[ruby-core:109961] [Ruby master Bug#19012] BasicSocket#recv* methods return an empty packet instead of nil on closed connections

Issue #19012 has been reported by byroot (Jean Boussier).

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Bug #19012: BasicSocket#recv* methods return an empty packet instead of nil on closed connections
https://bugs.ruby-lang.org/issues/19012

* Author: byroot (Jean Boussier)
* Status: Open
* Priority: Normal
* Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
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`man recvmsg(2)` states:

> Return Value
> These calls return the number of bytes received, or -1 if an error occurred. The return value will be 0 when the peer has performed an orderly shutdown.

But somehow the entire `receiv` family of methods in Ruby seem to interpret `0` as empty string instead of "EOF".

```ruby
require 'socket'

puts "=== pipes ==="
r, w = IO.pipe
r.read_nonblock(1, exception: false) # => :wait_readable
w.close
r.read_nonblock(1, exception: false) # => nil (EOF)

puts "=== sockets ===="
r, w = UNIXSocket.socketpair
r.read_nonblock(1, exception: false) # => :wait_readable
r.recvmsg_nonblock(1, exception: false) # => :wait_readable
r.recv_nonblock(1, exception: false) # => :wait_readable

w.close

r.read_nonblock(1, exception: false) # => nil (EOF)
r.recvmsg_nonblock(1, exception: false) # => ["", #<Addrinfo: empty-sockaddr SOCK_STREAM>, 128]]
r.recvmsg # => ["", #<Addrinfo: empty-sockaddr SOCK_STREAM>, 0]]
r.recv_nonblock(1, exception: false) # => ""
```

### Expected behavior

I would expect `recvmsg_nonblock`, `recvmsg`, `recv_nonblock` and `recv` to return `nil` when the connection is closed.





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