Fix the signature of loadfile

Lua 5.2+ uses loadfile(filename, mode, env), not loadfile(filename,
env). While this is a minor incompatibility, it'd be nice to be
consistent as much as possible.

We try to handle the incorrect case too, as obviously we don't want to
break existing programs.

src/main/resources/assets/computercraft/lua/bios.lua

@@ -539,23 +539,28 @@ function read( _sReplaceChar, _tHistory, _fnComplete, _sDefault )
     return sLine
 end
 
-function loadfile( _sFile, _tEnv )
-    expect(1, _sFile, "string")
-    expect(2, _tEnv, "table", "nil")
-
-    local file = fs.open( _sFile, "r" )
-    if file then
-        local func, err = load( file.readAll(), "@" .. fs.getName( _sFile ), "t", _tEnv )
-        file.close()
-        return func, err
+function loadfile( filename, mode, env )
+    -- Support the previous `loadfile(filename, env)` form instead.
+    if type(mode) == "table" and env == nil then
+        mode, env = nil, mode
     end
-    return nil, "File not found"
+
+    expect(1, filename, "string")
+    expect(2, mode, "string", "nil")
+    expect(3, env, "table", "nil")
+
+    local file = fs.open( filename, "r" )
+    if not file then return nil, "File not found" end
+
+    local func, err = load( file.readAll(), "@" .. fs.getName( filename ), mode, env )
+    file.close()
+    return func, err
 end
 
 function dofile( _sFile )
     expect(1, _sFile, "string")
 
-    local fnFile, e = loadfile( _sFile, _G )
+    local fnFile, e = loadfile( _sFile, nil, _G )
     if fnFile then
         return fnFile()
     else
@@ -571,7 +576,7 @@ function os.run( _tEnv, _sPath, ... )
     local tArgs = table.pack( ... )
     local tEnv = _tEnv
     setmetatable( tEnv, { __index = _G } )
-    local fnFile, err = loadfile( _sPath, tEnv )
+    local fnFile, err = loadfile( _sPath, nil, tEnv )
     if fnFile then
         local ok, err = pcall( function()
             fnFile( table.unpack( tArgs, 1, tArgs.n ) )
@@ -605,7 +610,7 @@ function os.loadAPI( _sPath )
 
     local tEnv = {}
     setmetatable( tEnv, { __index = _G } )
-    local fnAPI, err = loadfile( _sPath, tEnv )
+    local fnAPI, err = loadfile( _sPath, nil, tEnv )
     if fnAPI then
         local ok, err = pcall( fnAPI )
         if not ok then

src/main/resources/assets/computercraft/lua/rom/programs/shell.lua

@@ -56,7 +56,7 @@ local function createShellEnv( sDir )
                     sPath = fs.combine(sDir, sPath)
                 end
                 if fs.exists(sPath) and not fs.isDir(sPath) then
-                    local fnFile, sError = loadfile( sPath, tEnv )
+                    local fnFile, sError = loadfile( sPath, nil, tEnv )
                     if fnFile then
                         return fnFile, sPath
                     else

src/test/java/dan200/computercraft/core/ComputerTestDelegate.java

@@ -91,7 +91,7 @@ public class ComputerTestDelegate
         try( WritableByteChannel channel = mount.openChannelForWrite( "startup.lua" );
              Writer writer = Channels.newWriter( channel, StandardCharsets.UTF_8.newEncoder(), -1 ) )
         {
-            writer.write( "loadfile('test/mcfly.lua', _ENV)('test/spec') cct_test.finish()" );
+            writer.write( "loadfile('test/mcfly.lua', nil, _ENV)('test/spec') cct_test.finish()" );
         }
 
         computer = new Computer( new BasicEnvironment( mount ), term, 0 );

src/test/java/dan200/computercraft/core/computer/ComputerBootstrap.java

@@ -40,7 +40,7 @@ public class ComputerBootstrap
     {
         MemoryMount mount = new MemoryMount()
             .addFile( "test.lua", program )
-            .addFile( "startup", "assertion.assert(pcall(loadfile('test.lua', _ENV))) os.shutdown()" );
+            .addFile( "startup", "assertion.assert(pcall(loadfile('test.lua', nil, _ENV))) os.shutdown()" );
 
         run( mount, x -> { } );
     }

src/test/resources/test-rom/mcfly.lua

@@ -501,7 +501,7 @@ do
             if fs.isDir(file) then
                 run_in(file)
             elseif file:sub(-#suffix) == suffix then
-                local fun, err = loadfile(file, env)
+                local fun, err = loadfile(file, nil, env)
                 if not fun then
                     do_test { name = file:sub(#root_dir + 2), error = { message = err } }
                 else

src/test/resources/test-rom/spec/base_spec.lua

@@ -16,18 +16,43 @@ describe("The Lua base library", function()
     end)
 
     describe("loadfile", function()
+        local function make_file()
+            local tmp = fs.open("test-files/out.lua", "w")
+            tmp.write("return _ENV")
+            tmp.close()
+        end
+
         it("validates arguments", function()
             loadfile("")
-            loadfile("", {})
+            loadfile("", "")
+            loadfile("", "", {})
 
             expect.error(loadfile, nil):eq("bad argument #1 (expected string, got nil)")
-            expect.error(loadfile, "", false):eq("bad argument #2 (expected table, got boolean)")
+            expect.error(loadfile, "", false):eq("bad argument #2 (expected string, got boolean)")
+            expect.error(loadfile, "", "", false):eq("bad argument #3 (expected table, got boolean)")
         end)
 
         it("prefixes the filename with @", function()
             local info = debug.getinfo(loadfile("/rom/startup.lua"), "S")
             expect(info):matches { short_src = "startup.lua", source = "@startup.lua" }
         end)
+
+        it("loads a file with the global environment", function()
+            make_file()
+            expect(loadfile("test-files/out.lua")()):eq(_G)
+        end)
+
+        it("loads a file with a specific environment", function()
+            make_file()
+            local env = {}
+            expect(loadfile("test-files/out.lua", nil, env)()):eq(env)
+        end)
+
+        it("supports the old-style argument form", function()
+            make_file()
+            local env = {}
+            expect(loadfile("test-files/out.lua", env)()):eq(env)
+        end)
     end)
 
     describe("dofile", function()