[PATCH 7/8] pack-bitmap: reject pseudo-merge "sampleRate" of 0

[Taylor Blau <me@ttaylorr.com>]

The "bitmapPseudoMerge.*.sampleRate" configuration controls what
fraction of unstable commits are included in each pseudo-merge group.
The config validation accepts values in the range `[0, 1]`, but a value
of exactly 0 causes a division by zero in `select_pseudo_merges_1()`:

    if (j % (uint32_t)(1.0 / group->sample_rate))

When `sample_rate` is 0, `1.0 / 0.0` produces `+inf`, and casting
infinity to `uint32_t` is undefined behavior in C. On most platforms
this yields 0, making the subsequent modulo operation (`j % 0`) a
fatal arithmetic trap.

This path was not previously reachable because an earlier bug caused
all pseudo-merge candidates to be classified as "stable" (where the
sampling rate is not used), regardless of their actual commit date. Now
that the date classification is fixed, the unstable path is exercised
and the division by zero can fire.

Fix this by changing the validation to require a strict lower bound and
thus reject 0.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 pseudo-merge.c                  | 4 ++--
 t/t5333-pseudo-merge-bitmaps.sh | 2 +-
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/pseudo-merge.c b/pseudo-merge.c
index d79e5fb649a..75bed043602 100644
--- a/pseudo-merge.c
+++ b/pseudo-merge.c
@@ -169,8 +169,8 @@ static int pseudo_merge_config(const char *var, const char *value,
 		}
 	} else if (!strcmp(key, "samplerate")) {
 		group->sample_rate = git_config_double(var, value, ctx->kvi);
-		if (!(0 <= group->sample_rate && group->sample_rate <= 1)) {
-			warning(_("%s must be between 0 and 1, using default"), var);
+		if (!(0 < group->sample_rate && group->sample_rate <= 1)) {
+			warning(_("%s must be between 0 (exclusive) and 1, using default"), var);
 			group->sample_rate = DEFAULT_PSEUDO_MERGE_SAMPLE_RATE;
 		}
 	} else if (!strcmp(key, "threshold")) {
diff --git a/t/t5333-pseudo-merge-bitmaps.sh b/t/t5333-pseudo-merge-bitmaps.sh
index 63d2f64361d..46e8e6a8ea1 100755
--- a/t/t5333-pseudo-merge-bitmaps.sh
+++ b/t/t5333-pseudo-merge-bitmaps.sh
@@ -639,7 +639,7 @@ test_expect_success 'pseudo-merge commits are correctly classified by date' '
 	)
 '
 
-test_expect_failure 'sampleRate=0 does not cause division by zero' '
+test_expect_success 'sampleRate=0 does not cause division by zero' '
 	git init pseudo-merge-sample-rate-zero &&
 	test_when_finished "rm -fr pseudo-merge-sample-rate-zero" &&
 	(
-- 
2.54.0.rc1.73.g8f4e0170952