lua: introduce xlog.meta() method

This commit introduces the new method for xlog module: xlog.meta().
It opens an xlog file, reads and returns the meta block of the file,
which includes its filetype, instance_uuid and vclocks.

It's needed in order to introduce checking of names inside the config
module in the following commit.

Needed for #8978

@TarantoolBot document
Title: xlog.meta(file-name) method

Description: Open an xlog file, and return its meta block.
Possible errors: Failed to open a file or it does not contain properly
formatted meta block.

Example:

```lua
tarantool> xlog = require('xlog')
---
...

tarantool> xlog.meta('00000000000000000000.snap')
---
- filetype: SNAP
  prev_vclock: {}
  instance_uuid: 87b2e60f-275c-4efa-9b0e-e9562e309692
  vclock: {}
...
```

changelogs/unreleased/lua-xlog-meta.md

+## feature/lua/xlog
+
+* Introduced the `xlog.meta()` method for reading a meta block from an xlog
+  file.

src/box/lua/xlog.c

@@ -365,8 +365,48 @@ lbox_xlog_parser_open_pairs(struct lua_State *L)
 	return 3;
 }
 
+/** Opens an xlog file and returns its meta block. */
+static int
+lbox_xlog_parser_open_meta(struct lua_State *L)
+{
+	int args_n = lua_gettop(L);
+	if (args_n != 1 || !lua_isstring(L, 1))
+		luaL_error(L, "Usage: parser.open(log_filename)");
+
+	const char *filename = luaL_checkstring(L, 1);
+
+	/* Construct xlog cursor and object */
+	struct xlog_cursor *cur = xcalloc(1, sizeof(struct xlog_cursor));
+	if (xlog_cursor_open(cur, filename) < 0) {
+		return luaT_error(L);
+	}
+
+	lua_newtable(L);
+
+	lua_pushstring(L, "filetype");
+	lua_pushstring(L, cur->meta.filetype);
+	lua_settable(L, -3);
+
+	lua_pushstring(L, "instance_uuid");
+	luaT_pushuuidstr(L, &cur->meta.instance_uuid);
+	lua_settable(L, -3);
+
+	lua_pushstring(L, "vclock");
+	luaT_pushvclock(L, &cur->meta.vclock);
+	lua_settable(L, -3);
+
+	lua_pushstring(L, "prev_vclock");
+	luaT_pushvclock(L, &cur->meta.prev_vclock);
+	lua_settable(L, -3);
+
+	xlog_cursor_close(cur, false);
+	free(cur);
+	return 1;
+}
+
 static const struct luaL_Reg lbox_xlog_parser_lib [] = {
 	{ "pairs",	lbox_xlog_parser_open_pairs },
+	{ "meta",	lbox_xlog_parser_open_meta  },
 	{ NULL,		NULL                        }
 };
 

src/box/lua/xlog.lua

@@ -4,6 +4,11 @@ local function xlog_pairs(...)
     return fun.wrap(internal.pairs(...))
 end
 
+local function xlog_meta(...)
+    return internal.meta(...)
+end
+
 return {
     pairs = xlog_pairs,
+    meta = xlog_meta,
 }

test/box-luatest/xlog_reader_test.lua

@@ -2,11 +2,13 @@ local fio = require('fio')
 local server = require('luatest.server')
 local t = require('luatest')
 local xlog = require('xlog')
+local uuid = require('uuid')
 
 local g = t.group()
 
 g.before_all(function(cg)
-    cg.server = server:new()
+    g.box_cfg = { instance_uuid = uuid() }
+    cg.server = server:new({box_cfg = g.box_cfg})
     cg.server:start()
 end)
 
@@ -150,3 +152,14 @@ g.test_bad_xlog = function(cg)
         },
     })
 end
+
+g.test_xlog_meta = function(cg)
+    local glob = fio.glob(fio.pathjoin(cg.server.workdir, '*.snap'))
+    local snap_path = glob[#glob]
+
+    local meta = xlog.meta(snap_path)
+    t.assert_equals(meta.filetype, 'SNAP')
+    t.assert_equals(meta.instance_uuid, cg.box_cfg.instance_uuid)
+    t.assert_not_equals(meta.vclock, nil)
+    t.assert_not_equals(meta.prev_vclock, nil)
+end