t5326: extract `test_rev_exists`

To determine which source of data is used for the MIDX's reverse index
cache, introduce a helper which forces loading the reverse index, and
then looks for the special trace2 event introduced in a previous commit.

For now, this helper just looks for when the legacy MIDX .rev file was
loaded, but in a subsequent commit will become parameterized over the
the reverse index's source.

This function replaces checking for the existence of the .rev file. We
could write a similar helper to ensure that the .rev file is cleaned up
after repacking, but it will make subsequent tests more difficult to
write, and provides marginal value since we already check that the MIDX
.bitmap file is removed.

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

@@ -17,16 +17,29 @@ midx_pack_source () {
 	test-tool read-midx --show-objects .git/objects | grep "^$1 " | cut -f2
 }
 
+test_rev_exists () {
+	commit="$1"
+
+	test_expect_success 'reverse index exists' '
+		GIT_TRACE2_EVENT=$(pwd)/event.trace \
+			git rev-list --test-bitmap "$commit" &&
+
+		test_path_is_file $midx-$(midx_checksum $objdir).rev &&
+		grep "\"category\":\"load_midx_revindex\",\"key\":\"source\",\"value\":\"rev\"" event.trace
+	'
+}
+
 setup_bitmap_history
 
 test_expect_success 'create single-pack midx with bitmaps' '
 	git repack -ad &&
 	git multi-pack-index write --bitmap &&
 	test_path_is_file $midx &&
-	test_path_is_file $midx-$(midx_checksum $objdir).bitmap &&
-	test_path_is_file $midx-$(midx_checksum $objdir).rev
+	test_path_is_file $midx-$(midx_checksum $objdir).bitmap
 '
 
+test_rev_exists HEAD
+
 basic_bitmap_tests
 
 test_expect_success 'create new additional packs' '
@@ -52,10 +65,11 @@ test_expect_success 'create multi-pack midx with bitmaps' '
 	test_line_count = 25 packs &&
 
 	test_path_is_file $midx &&
-	test_path_is_file $midx-$(midx_checksum $objdir).bitmap &&
-	test_path_is_file $midx-$(midx_checksum $objdir).rev
+	test_path_is_file $midx-$(midx_checksum $objdir).bitmap
 '
 
+test_rev_exists HEAD
+
 basic_bitmap_tests
 
 test_expect_success '--no-bitmap is respected when bitmaps exist' '
@@ -66,7 +80,6 @@ test_expect_success '--no-bitmap is respected when bitmaps exist' '
 
 	test_path_is_file $midx &&
 	test_path_is_file $midx-$(midx_checksum $objdir).bitmap &&
-	test_path_is_file $midx-$(midx_checksum $objdir).rev &&
 
 	git multi-pack-index write --no-bitmap &&
 
@@ -206,10 +219,11 @@ test_expect_success 'setup partial bitmaps' '
 	test_commit loose &&
 	git multi-pack-index write --bitmap 2>err &&
 	test_path_is_file $midx &&
-	test_path_is_file $midx-$(midx_checksum $objdir).bitmap &&
-	test_path_is_file $midx-$(midx_checksum $objdir).rev
+	test_path_is_file $midx-$(midx_checksum $objdir).bitmap
 '
 
+test_rev_exists HEAD~
+
 basic_bitmap_tests HEAD~
 
 test_expect_success 'removing a MIDX clears stale bitmaps' '
@@ -224,7 +238,6 @@ test_expect_success 'removing a MIDX clears stale bitmaps' '
 
 		# Write a MIDX and bitmap; remove the MIDX but leave the bitmap.
 		stale_bitmap=$midx-$(midx_checksum $objdir).bitmap &&
-		stale_rev=$midx-$(midx_checksum $objdir).rev &&
 		rm $midx &&
 
 		# Then write a new MIDX.
@@ -234,9 +247,7 @@ test_expect_success 'removing a MIDX clears stale bitmaps' '
 
 		test_path_is_file $midx &&
 		test_path_is_file $midx-$(midx_checksum $objdir).bitmap &&
-		test_path_is_file $midx-$(midx_checksum $objdir).rev &&
-		test_path_is_missing $stale_bitmap &&
-		test_path_is_missing $stale_rev
+		test_path_is_missing $stale_bitmap
 	)
 '
 
@@ -257,7 +268,6 @@ test_expect_success 'pack.preferBitmapTips' '
 		git multi-pack-index write --bitmap &&
 		test_path_is_file $midx &&
 		test_path_is_file $midx-$(midx_checksum $objdir).bitmap &&
-		test_path_is_file $midx-$(midx_checksum $objdir).rev &&
 
 		test-tool bitmap list-commits | sort >bitmaps &&
 		comm -13 bitmaps commits >before &&
@@ -267,7 +277,6 @@ test_expect_success 'pack.preferBitmapTips' '
 			<before | git update-ref --stdin &&
 
 		rm -fr $midx-$(midx_checksum $objdir).bitmap &&
-		rm -fr $midx-$(midx_checksum $objdir).rev &&
 		rm -fr $midx &&
 
 		git -c pack.preferBitmapTips=refs/tags/include \
@@ -305,7 +314,6 @@ test_expect_success 'writing a bitmap with --refs-snapshot' '
 		grep "$(git rev-parse two)" bitmaps &&
 
 		rm -fr $midx-$(midx_checksum $objdir).bitmap &&
-		rm -fr $midx-$(midx_checksum $objdir).rev &&
 		rm -fr $midx &&
 
 		# Then again, but with a refs snapshot which only sees
@@ -350,7 +358,6 @@ test_expect_success 'write a bitmap with --refs-snapshot (preferred tips)' '
 		) >snapshot &&
 
 		rm -fr $midx-$(midx_checksum $objdir).bitmap &&
-		rm -fr $midx-$(midx_checksum $objdir).rev &&
 		rm -fr $midx &&
 
 		git multi-pack-index write --bitmap --refs-snapshot=snapshot &&