storage: fix core_version string in exports

exports_deply_funcs() used to log the core version as nil. The problem
is the fact, that table.concat was used for core version string, so the
complied exports always had nil instead of core_version.

table.concat doesn't work with non-array tables, as they have no defined
order. Let's explicitly define __tostring function for version and use
it in order to log core version.

Closes tarantool#465

NO_DOC=bugfix

test/unit-luatest/storage_exports_test.lua

@@ -135,3 +135,16 @@ test_group.test_deploy_privs = function(g)
         box.schema.role.drop('two')
     end)
 end
+
+test_group.test_core_version = function(g)
+    g.server:exec(function()
+        local vexports = require('vshard.storage.exports')
+        local exports = vexports.log[#vexports.log]
+        exports = vexports.compile(exports)
+
+        local version = _TARANTOOL
+        local pos = version:find('-g')
+        version = version:sub(1, pos - 1)
+        ilt.assert_equals(exports.core_version, version)
+    end)
+end

vshard/storage/exports.lua

@@ -43,7 +43,7 @@ local function exports_compile(exports)
     end
     return {
         vshard_version = exports.version,
-        core_version = table.concat(lvutil.core_version, '.'),
+        core_version = tostring(lvutil.core_version),
         funcs = compiled_funcs,
     }
 end

vshard/version.lua

@@ -75,6 +75,17 @@ local version_mt = {
     __le = function(l, r)
         return version_cmp(l, r) <= 0
     end,
+    __tostring = function(v)
+        local str = v.id_major .. '.' .. v.id_middle .. '.' .. v.id_minor
+        if v.rel_type then
+            str = str .. '-' .. v.rel_type
+            if v.rel_num ~= 0 then
+                str = str .. v.rel_num
+            end
+        end
+        str = str .. '-' .. v.id_commit
+        return str
+    end,
 }
 
 local function version_new(id_major, id_middle, id_minor, rel_type, rel_num,