t5326: demonstrate bitmap corruption after permutation

This patch demonstrates a cause of bitmap corruption that can occur when
the contents of the multi-pack index does not change, but the underlying
object order does.

In this example, we have a MIDX containing two packs, each with a
distinct set of objects (pack A corresponds to the tree, blob, and
commit from the first patch, and pack B corresponds to the second
patch).

First, a MIDX is written where the 'A' pack is preferred. As expected,
the bitmaps generated there are in-tact. But then, we generate an
identical MIDX with a different object order: this time preferring pack
'B'.

Due to a bug which will be explained and fixed in the following commit,
the MIDX is updated, but the .rev file is not, causing the .bitmap file
to be read incorrectly. Specifically, the .bitmap file will contain
correct data, but the auxiliary object order in the .rev file is stale,
causing readers to get confused by reading the new bitmaps using the old
object order.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Reviewed-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5326-multi-pack-bitmaps.sh

@@ -395,4 +395,35 @@ test_expect_success 'hash-cache values are propagated from pack bitmaps' '
 	)
 '
 
+test_expect_failure 'changing the preferred pack does not corrupt bitmaps' '
+	rm -fr repo &&
+	git init repo &&
+	test_when_finished "rm -fr repo" &&
+	(
+		cd repo &&
+
+		test_commit A &&
+		test_commit B &&
+
+		git rev-list --objects --no-object-names HEAD^ >A.objects &&
+		git rev-list --objects --no-object-names HEAD^.. >B.objects &&
+
+		A=$(git pack-objects $objdir/pack/pack <A.objects) &&
+		B=$(git pack-objects $objdir/pack/pack <B.objects) &&
+
+		cat >indexes <<-EOF &&
+		pack-$A.idx
+		pack-$B.idx
+		EOF
+
+		git multi-pack-index write --bitmap --stdin-packs \
+			--preferred-pack=pack-$A.pack <indexes &&
+		git rev-list --test-bitmap A &&
+
+		git multi-pack-index write --bitmap --stdin-packs \
+			--preferred-pack=pack-$B.pack <indexes &&
+		git rev-list --test-bitmap A
+	)
+'
+
 test_done