raises TypeError: timeout must be a real number or None on macOS/BSD (new kqueue wait path) #154836

---

NEWS entry (already in the commit, blurb format)

Misc/NEWS.d/next/Library/2026-07-19-20-00-00.gh-issue-154836.kqWait.rst:

Fix :meth:subprocess.Popen.wait raising :exc:TypeError (macOS and other
BSDs) or :exc:OverflowError (Linux) for very large timeout values such
as float('inf'), a 3.15 regression in the new event-driven wait. Also
fix :meth:select.kqueue.control masking :exc:OverflowError for
out-of-range timeouts as :exc:TypeError.

[python-cla-bot]

All commit authors signed the Contributor License Agreement.

[BHUVANSH855]

Thanks for the PR!

I noticed that the only failing required check is the Contributor License Agreement (CLA). It looks like the implementation, and the rest of the CI checks are passing.

Could you please sign the CLA? Once that's done, the required check should pass, and the PR will be ready for code review.

[calvinrp]

CLA signed, thanks

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @gpshead for commit 9b7cdf2 🤖

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F154837%2Fmerge

If you want to schedule another build, you need to add the 🔨 test-with-buildbots label again.

[gpshead]

the changes look good, running them through the bots now to get some additional kqueue coverage as well. there'll probably be some unrelated failure noise from a few of the less reliable buildbots.

[miss-islington-app]

Thanks @calvinrp for the PR, and @gpshead for merging it 🌮🎉.. I'm working now to backport this PR to: 3.15.
🐍🍒⛏🤖

[bedevere-app]

GH-154891 is a backport of this pull request to the 3.15 branch.