Re: suppressing useless wakeups in logical/worker.c

On Thu, Jan 26, 2023 at 01:54:08PM -0500, Tom Lane wrote:
> After looking closer, I see that TimestampDifferenceMilliseconds
> already explicitly states that its output is intended for WaitLatch
> and friends, which makes it perfectly sane for it to clamp the result
> to [0, INT_MAX] rather than depending on the caller to not pass
> out-of-range values.

+1

> * This is typically used to calculate a wait timeout for WaitLatch()
> * or a related function. The choice of "long" as the result type
> - * is to harmonize with that. It is caller's responsibility that the
> - * input timestamps not be so far apart as to risk overflow of "long"
> - * (which'd happen at about 25 days on machines with 32-bit "long").
> + * is to harmonize with that; furthermore, we clamp the result to at most
> + * INT_MAX milliseconds, because that's all that WaitLatch() allows.
> *
> - * 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. Alternatively, we could detect that the
operation will overflow and return either 0 or INT_MAX, but I assume
there's minimal use of this function with negative timestamps.

--
Nathan Bossart
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