Re: [PATCH 2/2] read-cache: fix incorrect count and progress bar stalling

From: René Scharfe <l.s.r@web.de>
Date: Jun 7, 2021, 19:20 UTC
Message-ID: <eaf2b6b0-4202-d5ea-87a2-b828fdbc60a1@web.de>
In-Reply-To: <87k0n54qb6.fsf@evledraar.gmail.com>

Am 07.06.21 um 17:58 schrieb Ævar Arnfjörð Bjarmason:

Show 20 quoted lines

>
> On Mon, Jun 07 2021, Derrick Stolee wrote:
>
>> On 6/7/2021 10:43 AM, Ævar Arnfjörð Bjarmason wrote:
>>> Fix a potential incorrect display of the number of items (off by one)
>>> and stalling of the progress bar in refresh_index().
>>>
>>> The off-by-one error is minor, we should say we're processing the 1st
>>> item, not the 0th. This along with the next change also allows us to
>>> remove the last display_progress() call outside the loop, as we'll
>>> always have reached 100% now.
>>
>> This "pre-announce the progress" seems correct and is unlikely
>> to have a user sitting at "100%" while the loop is actually doing
>> work on that last cache entry.
>
> I guess pre-announce v.s. post-announce is a matter of some philosophy,
> for O(n) when can we be said to be doing work on n[0]? We entered the
> for-loop and are doing work on that istate->cache[i] item, so I'd like
> to think of it more as post-announce :)

Say you have a single item to process and it takes a minute. The original code shows 0% for a minute, then 100% at the end. With your change you'd get 100% for a minute. Both would be annoying, but the latter would have me raging. "If you're done", I'd yell at the uncaring machine, "what are you still doing!?".

Showing only the completed items makes sense. That the next one is being processed is self-understood. Once all of them are done, 100% is shown and the progress line is finished.

So I think this pattern works:

	for (i = 0; i < nr; i++) {
		display_progress(p, i);
		/* work work work */
	}
	display_progress(p, nr);

Alternatively, if the work part doesn't contain continue statements:

	for (i = 0; i < nr; i++) {
		/* work work work */
		display_progress(p, i + 1);
	}

> In any case, I'm changing this to the established pattern we use in most
> other places in the codebase, this one was an odd one out.

Consistency is a good thing, but perhaps some of these other places should be changed. It doesn't matter much because most items git deals with are processed quickly, so an off-by-one error should barely be noticeable, but still it would be nice to get it right. It's hard to test, though.

René