[ruby-core:105152] [Ruby master Bug#18007] Help developers of C extensions meet requirements in "doc/extension.rdoc"

Issue #18007 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 c42208f8e24402fe1aa8747901fba275bfb0d56b merged revision(s) c0f4e4ca6d0f76985bca79314b232b787c8f008e.

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Bug #18007: Help developers of C extensions meet requirements in "doc/extension.rdoc"
https://bugs.ruby-lang.org/issues/18007#change-93559

* Author: mdalessio (Mike Dalessio)
* Status: Closed
* Priority: Normal
* Backport: 2.6: REQUIRED, 2.7: REQUIRED, 3.0: DONE
----------------------------------------
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 alloc function.

https://github.com/ruby/ruby/blob/6963f8f743b42f9004a0879cd66c550f18987352/doc/extension.rdoc#label-Write+the+C+Code says:
> Since Object.allocate allocates an ordinary T_OBJECT type (instead of T_DATA), it's important to either use rb_define_alloc_func() to overwrite it or rb_undef_alloc_func() to delete it.

(note: which matters when using TypedData_Make_Struct/TypedData_Wrap_Struct as the native pointer is supplied without calling the class alloc function).

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 alloc function remain for their class (and therefore `class.new` creating a T_OBJECT instead of T_DATA and not setting the data pointer).

## 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 alloc function.

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 alloc function, 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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