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socket: allow explicit buffer for recv and recv_nonblock

This reduces GC overhead and makes the API more consistent
with IO#read and IO#read_nonblock.

• ext/socket/basicsocket.c (bsock_recv): document outbuf
• ext/socket/unixsocket.c (unix_recvfrom): ditto
• ext/socket/init.c (rsock_strbuf, recvfrom_locktmp): new functions
  (rsock_s_recvfrom): support destination buffer as 3rd arg
  (rsock_s_recvfrom_nonblock): ditto
• string.c (rb_str_locktmp_ensure): export for internal ext
• test/socket/test_nonblock.rb: test recv_nonblock
• test/socket/test_unix.rb: test recv
  [ruby-core:69543] [Feature #11242]

Benchmark results:

         user     system      total        real

alloc 0.130000 0.280000 0.410000 ( 0.420656)
extbuf 0.100000 0.220000 0.320000 ( 0.318708)

-------------------8<--------------------
require 'socket'
require 'benchmark'
nr = 100000
msg = ' ' * 16384
size = msg.bytesize
buf = ' ' * size
UNIXSocket.pair(:DGRAM) do |a, b|
Benchmark.bmbm do |x|
x.report('alloc') do
nr.times do
b.send(msg, 0)
a.recv(size, 0)
end
end

x.report('extbuf') do
  nr.times do
    b.send(msg, 0)
    a.recv(size, 0, buf)
  end
end

end
end

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@50912 b2dd03c8-39d4-4d8f-98ff-823fe69b080e