Ruby Box - Ruby’s in-process separation of Classes and Modules

Ruby Box is designed to provide separated spaces in a Ruby process, to isolate application codes, libraries and monkey patches.

Known issues

• Experimental warning is shown when ruby starts with RUBY_BOX=1 (specify -W:no-experimental option to hide it)

• Installing native extensions may fail under RUBY_BOX=1 because of stack level too deep in extconf.rb

• require 'active_support/core_ext' may fail under RUBY_BOX=1

• Defined methods in a box may not be referred by built-in methods written in Ruby

TODOs

• Add the loaded box on iseq to check if another box tries running the iseq (add a field only when VM_CHECK_MODE?)

• Assign its own TOPLEVEL_BINDING in boxes

• Fix calling warn in boxes to refer $VERBOSE and Warning.warn in the box

• Make an internal data container class Ruby::Box::Entry invisible

• More test cases about $LOAD_PATH and $LOADED_FEATURES

How to use

Enabling Ruby Box

First, an environment variable should be set at the ruby process bootup: RUBY_BOX=1. The only valid value is 1 to enable Ruby Box. Other values (or unset RUBY_BOX) means disabling Ruby Box. And setting the value after Ruby program starts doesn’t work.

Using Ruby Box

Ruby::Box class is the entrypoint of Ruby Box.

box = Ruby::Box.new
box.require('something') # or require_relative, load

The required file (either .rb or .so/.dll/.bundle) is loaded in the box (box here). The required/loaded files from something will be loaded in the box recursively.

# something.rb

X = 1

class Something
  def self.x = X
  def x = ::X
end

Classes/modules, those methods and constants defined in the box can be accessed via box object.

X = 2
p X                 # 2
p ::X               # 2
p box::Something.x  # 1
p box::X            # 1

Instance methods defined in the box also run with definitions in the box.

s = box::Something.new

p s.x  # 1

Specifications

Ruby Box types

There are two box types:

• Root box

• User boxes

There is the root box, just a single box in a Ruby process. Ruby bootstrap runs in the root box, and all builtin classes/modules are defined in the root box. (See “Builtin classes and modules”.)

User boxes are to run user-written programs and libraries loaded from user programs. The user’s main program (specified by the ruby command line argument) is executed in the “main” box, which is a user box automatically created at the end of Ruby’s bootstrap, copied from the root box.

When Ruby::Box.new is called, an “optional” box (a user, non-main box) is created, copied from the root box. All user boxes are flat, copied from the root box.

Ruby Box class and instances

Ruby::Box is a class, as a subclass of Module. Ruby::Box instances are a kind of Module.

Classes and modules defined in boxes

The classes and modules, newly defined in a box box, are accessible via box. For example, if a class A is defined in box, it is accessible as box::A from outside of the box.

In the box box, A can be referred to as A (and ::A).

Built-in classes and modules reopened in boxes

In boxes, builtin classes/modules are visible and can be reopened. Those classes/modules can be reopened using class or module clauses, and class/module definitions can be changed.

The changed definitions are visible only in the box. In other boxes, builtin classes/modules and those instances work without changed definitions.

# in foo.rb
class String
  BLANK_PATTERN = /\A\s*\z/
  def blank?
    self =~ BLANK_PATTERN
  end
end

module Foo
  def self.foo = "foo"

  def self.foo_is_blank?
    foo.blank?
  end
end

Foo.foo.blank? #=> false
"foo".blank?   #=> false

# in main.rb
box = Ruby::Box.new
box.require('foo')

box::Foo.foo_is_blank? #=> false   (#blank? called in box)

"foo".blank?          # NoMethodError
String::BLANK_PATTERN # NameError

The main box and box are different boxes, so monkey patches in main are also invisible in box.

Builtin classes and modules

In the box context, “builtin” classes and modules are classes and modules:

• Accessible without any require calls in user scripts

• Defined before any user program start running

• Including classes/modules loaded by prelude.rb (including RubyGems Gem, for example)

Hereafter, “builtin classes and modules” will be referred to as just “builtin classes”.

Builtin classes referred via box objects

Builtin classes in a box box can be referred from other boxes. For example, box::String is a valid reference, and String and box::String are identical (String == box::String, String.object_id == box::String.object_id).

box::String-like reference returns just a String in the current box, so its definition is String in the box, not in box.

# foo.rb
class String
  def self.foo = "foo"
end

# main.rb
box = Ruby::Box.new
box.require('foo')

box::String.foo  # NoMethodError

Class instance variables, class variables, constants

Builtin classes can have different sets of class instance variables, class variables and constants between boxes.

# foo.rb
class Array
  @v = "foo"
  @@v = "_foo_"
  V = "FOO"
end

Array.instance_variable_get(:@v) #=> "foo"
Array.class_variable_get(:@@v)   #=> "_foo_"
Array.const_get(:V)              #=> "FOO"

# main.rb
box = Ruby::Box.new
box.require('foo')

Array.instance_variable_get(:@v) #=> nil
Array.class_variable_get(:@@v)   # NameError
Array.const_get(:V)              # NameError

Global variables

In boxes, changes on global variables are also isolated in the boxes. Changes on global variables in a box are visible/applied only in the box.

