Date: Thu, 26 Jan 2023 11:48:12 -0800
From: Nathan Bossart <nathandbossart@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Thomas Munro <thomas.munro@gmail.com>,
	"Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>,
	"pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Subject: Re: suppressing useless wakeups in logical/worker.c
Message-ID: <20230126194812.GA1702315@nathanxps13>
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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
Amazon Web Services: https://aws.amazon.com