17 Apr
2024
17 Apr
'24
1:13 a.m.
Issue #20215 has been updated by akr (Akira Tanaka).
I couldn't understand the motivation of this issue.
However, the state of the read side of unidirectional data flow (Unix pipe, half of a
stream socket, etc.) can be one of the following.
1. We can read 1 or more bytes immediately.
2. We can detect EOF immediately.
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.
`io.eof?` returns true on 2 and 4, false otherwise. (So, it blocks on 3 and 4.)
`io.wait_readable(0)` returns truthy on 1 and 2, falsy otherwise.
They are enough to distinguish the states.
If we don't want to block, it is impossible to distinguish 3 and 4, though.
----------------------------------------
Feature #20215: Introduce `IO#readable?`
https://bugs.ruby-lang.org/issues/20215#change-107934
* 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/