17 Apr
2024
17 Apr
'24
5:16 p.m.
Issue #20215 has been updated by akr (Akira Tanaka).
Thank you for the explanation of the motivation.
I feel it is reasonable.
However, mame-san still had a question yesterday: why don't you detect an error in
writing a request?
I guess it is because writing the request may not fail.
It needs two writes to detect the error.
(The first write in a client application causes the client kernel to send a packet to the
server.
The server kernel responds to it with a RST packet because the server socket is closed.
The client kernel receives the RST packet and detects we cannot send data in the
connection.
The second write in the client application will cause an error.)
Please point out if I am wrong.
My next question is why `io.eof?` and `io.wait_readable(0)` doesn't fulfill your
motivation.
My current guess is related to the buffering mechanism of IO class.
But I'm not certain.
----------------------------------------
Feature #20215: Introduce `IO#readable?`
https://bugs.ruby-lang.org/issues/20215#change-107980
* 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/