Merge branch 'pp/midx-write-skip-empty' into jch

The `git multi-pack-index write` command has been updated to
silently return success when there are no object entries to index.
This avoids writing empty `multi-pack-index` layers, which
previously caused subsequent incremental midx writes using the
`--bitmap` option to fail when attempting to load the missing
reverse index.

* pp/midx-write-skip-empty:
  midx-write: skip writes with no object entries

midx-write.c

@@ -1617,9 +1617,8 @@ static int write_midx_internal(struct write_midx_opts *opts)
 	}
 
 	if (!ctx.entries_nr) {
-		if (opts->flags & MIDX_WRITE_BITMAP)
-			warning(_("refusing to write multi-pack .bitmap without any objects"));
-		opts->flags &= ~(MIDX_WRITE_REV_INDEX | MIDX_WRITE_BITMAP);
+		result = 0;
+		goto cleanup;
 	}
 
 	if (ctx.incremental) {

t/t5319-multi-pack-index.sh

@@ -54,6 +54,41 @@ test_expect_success "don't write midx with no packs" '
 	test_path_is_missing pack/multi-pack-index
 '
 
+test_expect_success 'skip non-incremental MIDX with no objects' '
+	git init --bare empty.git &&
+	(
+		cd empty.git &&
+		git pack-objects objects/pack/pack </dev/null &&
+		ls objects/pack >files.expect &&
+
+		for bitmap in "" --bitmap
+		do
+			git multi-pack-index write $bitmap >out 2>&1 &&
+			test_must_be_empty out &&
+			test_path_is_missing objects/pack/multi-pack-index &&
+			ls objects/pack >files.actual &&
+			test_cmp files.expect files.actual || return 1
+		done &&
+
+		git multi-pack-index write --incremental --bitmap &&
+		test_dir_is_empty objects/pack/multi-pack-index.d &&
+
+		echo blob | git hash-object -w --stdin >in &&
+		git pack-objects objects/pack/pack <in &&
+		git multi-pack-index write --incremental --bitmap &&
+		test_line_count = 1 objects/pack/multi-pack-index.d/multi-pack-index-chain &&
+		git multi-pack-index verify &&
+
+		echo another | git hash-object -w --stdin >in &&
+		git pack-objects objects/pack/pack <in &&
+		git multi-pack-index write --bitmap &&
+		test_path_is_file objects/pack/multi-pack-index &&
+		midx="$(midx_checksum objects)" &&
+		test_path_is_file objects/pack/multi-pack-index-$midx.bitmap &&
+		git multi-pack-index verify
+	)
+'
+
 test_expect_success SHA1 'warn if a midx contains no oid' '
 	cp "$TEST_DIRECTORY"/t5319/no-objects.midx $objdir/pack/multi-pack-index &&
 	test_must_fail git multi-pack-index verify &&

t/t5326-multi-pack-bitmaps.sh

@@ -305,7 +305,7 @@ test_midx_bitmap_cases () {
 		)
 	'
 
-	test_expect_success 'no .bitmap is written without any objects' '
+	test_expect_success 'no MIDX or .bitmap is written without any objects' '
 		rm -fr repo &&
 		git init repo &&
 		test_when_finished "rm -fr repo" &&
@@ -318,13 +318,14 @@ test_midx_bitmap_cases () {
 			pack-$empty.idx
 			EOF
 
+			ls $objdir/pack >files.expect &&
 			git multi-pack-index write --bitmap --stdin-packs \
-				<packs 2>err &&
+				<packs >out 2>&1 &&
 
-			test_grep "bitmap without any objects" err &&
-
-			test_path_is_file $midx &&
-			test_path_is_missing $midx-$(midx_checksum $objdir).bitmap
+			test_must_be_empty out &&
+			test_path_is_missing $midx &&
+			ls $objdir/pack >files.actual &&
+			test_cmp files.expect files.actual
 		)
 	'
 

t/t5334-incremental-multi-pack-index.sh

@@ -195,4 +195,79 @@ test_expect_success 'non-incremental write with existing incremental chain' '
 	)
 '
 
+test_expect_success 'skip initial MIDX layer with no objects' '
+	git init empty &&
+	(
+		cd empty &&
+		git config maintenance.auto false &&
+		git pack-objects $packdir/pack </dev/null &&
+
+		for bitmap in --bitmap --no-bitmap
+		do
+			git multi-pack-index write --incremental "$bitmap" >out 2>&1 &&
+			test_must_be_empty out &&
+			test_dir_is_empty "$midxdir" || return 1
+		done &&
+
+		write_midx_layer &&
+		test_line_count = 1 "$midx_chain" &&
+		git multi-pack-index verify
+	)
+'
+
+test_expect_success 'skip MIDX layer with empty pack' '
+	git init empty-pack &&
+	(
+		cd empty-pack &&
+		git config maintenance.auto false &&
+		write_midx_layer &&
+
+		git pack-objects $packdir/pack </dev/null &&
+		cp "$midx_chain" chain.expect &&
+		ls "$packdir" "$midxdir" >files.expect &&
+
+		for bitmap in --bitmap --no-bitmap
+		do
+			git multi-pack-index write --incremental "$bitmap" >out 2>&1 &&
+			test_must_be_empty out &&
+			test_cmp chain.expect "$midx_chain" &&
+			ls "$packdir" "$midxdir" >files.actual &&
+			test_cmp files.expect files.actual || return 1
+		done &&
+
+		write_midx_layer &&
+		test_line_count = 2 "$midx_chain" &&
+		git multi-pack-index verify &&
+		git rev-list --test-bitmap 2.2
+	)
+'
+
+test_expect_success 'skip MIDX layer with duplicate pack' '
+	git init duplicate-pack &&
+	(
+		cd duplicate-pack &&
+		git config maintenance.auto false &&
+		write_midx_layer &&
+
+		git rev-parse HEAD^{tree} >in &&
+		git pack-objects $packdir/pack <in &&
+		cp "$midx_chain" chain.expect &&
+		ls "$packdir" "$midxdir" >files.expect &&
+
+		for bitmap in --bitmap --no-bitmap
+		do
+			git multi-pack-index write --incremental "$bitmap" >out 2>&1 &&
+			test_must_be_empty out &&
+			test_cmp chain.expect "$midx_chain" &&
+			ls "$packdir" "$midxdir" >files.actual &&
+			test_cmp files.expect files.actual || return 1
+		done &&
+
+		write_midx_layer &&
+		test_line_count = 2 "$midx_chain" &&
+		git multi-pack-index verify &&
+		git rev-list --test-bitmap 2.2
+	)
+'
+
 test_done