[PATCH v2 7/9] 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>
---
 Documentation/config/bitmap-pseudo-merge.adoc | 4 ++--
 pseudo-merge.c                                | 4 ++--
 t/t5333-pseudo-merge-bitmaps.sh               | 2 +-
 3 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/Documentation/config/bitmap-pseudo-merge.adoc b/Documentation/config/bitmap-pseudo-merge.adoc
index 1f264eca99b..6bf52c80ba7 100644
--- a/Documentation/config/bitmap-pseudo-merge.adoc
+++ b/Documentation/config/bitmap-pseudo-merge.adoc
@@ -47,8 +47,8 @@ will be updated more often than a reference pointing at an old commit.
 bitmapPseudoMerge.<name>.sampleRate::
 	Determines the proportion of non-bitmapped commits (among
 	reference tips) which are selected for inclusion in an
-	unstable pseudo-merge bitmap. Must be between `0` and `1`
-	(inclusive). The default is `1`.
+	unstable pseudo-merge bitmap. Must be greater than `0` and
+	less than or equal to `1`. The default is `1`.
 
 bitmapPseudoMerge.<name>.threshold::
 	Determines the minimum age of non-bitmapped commits (among
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 e27f9850dc4..3d0617a2e17 100755
--- a/t/t5333-pseudo-merge-bitmaps.sh
+++ b/t/t5333-pseudo-merge-bitmaps.sh
@@ -640,7 +640,7 @@ 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' '
 	test_when_finished "rm -fr pseudo-merge-sample-rate-zero" &&
 	git init pseudo-merge-sample-rate-zero &&
 	(
-- 
2.54.0.9.gb905fd5d0ae