[ruby-core:125077] [Ruby Feature#20215] Introduce `IO#readable?`

19 Mar 2026

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

akr (Akira Tanaka) wrote in #note-14:
...
My next question is why `io.eof?` and `io.wait_readable(0)` doesn't fulfill your motivation.
Might be worth answering this directly...

One reason is that `wait_readable` doesn't exist on StringIO and OpenSSL::SSL::SSLSocket, so it's not possible to combine with `eof?`

Another reason is that there seems to be a *fifth* possibility in the "state of the read side of unidirectional data flow"

1.   We can read 1 or more bytes immediately.
2.   We can detect EOF immediately.
2b.  We *think* we can detect EOF immediately (`wait_readable(0)` returns true) but calling `eof?` can wait indefinitely.
3.   We cannot determine data/EOF immediately. If we wait indefinitely, we can read 1 or more bytes.
4.   We cannot determine data/EOF immediately. If we wait indefinitely, we can detect EOF.

I'm not sure if 2b is a fixable bug in `wait_readable`, or if it's something that can only be handled by a new method that combines `wait_readable` + `eof?` while handling that edge case.

BTW I'm still against the name `readable?`; in addition to the confusion described in #note-7, there's also confusion with `wait_readable(0)` that returns truthy on 1 and 2, while `readable?` would return false on 2.

What about `nonblock_eof?`
false on 1.
true on 2.
nil on 2b, 3, 4.

----------------------------------------
Feature #20215: Introduce `IO#readable?`
https://bugs.ruby-lang.org/issues/20215#change-116781

* Author: ioquatix (Samuel Williams)
* Status: Open
----------------------------------------
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".

-- 
https://bugs.ruby-lang.org/