[ruby-core:116567] [Ruby master Feature#20215] Introduce `IO#readable?`

Issue #20215 has been updated by Dan0042 (Daniel DeLorme).


Actually if I think about it a little, I can work around that like this:

```ruby
loop do
  if $stdin.wait_readable(0)
    str = $stdin.gets or abort("<eof>")
    p str
  else
    puts "no input, let's wait a bit..."
    sleep 1
  end
end
```

And I now realize this is the same as `IO.select`; not sure why I didn't think of using that before.

Which brings me to understand what @ioquatix has in mind is a bit different from what I thought. It looks like it can be implemented like this?

```ruby
class IO
  def readable?
    if IO.select([self],[],[],0).nil?
      true #no data available but not eof
    else
      !eof? #will not hang due to check above
    end
  end
  #or, what about adding an argument to eof?
  def eof?(non_blocking=false)
    if non_blocking
      return nil if IO.select([self],[],[],0).nil?
    end
    super
  end
end
```

----------------------------------------
Feature #20215: Introduce `IO#readable?`
https://bugs.ruby-lang.org/issues/20215#change-106583

* Author: ioquatix (Samuel Williams)
* Status: Open
* Priority: Normal
----------------------------------------
There are some cases where, as an optimisation, it's useful to know whether more data is potentially available.

We already have `IO#eof?` but the problem with using `IO#eof?` is that it can block indefinitely for sockets.

Therefore, code which uses `IO#eof?` to determine if there is potentially more data, may hang.

```ruby
def make_request(path = "/")
  client = connect_remote_host
  # HTTP/1.0 request:
  client.write("GET #{path} HTTP/1.0\r\n\r\n")

  # Read response
  client.gets("\r\n") # => "HTTP/1.0 200 OK\r\n"

  # Assuming connection close, there are two things the server can do:
  # 1. peer.close
  # 2. peer.write(...); peer.close

  if client.eof? # <--- Can hang here!
    puts "Connection closed"
    # Avoid yielding as we know there definitely won't be any data.
  else
    puts "Connection open, data may be available..."
    # There might be data available, so yield.
    yield(client)
  end
ensure
  client&.close
end

make_request do |client|
  puts client.read # <--- Prefer to wait here.
end
```

The proposed `IO#readable?` is similar to `IO#eof?` but rather than blocking, would simply return false. The expectation is the user will subsequently call `read` which may then wait.

The proposed implementation would look something like this:

```ruby
class IO
  def readable?
    !self.closed?
  end
end

class BasicSocket
  # Is it likely that the socket is still connected?
  # May return false positive, but won't return false negative.
  def readable?
    return false unless super
    
    # If we can wait for the socket to become readable, we know that the socket may still be open.
    result = self.recv_nonblock(1, MSG_PEEK, exception: false)
    
    # No data was available - newer Ruby can return nil instead of empty string:
    return false if result.nil?
    
    # Either there was some data available, or we can wait to see if there is data avaialble.
    return !result.empty? || result == :wait_readable
    
  rescue Errno::ECONNRESET
    # This might be thrown by recv_nonblock.
    return false
  end
end
```

For `IO` itself, when there is buffered data, `readable?` would also return true immediately, similar to `eof?`. This is not shown in the above implementation as I'm not sure if there is any Ruby method which exposes "there is buffered data".



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