[ruby-core:120952] [Ruby master Feature#15663] Documenting autoload semantics

Issue #15663 has been updated by fxn (Xavier Noria).


BTW, I am _describing_ how it works, not implying I like it :).

----------------------------------------
Feature #15663: Documenting autoload semantics
https://bugs.ruby-lang.org/issues/15663#change-111847

* Author: Eregon (Benoit Daloze)
* Status: Open
----------------------------------------
The semantics of autoload are extremely complicated.

As far as I can see, they are unfortunately not documented.

ruby/spec tries to test [many aspects](https://github.com/ruby/spec/blob/master/core/module/autoload_spec.rb) of it, `test/ruby/test_autoload.rb` has a few tests, and e.g. zeitwerk tests [some other parts](https://github.com/fxn/zeitwerk/blob/master/test/lib/zeitwerk/test_ruby_compatibility.rb).
One could of course read the MRI source code, but I find it very hard to follow around `autoload`.

For the context, I'm trying to implement `autoload` as correct as possible in TruffleRuby and finding it very difficult given the inconsistencies (see below) and lack of documentation.

There is nowhere a document on how it should behave, and given the complexity of it I am not even sure MRI behaves as expected.
Could we create this document?
For instance, there is such a [document for refinements](https://github.com/ruby/ruby/blob/trunk/doc/syntax/refinements.rdoc).

Here is an example how confusing autoload can be, and I would love to hear the rationale or have some written semantics on why it is that way.

main.rb:
```ruby
require "pp"

$: << __dir__

Object.autoload(:Foo, "foo")

CHECK = -> state {
  checks = -> {
    {
      defined: defined?(Foo),
      const_defined: Object.const_defined?(:Foo),
      autoload?: Object.autoload?(:Foo),
      in_constants: Object.constants.include?(:Foo),
    }
  }

  pp when: state, **checks.call, other_thread: Thread.new { checks.call }.value
}

CHECK.call(:before_require)

if ARGV.first == "require"
  require "foo"
else
  Foo # trigger the autoload
end

CHECK.call(:after)

p Foo
```

foo.rb:
```ruby
CHECK.call(:during_before_defining)

module Foo
end

CHECK.call(:during_after_defining)
```

Here are the results for MRI 2.6.1:
```ruby
$ ruby main.rb        
{:when=>:before_require,
 :defined=>"constant",
 :const_defined=>true,
 :autoload?=>"foo",
 :in_constants=>true,
 :other_thread=>
  {:defined=>"constant",
   :const_defined=>true,
   :autoload?=>"foo",
   :in_constants=>true}}
{:when=>:during_before_defining,
 :defined=>nil,
 :const_defined=>false,
 :autoload?=>nil,
 :in_constants=>true,
 :other_thread=>
  {:defined=>"constant",
   :const_defined=>true,
   :autoload?=>"foo",
   :in_constants=>true}}
{:when=>:during_after_defining,
 :defined=>"constant",
 :const_defined=>true,
 :autoload?=>nil,
 :in_constants=>true,
 :other_thread=>
  {:defined=>"constant",
   :const_defined=>true,
   :autoload?=>"foo",
   :in_constants=>true}}
{:when=>:after,
 :defined=>"constant",
 :const_defined=>true,
 :autoload?=>nil,
 :in_constants=>true,
 :other_thread=>
  {:defined=>"constant",
   :const_defined=>true,
   :autoload?=>nil,
   :in_constants=>true}}
Foo
```

Looking at during_before_defining, the constant looks not defined during the autoload for the Thread loading it, but looks defined and as an autoload for other threads.

Now we can discover other subtle semantics, by using `require` on the autoload file instead of accessing the constant:

```ruby
$ ruby main.rb require 
{:when=>:before_require,
 :defined=>"constant",
 :const_defined=>true,
 :autoload?=>"foo",
 :in_constants=>true,
 :other_thread=>
  {:defined=>"constant",
   :const_defined=>true,
   :autoload?=>"foo",
   :in_constants=>true}}
{:when=>:during_before_defining,
 :defined=>nil,
 :const_defined=>false,
 :autoload?=>nil,
 :in_constants=>true,
 :other_thread=>
  {:defined=>nil, :const_defined=>false, :autoload?=>nil, :in_constants=>true}}
{:when=>:during_after_defining,
 :defined=>"constant",
 :const_defined=>true,
 :autoload?=>nil,
 :in_constants=>true,
 :other_thread=>
  {:defined=>"constant",
   :const_defined=>true,
   :autoload?=>nil,
   :in_constants=>true}}
{:when=>:after,
 :defined=>"constant",
 :const_defined=>true,
 :autoload?=>nil,
 :in_constants=>true,
 :other_thread=>
  {:defined=>"constant",
   :const_defined=>true,
   :autoload?=>nil,
   :in_constants=>true}}
Foo
```

Looking at during_before_defining, now the other threads seem to see the constant not defined, although it is still in `Object.constants`.
But of course, the constant cannot be removed, as otherwise that would not be thread-safe and other threads would raise NameError when accessing the constant.
In fact, we can see other threads actually wait for the constant, by changing to `Thread.new { Foo; checks.call }`, and then we get a deadlock:

```
Traceback (most recent call last):
	2: from main.rb:20:in `<main>'
	1: from main.rb:17:in `block in <main>'
main.rb:17:in `value': No live threads left. Deadlock? (fatal)
3 threads, 3 sleeps current:0x00007f0124004cb0 main thread:0x000055929cc2c470
* #<Thread:0x000055929cc5b348 sleep_forever>
   rb_thread_t:0x000055929cc2c470 native:0x00007f013381d700 int:0
   main.rb:17:in `value'
   main.rb:17:in `block in <main>'
   main.rb:20:in `<main>'
* #<Thread:[e-mail]:17 sleep_forever>
   rb_thread_t:0x000055929ce026d0 native:0x00007f0129007700 int:0
    depended by: tb_thread_id:0x000055929cc2c470
   main.rb:17:in `value'
   main.rb:17:in `block in <main>'
   foo.rb:1:in `<top (required)>'
   /home/eregon/.rubies/ruby-2.6.1/lib/ruby/2.6.0/rubygems/core_ext/kernel_require.rb:54:in `require'
   /home/eregon/.rubies/ruby-2.6.1/lib/ruby/2.6.0/rubygems/core_ext/kernel_require.rb:54:in `require'
   main.rb:17:in `block (2 levels) in <main>'
* #<Thread:[e-mail]:17 sleep_forever>
   rb_thread_t:0x00007f0124004cb0 native:0x00007f0128e05700 int:0
    depended by: tb_thread_id:0x000055929ce026d0
   main.rb:17:in `block (2 levels) in <main>'
```

This is quite weird. Is the second behavior a bug?
Why should other threads suddenly see the constant as "not defined" while it is loading via `require` in the main thread?
It's also inconsistent with the first case.

I would have thought `require autoload_path` would basically do the same as triggering the autoload of the constant (such as `Foo`). But the results above show they differ.

There are many more complex cases for autoload, such as [this spec](https://github.com/ruby/spec/blob/72bd058b5cf0a9d9de5a188052db2fba021581cc/core/module/autoload_spec.rb#L360-L375), or how is thread-safety is achieved when methods are defined incrementally in Ruby but the module is defined immediately.

Who is knowledgeable about `autoload` and could answer these questions?
Could we start a document specifying the semantics?



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