Subject: Re: [PATCH v2 8/8] progress.c: add & assert a "global_progress" variable

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Fri, 08 Oct 2021 09:15:19 +0200

On Thu, Oct 07 2021, Emily Shaffer wrote:

> On Tue, Sep 21, 2021 at 01:09:29AM +0200, Ævar Arnfjörð Bjarmason wrote:
>> 
>> The progress.c code makes a hard assumption that only one progress bar
>> be active at a time (see [1] for a bug where this wasn't the
>> case). Add a BUG() that'll trigger if we ever regress on that promise
>> and have two progress bars active at the same time.
>> 
>> There was an alternative test-only approach to doing the same
>> thing[2], but by doing this outside of a GIT_TEST_* mode we'll know
>> we've put a hard stop to this particular API misuse.
>> 
>> It will also establish scaffolding to address current fundamental
>> limitations in the progress output: The current output must be
>> "driven" by calls to the likes of display_progress(). Once we have a
>> global current progress object we'll be able to update that object via
>> SIGALRM. See [3] for early code to do that.
>> 
>> It's conceivable that this change will hit the BUG() condition in some
>> scenario that we don't currently have tests for, this would be very
>> bad. If that happened we'd die just because we couldn't emit some
>> pretty output.
>> 
>> See [4] for a discussion of why our test coverage is lacking; our
>> progress display is hidden behind isatty(2) checks in many cases, so
>> the test suite doesn't cover it unless individual tests are run in
>> "--verbose" mode, we might also have multi-threaded use of the API, so
>> two progress bars stopping and starting would only be visible due to a
>> race condition.
>> 
>> Despite that, I think that this change won't introduce such
>> regressions, because:
>> 
>>  1. I've read all the code using the progress API (and have modified a
>>     large part of it in some WIP code I have). Almost all of it is really
>>     simple, the parts that aren't[5] are complex in the display_progress() part,
>>     not in starting or stopping the progress bar.
>> 
>>  2. The entire test suite passes when instrumented with an ad-hoc
>>     Linux-specific mode (it uses gettid()) to die if progress bars are
>>     ever started or stopped on anything but the main thread[6].
>> 
>>     Extending that to die if display_progress() is called in a thread
>>     reveals that we have exactly two users of the progress bar under
>>     threaded conditions, "git index-pack" and "git pack-objects". Both
>>     uses are straightforward, and they don't start/stop the progress
>>     bar when threads are active.
>> 
>>  3. I've likewise done an ad-hoc test to force progress bars to be
>>     displayed with:
>> 
>>         perl -pi -e 's[isatty\((?:STDERR_FILENO|2)\)][1]g' $(git grep -l 'isatty\((STDERR_FILENO|2)\)')
>
> I think your ad-hoc test might be a little more compelling if it was
> easier to understand, which is to say, maybe if your Perl oneliner was
> on more than one line, or had comments, or was in a different language.
> Although you explain it right after, we kind of have to take your word
> for it.

I'll see if I can use sed or something, which is easy enough in this
case. I just write Perl out of habit for this sort of thing
(e.g. balanced braces & Perl-regexes make it much nicer).

>> 
>>     I.e. to replace all checks (not just for progress) of checking
>>     whether STDERR is connected to a TTY, and then monkeypatching
>>     is_foreground_fd() in progress.c to always "return 1". Running the
>>     tests with those applied, interactively and under -V reveals via:
>> 
>>         $ grep -e set_progress_signal -e clear_progress_signal test-results/*out
>> 
>>     That nothing our tests cover hits the BUG conditions added here,
>>     except the expected "BUG: start two concurrent progress bars" test
>>     being added here.
>> 
>>     That isn't entirely true since we won't be getting 100% coverage
>>     due to cascading failures from tests that expected no progress
>>     output on stderr. To make sure I covered 100% I also tried making
>>     the display() function in progress.c a NOOP on top of that (it's
>>     the calls to start_progress_delay() and stop_progress()) that
>>     matter.
>> 
>>     That doesn't hit the BUG() either. Some tests fail in that mode
>>     due to a combination of the overzealous isatty(2) munging noted
>>     above, and the tests that are testing that the progress output
>>     itself is present (but for testing I'd made display() a NOOP).
>> 
>> Between those three points I think it's safe to go ahead with this
>> change.
>
> One worry I had was that we might be painting ourselves into a corner
> here if we did want to support the ability to do multiple progress bars
> simultaneously (for example if we want to pull from multiple CDNs at the
> same time when we're using promisor packfiles, and we expect those packs
> to be large enough that we'd need to show a progress bar for each one).
> However, I think the pattern - hang onto a pointer to the progress
> objects, and complain if we get a signal and there are any still valid -
> still holds well enough, so I'm ok with this change.

Yes, and that's a thing I'd really like the progress code to be able to
do too, and I've got some follow-up patches, but (somewhat
paradoxically) in order to display multiple progress bars you need to
first have a step like this to ensure that there is only ever one
progress bar.

