[ruby-core:113050] [Ruby master Feature#19560] IO#close_on_fork= and IO#close_on_fork?

Issue #19560 has been reported by byroot (Jean Boussier).

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Feature #19560: IO#close_on_fork= and IO#close_on_fork?
https://bugs.ruby-lang.org/issues/19560

* Author: byroot (Jean Boussier)
* Status: Open
* Priority: Normal
----------------------------------------
### Context

Forking setups are extremely common in the Ruby ecosystem, as they remain the primary way to get parallelism with MRI.

Generally speaking it works very well, however there are two main issues library authors and application owners need to be careful of:

  - Restarting threads
  - Closing inherited connections and other file descriptors.

I believe we could make the second one much easier.

### O_CLOFORK

A couple years ago, [a new flag was added to the POSIX spec: `O_CLOFORK`](https://austingroupbugs.net/view.php?id=1318). Similar to `O_CLOEXEC`, this file descriptor flag make it so the file descriptor is automatically closed upon forking.

Unfortunately its support is relatively limited for now. It's supported on macOS and some relatively exotic unixes, but not in Linux nor most BSDs.

[The feature was discussed on Linux mailing list](https://lore.kernel.org/lkml/20200525081626.GA16796@amd/T/#m5b8b20ea6e4ac1eb3bc5353c150ff97b8053b727), but it seem to have encountered some strong opposition, so it's unclear if we can hope for it to be added.


That said, I don't think it would be too hard for Ruby to shim this feature by closing all IOs with `close_on_fork?` right after fork.

### Ruby shim

This can be implemented as a Ruby shim starting in Ruby 3.1 using the `Process._fork` callback

```ruby
class IO
  def close_on_fork=(enabled)
    if enabled
      ::CloseIOOnFork::IOS[self] = true
    end
    @close_on_fork = enabled
  end

  def close_on_fork?
    @close_on_fork
  end
end

module CloseIOOnFork
  IOS = ObjectSpace::WeakMap.new

  def _fork
    pid = super
    if pid == 0 # child
      ::CloseIOOnFork::IOS.each_key do |io|
        io.close if io.close_on_fork?
      end
    end
    pid
  end
end
Process.singleton_class.prepend(CloseIOOnFork)

rd, rw = IO.pipe
rw.close_on_fork = true
pid = fork do
  p rw.closed? # => true
end
Process.wait(pid)
```

### Usage

With such feature, many network client would mostly just need to set this flag on their sockets, and just properly handle unexpectedly closed connections, which most already do.





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