loaders: add require_first function (internal)

Usage example:

```lua
local loaders = require('internal.loaders')
local vshard = loaders.require_first('vshard-ee', 'vshard')
```

The function is for internal use.

It would be nice to have something of this kind in the public API, but
I'm not going to solve it within this patch.

Needed for https://github.com/tarantool/tarantool-ee/issues/815

NO_DOC=no public API changes
NO_CHANGELOG=see NO_DOC

src/lua/loaders.lua

@@ -520,6 +520,31 @@ _G.require = function(modname)
     return raw_require(modname)
 end
 
+-- Load first available module from the given ones.
+--
+-- It just calls `require()` on the provided arguments (in the
+-- given order) and once this call succeeds, the function returns
+-- its result.
+--
+-- If neither of the given modules can be `require`'d, an error is
+-- raised.
+local function require_first(...)
+    if select('#', ...) == 0 then
+        error('loaders.require_first expects at least one argument', 0)
+    end
+
+    for i = 1, select('#', ...) do
+        local module_name = select(i, ...)
+        local ok, res = pcall(require, module_name)
+        if ok then
+            return res
+        end
+    end
+
+    local modules_str = ('"%s"'):format(table.concat({...}, '", "'))
+    error(('neither of modules %s are found'):format(modules_str), 0)
+end
+
 return {
     ROCKS_LIB_PATH = ROCKS_LIB_PATH,
     ROCKS_LUA_PATH = ROCKS_LUA_PATH,
@@ -545,4 +570,5 @@ return {
     --
     -- Usage: loaders.no_package_loaded[modname] = true
     no_package_loaded = no_package_loaded,
+    require_first = require_first,
 }

test/app-luatest/require_first_test.lua

+local t = require('luatest')
+local treegen = require('test.treegen')
+local justrun = require('test.justrun')
+
+local g = t.group()
+
+g.before_all(treegen.init)
+g.after_all(treegen.clean)
+
+-- A few test cases for the loaders.require_first() function.
+--
+-- Note: The loaders module is internal.
+g.test_basic = function(g)
+    local dir = treegen.prepare_directory(g, {}, {})
+    treegen.write_script(dir, 'a.lua', 'return {whoami = "a"}')
+    treegen.write_script(dir, 'b.lua', 'return {whoami = "b"}')
+
+    treegen.write_script(dir, 'main.lua', string.dump(function()
+        local loaders = require('internal.loaders')
+        local t = require('luatest')
+
+        for i, case in ipairs({
+            -- Positive test cases.
+            {args = {'a', 'b'}, exp = {whoami = 'a'}},
+            {args = {'b', 'a'}, exp = {whoami = 'b'}},
+            {args = {'a', 'c'}, exp = {whoami = 'a'}},
+            {args = {'c', 'a'}, exp = {whoami = 'a'}},
+            -- Negative test cases.
+            {args = {}, err = 'loaders.require_first expects at least one ' ..
+                'argument'},
+            {args = {'c'}, err = 'neither of modules "c" are found'},
+            {args = {'c', 'd'}, err = 'neither of modules "c", "d" are found'},
+        }) do
+            if case.exp ~= nil then
+                assert(case.err == nil)
+                local res = loaders.require_first(unpack(case.args))
+                t.assert_equals(res, case.exp, ('case #%03d'):format(i))
+            else
+                assert(case.err ~= nil)
+                t.assert_error_msg_equals(case.err, loaders.require_first,
+                    unpack(case.args))
+            end
+        end
+    end))
+
+    local opts = {nojson = true, stderr = true}
+    local res = justrun.tarantool(dir, {}, {'main.lua'}, opts)
+    t.assert_equals(res.exit_code, 0, {res.stdout, res.stderr})
+end