[Rd] R_SocketWaitMultiple(): Thinko leading to negative select() timeouts(?)

[Peter Dalgaard]

Well, we have had mysterious stalls.... E.g. the popup for the CRAN mirror selection can be quite slow on Mac, and given the low reproducibility and difficulty in catching the problem in the act, the cause can go undetected for ages (not saying that this is actually caused by R_SocketWaitMultiple() of course.)

-pd

> On 26 Mar 2026, at 09.34, Martin Maechler <maechler using stat.math.ethz.ch> wrote:
> 
>>>>>> Henrik Bengtsson 
>>>>>>    on Tue, 24 Mar 2026 20:54:00 -0700 writes:
> 
>> SUMMARY:
>> I think there is a thinko in R_SocketWaitMultiple() that can result in:
> 
>> howmany = R_SelectEx(maxfd+1, &rfd, &wfd, NULL, &tv, NULL);
> 
>> being called with a negative timeout 'tv' (tv.tv_sec and tv.tv_usec
>> negative). I think the bug is that:
> 
>> if (mytimeout < 0 || R_wait_usec / 1e-6 < mytimeout - used)
> 
>> should be:
> 
>> if (mytimeout < 0 || R_wait_usec / 1e6 < mytimeout - used)
> 
> or as you say below
> 
>     if (mytimeout < 0 || R_wait_usec * 1e-6 < mytimeout - used)
> 
> I've committed the latter,  as indeed, in
> <R>/src/modules/internet/Rsock.c   microseconds = μ sec --> 'usec"'
> are always multiplied by 1e-6 when transformed to  seconds
> (well, with one exception where *integer division*
> delta / 1000000  is used).
> 
> This "only" affects "Unix" ... it's still interesting that this
> has never be seen or noticed as having a negative effect in all
> the 23 years that this part of socket code exists...
> 
> Thanks a lot, Henrik!
> 
> Martin
> 
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-- 
Peter Dalgaard, Professor,
Center for Statistics, Copenhagen Business School
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