# foo.rb
$foo = "foo"
$VERBOSE = nil

puts "This appears: '#{$foo}'"

# main.rb
p $foo      #=> nil
p $VERBOSE  #=> false

box = Ruby::Box.new
box.require('foo')  # "This appears: 'foo'"

p $foo      #=> nil
p $VERBOSE  #=> false

Top level constants

Usually, top level constants are defined as constants of Object. In boxes, top level constants are constants of Object in the box. And the box object box‘s constants are strictly equal to constants of Object.

# foo.rb
FOO = 100

FOO         #=> 100
Object::FOO #=> 100

# main.rb
box = Ruby::Box.new
box.require('foo')

box::FOO      #=> 100

FOO          # NameError
Object::FOO  # NameError

Top level methods

Top level methods are private instance methods of Object, in each box.

# foo.rb
def yay = "foo"

class Foo
  def self.say = yay
end

Foo.say #=> "foo"
yay     #=> "foo"

# main.rb
box = Ruby::Box.new
box.require('foo')

box::Foo.say  #=> "foo"

yay  # NoMethodError

There is no way to expose top level methods in boxes to others. (See “Expose top level methods as a method of the box object” in “Discussions” section below)

Ruby Box scopes

Ruby Box works in file scope. One .rb file runs in a single box.

Once a file is loaded in a box box, all methods/procs defined/created in the file run in box.

Utility methods

Several methods are available for trying/testing Ruby Box.

• Ruby::Box.current returns the current box

• Ruby::Box.enabled? returns true/false to represent RUBY_BOX=1 is specified or not

• Ruby::Box.root returns the root box

• Ruby::Box.main returns the main box

• Ruby::Box#eval evaluates a Ruby code (String) in the receiver box, just like calling load with a file

Implementation details

ISeq inline method/constant cache

As described above in “Ruby Box scopes”, an “.rb” file runs in a box. So method/constant resolution will be done in a box consistently.

That means ISeq inline caches work well even with boxes. Otherwise, it’s a bug.

Method call global cache (gccct)

rb_funcall() C function refers to the global cc cache table (gccct), and the cache key is calculated with the current box.

So, rb_funcall() calls have a performance penalty when Ruby Box is enabled.

Current box and loading box

The current box is the box that the executing code is in. Ruby::Box.current returns the current box object.

The loading box is an internally managed box to determine the box to load newly required/loaded files. For example, box is the loading box when box.require("foo") is called.

Discussions

More builtin methods written in Ruby

If Ruby Box is enabled by default, builtin methods can be written in Ruby because it can’t be overridden by users’ monkey patches. Builtin Ruby methods can be JIT-ed, and it could bring performance reward.

Monkey patching methods called by builtin methods

Builtin methods sometimes call other builtin methods. For example, Hash#map calls Hash#each to retrieve entries to be mapped. Without Ruby Box, Ruby users can overwrite Hash#each and expect the behavior change of Hash#map as a result.

But with boxes, Hash#map runs in the root box. Ruby users can define Hash#each only in user boxes, so users cannot change Hash#map‘s behavior in this case. To achieve it, users should override bothHash#map and Hash#each (or only Hash#map).

It is a breaking change.

Users can define methods using Ruby::Box.root.eval(...), but it’s clearly not ideal API.

Assigning values to global variables used by builtin methods

Similar to monkey patching methods, global variables assigned in a box is separated from the root box. Methods defined in the root box referring a global variable can’t find the re-assigned one.

Context of $LOAD_PATH and $LOADED_FEATURES

Global variables $LOAD_PATH and $LOADED_FEATURES control require method behaviors. So those variables are determined by the loading box instead of the current box.

This could potentially conflict with the user’s expectations. We should find the solution.

Expose top level methods as a method of the box object

Currently, top level methods in boxes are not accessible from outside of the box. But there might be a use case to call other box’s top level methods.

Split root and builtin box

Currently, the single “root” box is the source of classext CoW. And also, the “root” box can load additional files after starting main script evaluation by calling methods which contain lines like require "openssl".

That means, user boxes can have different sets of definitions according to when it is created.

[root]
 |
 |----[main]
 |
 |(require "openssl" called in root)
 |
 |----[box1] having OpenSSL
 |
 |(remove_const called for OpenSSL in root)
 |
 |----[box2] without OpenSSL

This could cause unexpected behavior differences between user boxes. It should NOT be a problem because user scripts which refer to OpenSSL should call require "openssl" by themselves. But in the worst case, a script (without require "openssl") runs well in box1, but doesn’t run in box2. This situation looks like a “random failure” to users.

An option possible to prevent this situation is to have “root” and “builtin” boxes.

• root

• The box for the Ruby process bootstrap, then the source of CoW

• After starting the main box, no code runs in this box

• builtin

• The box copied from the root box at the same time with “main”

• Methods and procs defined in the “root” box run in this box

• Classes and modules required will be loaded in this box

This design realizes a consistent source of box CoW.

Separate cc_tbl and callable_m_tbl, cvc_tbl for less classext CoW

The fields of rb_classext_t contains several cache(-like) data, cc_tbl(callcache table), callable_m_tbl(table of resolved complemented methods) and cvc_tbl(class variable cache table).

The classext CoW is triggered when the contents of rb_classext_t are changed, including cc_tbl, callable_m_tbl, and cvc_tbl. But those three tables are changed by just calling methods or referring class variables. So, currently, classext CoW is triggered much more times than the original expectation.

If we can move those three tables outside of rb_classext_t, the number of copied rb_classext_t will be much less than the current implementation.