Subject: Re: suppressing useless wakeups in logical/worker.c

From: Tom Lane <[email-masked]>
Date: Thu, 26 Jan 2023 15:04:30 -0500

Nathan Bossart <[email-masked]> writes:
> On Thu, Jan 26, 2023 at 01:54:08PM -0500, Tom Lane wrote:
>> - * Both inputs must be ordinary finite timestamps (in current usage,
>> - * they'll be results from GetCurrentTimestamp()).
>> + * At least one input must be an ordinary finite timestamp, else the "diff"
>> + * calculation might overflow.  We do support stop_time == TIMESTAMP_INFINITY,
>> + * which will result in INT_MAX wait time.

> I wonder if we should explicitly reject negative timestamps to eliminate
> any chance of int64 overflow, too.

Hmm.  I'm disinclined to add an assumption that the epoch is in the past,
but I take your point that the subtraction would overflow with
TIMESTAMP_INFINITY and a negative finite timestamp.  Maybe we should
make use of pg_sub_s64_overflow()?

BTW, I just noticed that the adjacent function TimestampDifference
has a lot of callers that would be much happier using
TimestampDifferenceMilliseconds.

			regards, tom lane

---

From: Nathan Bossart <[email-masked]>
Date: Thu, 26 Jan 2023 12:23:01 -0800

> make use of pg_sub_s64_overflow()?

That would be my vote.  I think the 'diff <= 0' check might need to be
replaced with something like 'start_time > stop_time' so that we return 0
for the underflow case.

-- 
Nathan Bossart
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