[ruby-core:117541] [Ruby master Feature#20215] Introduce `IO#readable?`

17 Apr 2024

      Issue #20215 has been updated by ioquatix (Samuel Williams).
...
Even if client.eof? returns false without blocking, it could still result in an EOF with zero read bytes. Therefore, it would be better to do read without unnecessary checks.
I understand, thanks for your question. I may not be able to answer this well before the meeting, so I'll try to come up with a clear justification, but if the time frame is too tight, I'll discuss it next time instead.

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Feature #20215: Introduce `IO#readable?`
https://bugs.ruby-lang.org/issues/20215#change-107933

* Author: ioquatix (Samuel Williams)
* Status: Open
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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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