The user only has one terminal, so we'll need to serialize our N
progress bars to one "emitter", we'll need to teach the progress
accounting to either have N parallel progress lines, or to simply make N
number of "slave" "struct progress *" hang off it. I'm leaning towards
the latter.

> There are a couple patches in the middle which I didn't reply to, but I
> did read them, and they were so tiny and mechanical that I did not have
> useful comments to add.
>
> Thanks, it's nice to see progress here (ha ha ha).

:)

> Preferably with the BUG() message nit below,
> Reviewed-by: Emily Shaffer <[email-masked]>
>
>> 
>> 1. 6f9d5f2fda1 (commit-graph: fix progress of reachable commits, 2020-07-09)
>> 2. https://lore.kernel.org/git/[email-masked]
>> 3. https://lore.kernel.org/git/[email-masked]/
>> 4. https://lore.kernel.org/git/[email-masked]/
>> 5. b50c37aa44d (Merge branch 'ab/progress-users-adjust-counters' into
>>    next, 2021-09-10)
>> 6. https://lore.kernel.org/git/[email-masked]/
>> 
>> Signed-off-by: Ævar Arnfjörð Bjarmason <[email-masked]>
>> ---
>>  progress.c                  | 17 +++++++++++++----
>>  t/t0500-progress-display.sh | 11 +++++++++++
>>  2 files changed, 24 insertions(+), 4 deletions(-)
>> 
>> diff --git a/progress.c b/progress.c
>> index 1ab7d19deb8..14a023f4b43 100644
>> --- a/progress.c
>> +++ b/progress.c
>> @@ -46,6 +46,7 @@ struct progress {
>>  };
>>  
>>  static volatile sig_atomic_t progress_update;
>> +static struct progress *global_progress;
>>  
>>  /*
>>   * These are only intended for testing the progress output, i.e. exclusively
>> @@ -221,11 +222,15 @@ void progress_test_force_update(void)
>>  	progress_interval(SIGALRM);
>>  }
>>  
>> -static void set_progress_signal(void)
>> +static void set_progress_signal(struct progress *progress)
>>  {
>>  	struct sigaction sa;
>>  	struct itimerval v;
>>  
>> +	if (global_progress)
>> +		BUG("should have no global_progress in set_progress_signal()");
>> +	global_progress = progress;
>
> Can we make the BUG() message a little clearer? Even in the context of
> the code, it's not clear that what this BUG() really means is "hey, you
> forgot to call stop_progress on something" or "hey, you can't have two
> progress bars at the same time". Even if you were to change the name of
> 'global_progress' to 'existing_progress_bar' or something, I think that
> would make the message more understandable.

Willdo, thanks.

>> +
>>  	if (progress_testing)
>>  		return;
>>  
>> @@ -243,10 +248,14 @@ static void set_progress_signal(void)
>>  	setitimer(ITIMER_REAL, &v, NULL);
>>  }
>>  
>> -static void clear_progress_signal(void)
>> +static void clear_progress_signal(struct progress *progress)
>>  {
>>  	struct itimerval v = {{0,},};
>>  
>> +	if (!global_progress)
>> +		BUG("should have a global_progress in clear_progress_signal()");
>> +	global_progress = NULL;
>> +
>>  	if (progress_testing)
>>  		return;
>>  
>> @@ -270,7 +279,7 @@ static struct progress *start_progress_delay(const char *title, uint64_t total,
>>  	strbuf_init(&progress->counters_sb, 0);
>>  	progress->title_len = utf8_strwidth(title);
>>  	progress->split = 0;
>> -	set_progress_signal();
>> +	set_progress_signal(progress);
>>  	trace2_region_enter("progress", title, the_repository);
>>  	return progress;
>>  }
>> @@ -374,7 +383,7 @@ void stop_progress_msg(struct progress **p_progress, const char *msg)
>>  		display(progress, progress->last_value, buf);
>>  		free(buf);
>>  	}
>> -	clear_progress_signal();
>> +	clear_progress_signal(progress);
>>  	strbuf_release(&progress->counters_sb);
>>  	if (progress->throughput)
>>  		strbuf_release(&progress->throughput->display);
>> diff --git a/t/t0500-progress-display.sh b/t/t0500-progress-display.sh
>> index ffa819ca1db..124d33c96b3 100755
>> --- a/t/t0500-progress-display.sh
>> +++ b/t/t0500-progress-display.sh
>> @@ -296,6 +296,17 @@ test_expect_success 'cover up after throughput shortens a lot' '
>>  	test_cmp expect out
>>  '
>>  
>> +test_expect_success 'BUG: start two concurrent progress bars' '
>> +	cat >in <<-\EOF &&
>> +	start 0 one
>> +	start 0 two
>> +	EOF
>> +
>> +	test_must_fail test-tool progress \
>> +		<in 2>stderr &&
>> +	grep -E "^BUG: .*: should have no global_progress in set_progress_signal\(\)$" stderr
>> +'
>> +
>>  test_expect_success 'progress generates traces' '
>>  	cat >in <<-\EOF &&
>>  	start 40
>> -- 
>> 2.33.0.1098.gf02a64c1a2d
>>