Merge branch 'tb/midx-no-bitmap-for-no-objects'

When there is no object to write .bitmap file for, "git
multi-pack-index" triggered an error, instead of just skipping,
which has been corrected.

* tb/midx-no-bitmap-for-no-objects:
  midx: prevent writing a .bitmap without any objects

midx.c

@@ -1077,6 +1077,9 @@ static int write_midx_bitmap(char *midx_name, unsigned char *midx_hash,
 	char *bitmap_name = xstrfmt("%s-%s.bitmap", midx_name, hash_to_hex(midx_hash));
 	int ret;
 
+	if (!ctx->entries_nr)
+		BUG("cannot write a bitmap without any objects");
+
 	if (flags & MIDX_WRITE_BITMAP_HASH_CACHE)
 		options |= BITMAP_OPT_HASH_CACHE;
 
@@ -1401,6 +1404,12 @@ static int write_midx_internal(const char *object_dir,
 		goto cleanup;
 	}
 
+	if (!ctx.entries_nr) {
+		if (flags & MIDX_WRITE_BITMAP)
+			warning(_("refusing to write multi-pack .bitmap without any objects"));
+		flags &= ~(MIDX_WRITE_REV_INDEX | MIDX_WRITE_BITMAP);
+	}
+
 	cf = init_chunkfile(f);
 
 	add_chunk(cf, MIDX_CHUNKID_PACKNAMES, pack_name_concat_len,

t/t5326-multi-pack-bitmaps.sh

@@ -266,6 +266,28 @@ test_expect_success 'hash-cache values are propagated from pack bitmaps' '
 	)
 '
 
+test_expect_success 'no .bitmap is written without any objects' '
+	rm -fr repo &&
+	git init repo &&
+	test_when_finished "rm -fr repo" &&
+	(
+		cd repo &&
+
+		empty="$(git pack-objects $objdir/pack/pack </dev/null)" &&
+		cat >packs <<-EOF &&
+		pack-$empty.idx
+		EOF
+
+		git multi-pack-index write --bitmap --stdin-packs \
+			<packs 2>err &&
+
+		grep "bitmap without any objects" err &&
+
+		test_path_is_file $midx &&
+		test_path_is_missing $midx-$(midx_checksum $objdir).bitmap
+	)
+'
+
 test_expect_success 'graceful fallback when missing reverse index' '
 	rm -fr repo &&
 	git init repo &&