[ruby-core:104990] [Ruby master Feature#18007] Help developers of C extensions meet requirements in "doc/extension.rdoc"

Issue #18007 has been updated by nobu (Nobuyoshi Nakada).


@naruse objects to that warning appearing every time that instance is created.
So I propose to undefine the allocator in that check, so an exception will be raised when `new` or `allocate` are called later.

```C
static inline void
rb_data_object_check(VALUE klass)
{
    if (klass != rb_cObject && (rb_get_alloc_func(klass) == rb_class_allocate_instance)) {
        rb_undef_alloc_func(klass);
#if 0 // TODO: enable at the next release
        rb_warn("allocator of T_DATA class %"PRIsVALUE" got undefined", klass);
#endif
    }
}
```

----------------------------------------
Feature #18007: Help developers of C extensions meet requirements in "doc/extension.rdoc"
https://bugs.ruby-lang.org/issues/18007#change-93389

* Author: mdalessio (Mike Dalessio)
* Status: Open
* Priority: Normal
----------------------------------------
A pull request for this feature has been submitted at https://github.com/ruby/ruby/pull/4604

## Problem being solved

This option is intended to help developers of C extensions to check if their code meets the requirements explained in "doc/extension.rdoc". Specifically, I want to ensure that `T_DATA` object classes undefine or redefine the `allocate` method.

There is currently no easy way for an author of a C extension to easily see where they have made the mistake of letting the default `allocate` class method remain.

## Description of the solution

Compiled with this option, Ruby will warn when a `T_DATA` object is created whose class has not undefined or redefined the `allocate` method.

A new function is defined, `rb_data_object_check`. That function is called from `rb_data_object_wrap()` and
`rb_data_typed_object_wrap()` (which implement the `Data_Wrap_Struct` family of macros).

The warning, when emitted, looks like this:

```
warning: T_DATA class Nokogiri::XML::Document should undefine or redefine the allocate method, please see doc/extension.rdoc
```

## Examples of this problem in the wild

Using this option, I found that [many of Nokogiri's classes needed to undefine `allocate`](https://github.com/sparklemotion/nokogiri/commit/c5ba3a5).

This PR also updates these core Ruby classes by undefining `allocate`:

- `ObjectSpace::InternalObjectWrapper`
- `Socket::Ifaddr`

## Questions for reviewers

__Does this check really need to be behind a configuration option?__ Performance impact is very small (see benchmarks below), but I put it behind a flag because I am worried that there may be a many C extensions that would emit warnings at runtime, and the users of those extensions cannot fix the problem and so would mostly just be annoyed.

__Should this warning be emitted with the `deprecated` category?__

## Benchmarking

I benchmarked this code by allocating `Nokogiri::XML::NodeSet`s in a loop. This is a class with a [relatively simple `allocate` function](https://github.com/sparklemotion/nokogiri/blob/6d688d8c0f3351797e9576d3710acf458582bb30/ext/nokogiri/xml_node_set.c#L441-L464).

The runs cover the four combinations of enabled/disabled, and warnings/no-warnings.

```
ruby 3.1.0dev (2021-06-25T04:02:18Z flavorjones-extens.. de943189aa) [x86_64-linux]
Warming up --------------------------------------
disabled, warn=false   490.143k i/100ms
Calculating -------------------------------------
disabled, warn=false      4.863M (1.5%) i/s -     49.014M in  10.081177s

ruby 3.1.0dev (2021-06-25T04:02:18Z flavorjones-extens.. de943189aa) [x86_64-linux]
Warming up --------------------------------------
 disabled, warn=true   483.070k i/100ms
Calculating -------------------------------------
 disabled, warn=true      4.839M (ア 1.4%) i/s -     48.790M in  10.083899s

Comparison:
disabled, warn=false:  4863064.0 i/s
 disabled, warn=true:  4839310.1 i/s - same-ish: difference falls within error


ruby 3.1.0dev (2021-06-25T04:02:18Z flavorjones-extens.. de943189aa) [x86_64-linux]
Warming up --------------------------------------
 enabled, warn=false   484.398k i/100ms
Calculating -------------------------------------
 enabled, warn=false      4.840M (ア 1.9%) i/s -     48.440M in  10.011854s

Comparison:
disabled, warn=false:  4863064.0 i/s
 enabled, warn=false:  4840123.2 i/s - same-ish: difference falls within error
 disabled, warn=true:  4839310.1 i/s - same-ish: difference falls within error


ruby 3.1.0dev (2021-06-25T04:02:18Z flavorjones-extens.. de943189aa) [x86_64-linux]
Warming up --------------------------------------
  enabled, warn=true   492.200k i/100ms
Calculating -------------------------------------
  enabled, warn=true      4.866M (ア 2.1%) i/s -     48.728M in  10.017455s

Comparison:
  enabled, warn=true:  4866434.8 i/s
disabled, warn=false:  4863064.0 i/s - same-ish: difference falls within error
 enabled, warn=false:  4840123.2 i/s - same-ish: difference falls within error
 disabled, warn=true:  4839310.1 i/s - same-ish: difference falls within error
```

My conclusion is that the performance impact is very small, and we could omit the option if the Ruby core maintainers decide this behavior should be on by default.



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