[PATCH 3/7] progress: catch backwards counting with GIT_TEST_CHECK_PROGRESS

["SZEDER Gábor" <szeder.dev@gmail.com>]

We had to fix a buggy progress line recently, where the progress
counter counted backwards, see 8e118e8490 (pack-objects: update
"nr_seen" progress based on pack-reused count, 2021-04-11).

Extend GIT_TEST_CHECK_PROGRESS to catch these cases as well, i.e.
trigger a BUG() when the counter passed to display_progress() is
smaller than the previous value.

Note that we allow subsequent display_progress() calls with the same
counter value, because:

  - Strictly speaking, it's not wrong to do so.

  - Forbidding it might make the code calling display_progress() more
    complex; I suspect that would be the case with e.g. the "Updating
    index flags" progress line in 'unpack-trees.c', where the counter
    is increased in recursive function calls.

  - We would need to special case the internal display() call in
    stop_progress_msg(), because it uses the same counter value as the
    last display_progress() call, which would trigger this BUG().

't0500-progress-display.sh' countains a few tests that check how
shortened progress lines are covered up, and one of them ('progress
shortens - crazy caller') shortens the progress line by counting
backwards.  From now on that test would trigger this BUG(), so remove
it; the other test cases cover shortening progress lines sufficiently.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
---
 progress.c                  |  6 ++++++
 t/t0500-progress-display.sh | 35 +++++++++++++----------------------
 2 files changed, 19 insertions(+), 22 deletions(-)

diff --git a/progress.c b/progress.c
index 549e8d1fe7..034d50cd6b 100644
--- a/progress.c
+++ b/progress.c
@@ -115,6 +115,12 @@ static void display(struct progress *progress, uint64_t n, const char *done)
 	int show_update = 0;
 	int last_count_len = counters_sb->len;
 
+	if (test_check_progress && progress->last_value != -1 &&
+	    n < progress->last_value)
+		BUG("progress \"%s\" counts backwards %"PRIuMAX" -> %"PRIuMAX,
+		    progress->title, (uintmax_t)progress->last_value,
+		    (uintmax_t)n);
+
 	progress->last_value = n;
 
 	if (progress->delay && (!progress_update || --progress->delay))
diff --git a/t/t0500-progress-display.sh b/t/t0500-progress-display.sh
index 641fa0964e..a73dd45153 100755
--- a/t/t0500-progress-display.sh
+++ b/t/t0500-progress-display.sh
@@ -153,28 +153,6 @@ EOF
 	test_cmp expect out
 '
 
-# Progress counter goes backwards, this should not happen in practice.
-test_expect_success 'progress shortens - crazy caller' '
-	cat >expect <<-\EOF &&
-	Working hard:  10% (100/1000)<CR>
-	Working hard:  20% (200/1000)<CR>
-	Working hard:   0% (1/1000)  <CR>
-	Working hard: 100% (1000/1000)<CR>
-	Working hard: 100% (1000/1000), done.
-	EOF
-
-	cat >in <<-\EOF &&
-	progress 100
-	progress 200
-	progress 1
-	progress 1000
-	EOF
-	test-tool progress --total=1000 "Working hard" <in 2>stderr &&
-
-	show_cr <stderr >out &&
-	test_cmp expect out
-'
-
 test_expect_success 'progress display with throughput' '
 	cat >expect <<-\EOF &&
 	Working hard: 10<CR>
@@ -324,13 +302,26 @@ test_expect_success 'GIT_TEST_CHECK_PROGRESS catches non-matching total' '
 	grep "BUG:.*total progress does not match" stderr
 '
 
+test_expect_success 'GIT_TEST_CHECK_PROGRESS catches backwards counting' '
+	cat >in <<-\EOF &&
+	progress 2
+	progress 1
+	EOF
+
+	test_must_fail env GIT_TEST_CHECK_PROGRESS=1 \
+		test-tool progress --total=3 "Working hard" <in 2>stderr &&
+	grep "BUG:.*counts backwards" stderr
+'
+
 test_expect_success 'tolerate bogus progress without GIT_TEST_CHECK_PROGRESS' '
 	cat >expect <<-\EOF &&
+	Working hard:  66% (2/3)<CR>
 	Working hard:  33% (1/3)<CR>
 	Working hard:  33% (1/3), done.
 	EOF
 
 	cat >in <<-\EOF &&
+	progress 2
 	progress 1
 	EOF
 	(
-- 
2.32.0.289.g44fbea0957