[ruby-core:105462] [Ruby master Bug#18141] Marshal load with proc yield objects before they are fully initialized

Issue #18141 has been updated by byroot (Jean Boussier).

Status changed from Closed to Open
Description updated
Subject changed from Marshal load with proc yield strings before they are fully initialized  to Marshal load with proc yield objects before they are fully initialized 

I took the liberty to re-open this issue and to rewrite it to be more generic. 

I wonder if it wouldn't be simpler to revert the string only fix (https://github.com/ruby/ruby/pull/4797), and then to merge the more general one (https://github.com/ruby/ruby/pull/4866), this way it would be simpler to backport.

Any opinions?

----------------------------------------
Bug #18141: Marshal load with proc yield objects before they are fully initialized 
https://bugs.ruby-lang.org/issues/18141#change-93912

* Author: byroot (Jean Boussier)
* Status: Open
* Priority: Normal
* Backport: 2.6: REQUIRED, 2.7: REQUIRED, 3.0: REQUIRED
----------------------------------------
I assume this is a bug because I can't find any spec or test for this behaviour:

Consider the following script:
```ruby
payload = Marshal.dump("foo")

Marshal.load(payload, -> (obj) {
  if obj.is_a?(String)
    p [obj, obj.encoding]
  end
  obj
})
p [:final, string, string.encoding]
```

outputs:
```ruby
["foo", #<Encoding:ASCII-8BIT>]
[:final, "foo", #<Encoding:UTF-8>]
```

So `Marshal` call the proc before the string get its encoding assigned, this is because the encoding is stored alongside as a `TYPE_IVAR`. I think in such cases `Marshal` should delay calling the proc until the object is fully restored.

A corollary to this behaviour is that the following code:

```ruby
Marshal.load(payload, :freeze.to_proc)
```

raises with `can't modify frozen String: "foo" (FrozenError)`.

The same happens with any instance variable on `Array` or `Hash`

```ruby
foo = {}
foo.instance_variable_set(:@bar, 42)

payload = Marshal.dump(foo)

object = Marshal.load(payload, ->(obj) {
  if obj.is_a?(Hash)
    p [obj, obj.instance_variable_get(:@bar)]
    obj.freeze
  end
  obj
})
```

```
[{}, nil]
/tmp/marshal.rb:6:in `load': can't modify frozen Hash: {} (FrozenError)
	from /tmp/marshal.rb:6:in `<main>
```



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