[ruby-core:108338] [Ruby master Feature#18630] Introduce general `IO#timeout` and `IO#timeout=`for all (non-)blocking operations.

Issue #18630 has been updated by Eregon (Benoit Daloze).


In relation to #18668, this proposal needs IO#nonblock= and IO#nonblock? because IO#timeout= only applies to IO#nonblock?=true IO instances. And so to use it sucessfully, a user needs to ensure the relevant IO is in nonblock=true mode + some other conditions:

https://github.com/ruby/ruby/pull/5653/files#diff-92194f057884b3287a3a6bf84e6e3b2bf433a556b68562799252a091744e7854R856-R867 says:
```
 *  call-seq:
 *    timeout = duration -> duration
 *    timeout = nil -> nil
 *
 *  Set the internal timeout to the specified duration or nil. The timeout
 *  applies to all blocking operations provided the IO is in non-blocking mode:
 *  +io.nonblock? => true+.
 *
 *  This affects the following methods (but is not limited to): #gets, #puts,
 *  #read, #write, #wait_readable and #wait_writable. This also affects
 *  blocking socket operations like Socket#accept and Socket#connect.
```


----------------------------------------
Feature #18630: Introduce general `IO#timeout` and `IO#timeout=`for all (non-)blocking operations.
https://bugs.ruby-lang.org/issues/18630#change-97363

* Author: ioquatix (Samuel Williams)
* Status: Open
* Priority: Normal
----------------------------------------
I would like us to consider introducing a general IO timeout for all (non-)blocking operations, specified per-IO instance. It's useful for ensuring programs don't stop responding or spend an unreasonable amount of time waiting for IO operations.

There are effectively two kinds of interfaces that we need to address:

- Those that already have a timeout argument (e.g. `wait_readable`) and we follow the existing semantics.
- Those that don't have a timeout argument or timeout semantics (e.g. `puts`, `gets`), and thus probably need to raise an exception on timeout.

We have three possible kinds of exceptions we could raise:

- `Errno::ETIMEDOUT`
- `Timeout::Error` (from `timeout.rb`)
- Introduce `IO::Timeout` or something similar.

Timeout isn't necessarily an error condition. There are different arguments for whether we should define:

```ruby
class IO::Timeout < Exception
end

# or

class IO::Timeout < StandardError
end
```

I believe the latter (`StandardError`) is more practical but I'm open to either option. I might have more specific arguments later why one is better than the other after testing in a practical system.

There is already a PR to try it out: https://github.com/ruby/ruby/pull/5653



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