[PATCH v2 3/3] pack-bitmap: add loading corrupt bitmap_index test

["Lidong Yan via GitGitGadget" <gitgitgadget@gmail.com>]

From: Lidong Yan <502024330056@smail.nju.edu.cn>

This patch add "load corrupt bitmap" test case in t5310-pack-bitmaps.sh.

This test case intentionally corrupt the "xor_offset" field of the first
entry. To find position of first entry in *.bitmap, we need to skip 4
ewah_bitmaps before entries. And I add a function `skip_ewah_bitmap()`
to do this.

Signed-off-by: Lidong Yan <502024330056@smail.nju.edu.cn>
---
 t/t5310-pack-bitmaps.sh | 50 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 50 insertions(+)

diff --git a/t/t5310-pack-bitmaps.sh b/t/t5310-pack-bitmaps.sh
index a62b463eaf09..537a507957bb 100755
--- a/t/t5310-pack-bitmaps.sh
+++ b/t/t5310-pack-bitmaps.sh
@@ -26,6 +26,18 @@ has_any () {
 	grep -Ff "$1" "$2"
 }
 
+skip_ewah_bitmap() {
+	local bitmap="$1" &&
+	local offset="$2" &&
+	local size= &&
+
+	offset=$(($offset + 4)) &&
+	size=0x$(od -An -v -t x1 -j $offset -N 4 $bitmap | tr -d ' \n') &&
+	size=$(($size * 8)) &&
+	offset=$(($offset + 4 + $size + 4)) &&
+	echo $offset
+}
+
 # Since name-hash values are stored in the .bitmap files, add a test
 # that checks that the name-hash calculations are stable across versions.
 # Not exhaustive, but these hashing algorithms would be hard to change
@@ -486,6 +498,44 @@ test_bitmap_cases () {
 			grep "ignoring extra bitmap" trace2.txt
 		)
 	'
+
+	# A `.bitmap` file has the following structure:
+	# | Header | Commits | Trees | Blobs | Tags | Entries... |
+	#
+	# - The header is 32 bytes long when using SHA-1.
+	# - Commits, Trees, Blobs, and Tags are all stored as EWAH bitmaps.
+	#
+	# This test intentionally corrupts the `xor_offset` field of the first entry
+	# to verify robustness against malformed bitmap data.
+	test_expect_success 'load corrupt bitmap' '
+		rm -fr repo &&
+		git init repo &&
+		test_when_finished "rm -fr repo" &&
+		(
+			cd repo &&
+			git config pack.writeBitmapLookupTable '"$writeLookupTable"' &&
+
+			test_commit base &&
+
+			git repack -adb &&
+			bitmap="$(ls .git/objects/pack/pack-*.bitmap)" &&
+			chmod +w "$bitmap" &&
+
+			hdr_sz=$((12 + $(test_oid rawsz))) &&
+			offset=$(skip_ewah_bitmap $bitmap $hdr_sz) &&
+			offset=$(skip_ewah_bitmap $bitmap $offset) &&
+			offset=$(skip_ewah_bitmap $bitmap $offset) &&
+			offset=$(skip_ewah_bitmap $bitmap $offset) &&
+			offset=$((offset + 4)) &&
+
+			printf '\161' |
+				dd of=$bitmap count=1 bs=1 conv=notrunc seek=$offset &&
+
+			git rev-list --count HEAD > expect &&
+			git rev-list --use-bitmap-index --count HEAD > actual &&
+			test_cmp expect actual
+		)
+	'
 }
 
 test_bitmap_cases
-- 
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