Fix socket.recv documentation.

Product: Tarantool
Root document: https://www.tarantool.io/en/doc/latest/reference/reference_lua/socket/#lua-function.socket_object.recv
SME: @ nshy

Info

https://www.tarantool.io/en/doc/latest/reference/reference_lua/socket/#lua-function.socket_object.recv

What's wrong?

Method returns nil on error with socket._errno set.

Code:

socket = require('socket')
s = socket('AF_UNIX', 'SOCK_DGRAM', 0)
-- should be error as socket is nonblocking
r = s:recv(1)
print(r)
print(s._errno)

Output

nil
11

Also in case of EOF it just receives empty string. There is no special EOF status.

socket = require('socket')
fio = require('fio')
fio.unlink('test.sock')
s = socket('AF_UNIX', 'SOCK_STREAM', 0)
assert(s)
assert(s:bind('unix/', 'test.sock'))
assert(s:listen())
c = socket.tcp_connect('unix/', 'test.sock')
assert(c)
print(c)
sc = s:accept()
assert(sc)
sc:write("4444")
d = c:recv(2)
assert(d)
print(d)
sc:shutdown(socket.SHUT_WR)
d = c:recv(2)
assert(d)
print(d)
d = c:recv(2)
assert(d)
assert(string.len(d) == 0)
print("EOF")
d = c:recv(2)
assert(d)
assert(string.len(d) == 0)
print("EOF")

Output:

fd 12, aka unix/:, peer unix/:test.sock
44
44
EOF
EOF

Open the socket.recv documentation at the linked Tarantool reference page and compare its description with the provided Lua examples. Document that errors return nil with socket._errno set, and that EOF returns an empty string without a special status; the examples should support the corrected behavior.