Bug #18007
Updated by Eregon (Benoit Daloze) over 3 years ago
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.
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 `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 (± 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 (± 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 (± 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.