On Thu, Oct 14, 2021 at 12:28:26AM +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\(2\)][1]g' $(git grep -l -F 'isatty(2)') > > 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. > > 1. 6f9d5f2fda1 (commit-graph: fix progress of reachable commits, 2020-07-09) > 2. https://lore.kernel.org/git/20210620200303.2328957-3-szeder.dev@xxxxxxxxx > 3. https://lore.kernel.org/git/patch-18.25-e21fc66623f-20210623T155626Z-avarab@xxxxxxxxx/ > 4. https://lore.kernel.org/git/cover-00.25-00000000000-20210623T155626Z-avarab@xxxxxxxxx/ > 5. b50c37aa44d (Merge branch 'ab/progress-users-adjust-counters' into > next, 2021-09-10) > 6. https://lore.kernel.org/git/877dffg37n.fsf@xxxxxxxxxxxxxxxxxxx/ > > Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@xxxxxxxxx> I find it much nicer to understand now, thanks. The BUG() change in particular is excellent. Reviewed-by: Emily Shaffer <emilyshaffer@xxxxxxxxxx>
