According to the documentation select.poll.poll() is blocked for infinite timeout if the timeout argument is negative. But due to rounding it is possible that timeout < 0, but an integer ms passed to the poll() syscall is 0, and poll() is not blocked.

I suggest to implement ROUND_UP in pytime and use it in all functionsaccepting a timeout, not only poll. Search for ROUND_CEILING in the code tofind them. But functions accepting time like clock_settime() must continueto use ROUND_CEILING.Does someone want to work on such patch? The new rounding mode must be testin test_time, you should just add the new mode at the top once if I recallcorrectly.---select.poll currently uses ROUND_CEILING: round towards +infinity.It looks like you would prefer ROUND_UP: round away from zero, right?In the history of CPython, the rounding mode of functions accepting time,timeout, duration, etc. changed many times, mostly because the C codechanged in subtle ways to fix different bugs. I mean that the exactrounding mode wasn't really chosen on purpose. I'm now trying to get betterdefined rouding modes in pytime, but it seems poll uses the wrong one.https://haypo.github.io/pytime.htmlI think that my problem is that I wanted to reuse the same rounding modesfor different use cases. It seems like clocks need ROUND_CEILING buttimeouts need ROUND_UP. Well, in the case of poll, the previous code beforemoving to pytime was using ceil() which uses ROUND_CEILING.

This particular issue can be fixed by adding the condition (PyFloat_Check(timeout_obj) && PyFloat_AsDouble(timeout_obj) < 0). The problem is only with writing good test for infinity timeout.This test could be also used for testingissue31334.

I have added a Pull Request fixing this issue. The current implementation is checking in the syscall if the value for ms before rounding was negative.To test for this, I call poll.poll in a thread and check that the thread is alive after a join with timeout (so this means it has blocked). Then to clean up, I write to a pipe and therefore it unblocks.The implementation is available in the PR:https://github.com/python/cpython/pull/4003

Thank you Pablo. The initial test leaked a thread, now it is much better.Could you please make the test testing different timeout values: None, -1, -1.0, -0.1, -1e-100?

I have updated both the test and the implementation to address your feedback.

Ok, so it seems like we need 3 rounding modes:* _PyTime_ROUND_FLOOR: read a clock, like datetime.datetime.now(). We need to round nanoseconds since datetime.datetime only supports 1 us resolution* _PyTime_ROUND_HALF_EVEN: "round from a Python float" like round(float), used by datetime.datetime.fromtimestamp()* _PyTime_ROUND_UP: round timeouts, socket.settimeout(), lock.acquire(timeout), poll(timeout), etc._PyTime_ROUND_UP and _PyTime_ROUND_CEILING are the same for positive numbers, but using _PyTime_ROUND_CEILING causes this bug: values in ]-0.5; 0.0[ are rounding to zero which gives the wrong behaviour. It seems like _PyTime_ROUND_CEILING is not needed in Python currently.

> It seems like _PyTime_ROUND_CEILING is not needed in Python currently.Oops, my pendingPR 3983 uses _PyTime_ROUND_CEILING :-) Please keep it.

New changeset2c15b29aea5d6b9c61aa42d2c24a07ff1edb4b46 by Serhiy Storchaka (Pablo Galindo) in branch 'master':bpo-31786: Make functions in the select module blocking when timeout is a small negative value. (#4003)https://github.com/python/cpython/commit/2c15b29aea5d6b9c61aa42d2c24a07ff1edb4b46

New changeset95602b368b87da3702a0f340ded2a23e823bb104 by Serhiy Storchaka (Pablo Galindo) in branch '3.6':[3.6]bpo-31786: Make functions in the select module blocking when timeout is a small negative value. (GH-4003). (#4022)https://github.com/python/cpython/commit/95602b368b87da3702a0f340ded2a23e823bb104

New changeseted267e3305a54eddae8106bdaae2c62d4c3b7db6 by Serhiy Storchaka in branch '2.7':[2.7]bpo-31786: Make functions in the select module blocking when timeout is a small negative value. (GH-4003). (#4031)https://github.com/python/cpython/commit/ed267e3305a54eddae8106bdaae2c62d4c3b7db6

Thank you for your contribution Pablo.The issue is fixed in 3.6 and 3.7. It is hard to fix it in 2.7.