Issue #18141 has been updated by nagachika (Tomoyuki Chikanaga).

Backport changed from 2.6: REQUIRED, 2.7: REQUIRED, 3.0: REQUIRED to 2.6: REQUIRED, 2.7: REQUIRED, 3.0: DONE

ruby_3_0 fe9d33beb78d5c7932a5c2ca3953045c0ae751d5 merged revision(s) 89242279e61b023a81c58065c62a82de8829d0b3,529fc204af84f825f98f83c34b004acbaa802615.

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Bug #18141: Marshal load with proc yield objects before they are fully initialized 
https://bugs.ruby-lang.org/issues/18141#change-94100

* Author: byroot (Jean Boussier)
* Status: Closed
* Priority: Normal
* Backport: 2.6: REQUIRED, 2.7: REQUIRED, 3.0: DONE
----------------------------------------
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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