[ruby-core:116198] [Ruby master Feature#19057] Hide implementation of `rb_io_t`.

Issue #19057 has been updated by ioquatix (Samuel Williams).


Update from 5 months ago:

- unicorn has merged a fix but it is not released: https://yhbt.net/unicorn.git/63c85c4282d15e22bd32a905883d2d0e149619d1/s/
- kgio has merged a fix but it is not released: https://yhbt.net/kgio.git/dbf5290cf9f89174f6b35a597af9a4226633d79b/s/
- raindrops merged a fix but it is not released: https://yhbt.net/raindrops.git/b3212417cc3e7cc44aa9e1ffe89b0d62ef3fdab5/s/

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Feature #19057: Hide implementation of `rb_io_t`.
https://bugs.ruby-lang.org/issues/19057#change-106214

* Author: ioquatix (Samuel Williams)
* Status: Assigned
* Priority: Normal
* Assignee: ioquatix (Samuel Williams)
* Target version: 3.4
----------------------------------------
In order to make improvements to the IO implementation like <https://bugs.ruby-lang.org/issues/18455>, we need to add new fields to `struct rb_io_t`.

By the way, ending types in `_t` is not recommended by POSIX, so I'm also trying to rename the internal implementation to drop `_t` where possible during this conversion.

Anyway, we should try to hide the implementation of `struct rb_io`. Ideally, we don't expose any of it, but the problem is backwards compatibility.

So, in order to remain backwards compatibility, we should expose some fields of `struct rb_io`, the most commonly used one is `fd` and `mode`, but several others are commonly used.

There are many fields which should not be exposed because they are implementation details.

## Current proposal

The current proposed change <https://github.com/ruby/ruby/pull/6511> creates two structs:

```c
// include/ruby/io.h
#ifndef RB_IO_T
struct rb_io {
  int fd;
  // ... public fields ...
};
#else
struct rb_io;
#endif

// internal/io.h
#define RB_IO_T
struct rb_io {
  int fd;
  // ... public fields ...
  // ... private fields ...
};
```

However, we are not 100% confident this is safe according to the C specification. My experience is not sufficiently wide to say this is safe in practice, but it does look okay to both myself, and @Eregon + @tenderlovemaking have both given some kind of approval.

That being said, maybe it's not safe.

There are two alternatives:

## Hide all details

We can make public `struct rb_io` completely invisible.

```c
// include/ruby/io.h
#define RB_IO_HIDDEN
struct rb_io;
int rb_ioptr_descriptor(struct rb_io *ioptr); // accessor for previously visible state.

// internal/io.h
struct rb_io {
  // ... all fields ...
};
```

This would only be forwards compatible, and code would need to feature detect like this:

```c
#ifdef RB_IO_HIDDEN
#define RB_IOPTR_DESCRIPTOR rb_ioptr_descriptor
#else
#define RB_IOPTR_DESCRIPTOR(ioptr) rb_ioptr_descriptor(ioptr)
#endif
```

## Nested public interface

Alternatively, we can nest the public fields into the private struct:

```c
// include/ruby/io.h
struct rb_io_public {
  int fd;
  // ... public fields ...
};

// internal/io.h
#define RB_IO_T
struct rb_io {
  struct rb_io_public public;
  // ... private fields ...
};
```

## Considerations

I personally think the "Hide all details" implementation is the best, but it's also the lest compatible. This is also what we are ultimately aiming for, whether we decide to take an intermediate "compatibility step" is up to us.

I think "Nested public interface" is messy and introduces more complexity, but it might be slightly better defined than the "Current proposal" which might create undefined behaviour. That being said, all the tests are passing.





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