Add strict.lua to replace "Module:No globals"

This is being backported because many users copy lua modules from
Wikipedia, and thus benefit from forwards-compatibility.

For the most part, it is a good idea to avoid global variables and use
`local` variables instead. Quoting from the ScopeTutorial[1], "The
general rule is to always use local variables, unless it's necessary for
every part of your program to be able to access the variable (which is
very rare)."

Wikimedia module authors have written "Module:No globals", which errors
on the use of any global variable. On the English Wikipedia, this is
used on 32% of pages (18 million). Wikidata[2] indicates that it's been
copied to 334 other wikis.

Lua itself distributes an extra named "strict.lua"[3], which is what
this is based off of. Similar to bit32.lua, this is a pure-Lua library
that can be imported/enabled with `require( "strict" )` at the top of a
module.

The two changes I made from Lua's strict is to exempt the `arg` key,
which is used internally by Scribunto, and remove `what()`, since we
don't enable access to `debug.getinfo()` for security reasons.

[1] https://lua-users.org/wiki/ScopeTutorial
[2] https://www.wikidata.org/wiki/Q16748603
[3] http://www.lua.org/extras/5.1/strict.lua

(Cherry-picked from 829c53ef05)

Bug: T209310
Change-Id: I46ee6f630ac6b26c68c31becd1f3b9d961bcab29

COPYING

@@ -5,6 +5,7 @@ Some included files and subdirectories are licensed with an MIT license, not
 with the GPL. These are:
 
 * engines/LuaCommon/lualib/package.lua (as described in that file)
+* engines/LuaCommon/lualib/strict.lua (as described in that file)
 * engines/LuaCommon/lualib/luabit/ (as described in readme.txt in that directory)
 * engines/LuaCommon/lualib/ustring/ (as described in README in that directory)
 * engines/LuaStandalone/binaries/ (as described in README in that directory)

includes/engines/LuaCommon/lualib/strict.lua

@@ -0,0 +1,31 @@
+-- strict.lua (fork of http://www.lua.org/extras/5.1/strict.lua)
+-- checks uses of undeclared global variables
+-- All global variables must be 'declared' through a regular assignment
+-- (even assigning nil will do) in a main chunk before being used
+-- anywhere or assigned to inside a function.
+-- distributed under the Lua license: http://www.lua.org/license.html
+-- Scribunto modifications:
+-- * exempt arg, used by require()
+-- * remove what(), since debug.getinfo isn't allowed
+
+local error, rawset, rawget = error, rawset, rawget
+
+local mt = getmetatable(_G)
+if mt == nil then
+  mt = {}
+  setmetatable(_G, mt)
+end
+
+mt.__newindex = function (t, n, v)
+  if n ~= "arg" then
+    error("assign to undeclared variable '"..n.."'", 2)
+  end
+  rawset(t, n, v)
+end
+
+mt.__index = function (t, n)
+  if n ~= "arg" then
+    error("variable '"..n.."' is not declared", 2)
+  end
+  return rawget(t, n)
+end

tests/phpunit/engines/LuaCommon/CommonTests.lua

@@ -166,6 +166,22 @@ function test.noLeaksViaPackageLoaded()
 	return 'ok'
 end
 
+function test.strictGood()
+	require( 'strict' )
+	local foo = "bar"
+	return foo
+end
+
+function test.strictBad1()
+	require( 'strict' )
+	return bar
+end
+
+function test.strictBad2()
+	require( 'strict' )
+	bar = "foo"
+end
+
 test.loadData = {}
 
 function test.loadData.get( ... )
@@ -300,6 +316,19 @@ return testframework.getTestProvider( {
 	  expect = { 'ok' },
 	},
 
+	{ name = 'strict on good code raises no errors',
+		func = test.strictGood,
+		expect = { 'bar' },
+	},
+	{ name = 'strict on code reading from a global errors',
+		func = test.strictBad1,
+		expect = "variable 'bar' is not declared",
+	},
+	{ name = 'strict on code setting from a global errors',
+		func = test.strictBad2,
+		expect = "assign to undeclared variable 'bar'",
+	},
+
 	{ name = 'mw.loadData, returning non-table',
 	  func = mw.loadData, args = { 'Module:CommonTests-data-fail1' },
 	  expect = "Module:CommonTests-data-fail1 returned string, table expected",