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

Am 14.06.21 um 13:07 schrieb Ævar Arnfjörð Bjarmason:

On Thu, Jun 10 2021, René Scharfe wrote:

quoted

Am 09.06.21 um 00:12 schrieb Ævar Arnfjörð Bjarmason:

quoted

On Tue, Jun 08 2021, René Scharfe wrote:

quoted

I wonder (only in a semi-curious way, though) if we can detect
off-by-one errors by adding an assertion to display_progress() that
requires the first update to have the value 0, and in stop_progress()
one that requires the previous display_progress() call to have a value
equal to the total number of work items.  Not sure it'd be worth the
hassle..

That's intentional. We started eating 3 apples, got to one, but now our
house is on fire and we're eating no more apples today, even if we
planned to eat 3 when we sat down.

The progress bar reflects this unexpected but recoverable state:

    $ perl -wE 'for (0..1) { say "update"; say "progress $_" }' |
      ./helper/test-tool progress --total=3 Apples 2>&1 |
      cat -v | perl -pe 's/\^M\K/\n/g'
    Apples:   0% (0/3)^M
    Apples:  33% (1/3)^M
    Apples:  33% (1/3), done.

We're at 1/3, but we're done. No more apples.

This isn't just some hypothetical, e.g. consider neeing to unlink() or
remove files/directories one at a time in a directory and getting the
estimated number from st_nlink (yeah yeah, unportable, but it was the
first thing I thought of).

We might think we're processing 10 entries, but another other processes
might make our progress bar end at more or less than the 100% we
expected. That's OK, not something we should invoke BUG() about.

It doesn't have to be a BUG; a warning would suffice.  And I hope not
finishing the expected number of items due to a catastrophic event is
rare enough that an additional warning wouldn't cause too much pain.

It's not a catastrophic event, just a run of the mill race condition
we'll expect if we're dealing with the real world.

E.g. you asked to unlink 1000 files, we do so, we find 10 are unlinked
already, or the command is asked to recursively unlink all files in a
directory tree, and new ones have showed up.

In those cases we should just just shrug and move on, no need for a
warning. We just don't always have perfect information about future
state at the start of the loop.

If a planned work item is cancelled then it can still be counted as
done.  Or the total could be adjusted, but that might look awkward.

quoted

Loops that *regularly* end early are not a good fit for progress
percentages, I think.

Arguably yes, but in these fuzzy cases not providing a "total" means
showing no progress at all, just a counter. Perhaps we should have some
other "provide total, and it may be fuzzy" flag. Not providing it might
run into your proposed BUG(), my point was that the current API
providing this flexibility is intentional.

Your patch turns a loop that doesn't immediately report skipped items
into one with contiguous progress updates.  That's a good way to deal
with the imagined restrictions for error detection.  Another would be
to make the warnings optional.

quoted

quoted

Similarly, the n=0 being distinguishable from the first
display_progress() is actually useful in practice. It's something I've
seen git.git emit (not recently, I patched the relevant code to emit
more granular progress).

It's useful to know that we're stalling on the setup code before the
for-loop, not on the first item.

Hmm, preparations that take a noticeable time might deserve their own
progress line.

Sure, and I've split some of those up in the past, but this seems like
ducking/not addressing the point that the API use we disagree on has
your preferred use conflating these conditions, but mine does not...

Subtle.  If preparation takes a long time and each item less than that
then the progress display is likely to jump from "0/n" to "i/n", where
i > 1, and the meaning of "1/n" becomes moot.

The progress display could show just the title before the first
display_progress() call to make the distinction clear.  Would it really
be useful, though?  Perhaps a trace2 region started by the first
display_progress() call and ended by the last one (n == total) would
be better.

quoted

Anyway, if no guard rails can be built then we have to rely on our math
skills alone.  Off-by-one errors may look silly, but are no joke -- they
are surprisingly easy to make.

...which, regardless of whether one views a progress of "1/5 items" has
"finished 1/5" or "working on 1/5", which I think *in general* is an
arbitrary choice, I think the progress.c API we have in git.git clearly
fits the usage I'm describing better.

How so?  start_progress() specifies a title and the total number of
items, display_progress() reports some other number that is shown in
relation to the total, and stop_progress() finishes the progress line.
This API is not documented and thus its meaning is (strictly speaking)
left unspecified.

It can be used to show a classic "percent-done progress indicator", as
https://dl.acm.org/doi/10.1145/1165385.317459 calls it.  That's how I
read a growing percentage shown by a program, and "done" I understand
as "has been done" (completed), not as "is being done".

Wikipedia sent me to
https://chrisharrison.net/projects/progressbars/ProgBarHarrison.pdf,
which has some fun ideas on how to warp the perception of time for
users staring at a progress bar, but also states typical ones grow
with the amount of completed work.

René