box - Documentation for Ruby 4.0
Description: box: 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 applica
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# 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 three box types:

- Master box

- Root box

- User boxes

Ruby bootstrap runs in the root box, and a

There is the root box, just a single box in a Ruby process. All builtin classes/modules are defined and run 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. The files specified with `-r` command line option will be required in the main box.

Calling `Ruby::Box.new` creates an “optional” box (a user, non-main box), technically equal to the main box.

Ruby also has the master box. The master box is the “master copy” of all boxes. Boxes will be created as a copy of the master box. The master box is only for the source of box copies, and no code runs in the master box.

    [master]
     |
     |----[root]
     |
     |----[main]
     |
     |----[user box 1]
     |
     |----[user box 2]
     ...

### 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` above 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

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

Builtin classes and modules are loaded in all boxes, and run in the root box.

### Exceptional non-built-in classes/modules

There are some exceptional classes/modules that are enabled in default, but aren’t built-in classes. Those classes/modules are:

- `RubyGems`

- `ErrorHighlight`

- `DidYouMean`

- `SyntaxSuggest`

Those classes/modules (part of default gems) are loaded in each boxes independently. If a user box’s code calls RubyGems, it calls the RubyGems inside the box itself, instead of the root box’s one.

### 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 both`Hash#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.

#### 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.