socket: avoid fcntl for read/write_nonblock on Linux

On platforms where MSG_DONTWAIT works reliably on all sockets
(so far, I know of Linux), we can avoid fcntl syscalls and
implement IO#write_nonblock and IO#read_nonblock in terms of the
socket-specific send and recv family of syscalls.

This avoids side effects on the socket, and also encourages
generic code to be written in cases where IO wrappers like
OpenSSL::SSL::SSLSocket are used.

Perhaps in the future, side-effect-free non-blocking I/O can
be standard on all files and OSes: https://cr.yp.to/unix/nonblock.html

* ext/socket/lib/socket.rb (read_nonblock, write_nonblock):
  Linux-specific wrapper without side effects
  [ruby-core:80780] [Feature #13362]
* test/socket/test_basicsocket.rb (test_read_write_nonblock):
  new test

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58400 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

1 files changed, 18 insertions, 0 deletions

diff --git a/ext/socket/lib/socket.rb b/ext/socket/lib/socket.rb
index de5eda3991..8127009fff 100644
--- a/ext/socket/lib/socket.rb
+++ b/ext/socket/lib/socket.rb
@@ -442,6 +442,24 @@ class BasicSocket < IO
                        scm_rights: false, exception: true)
     __recvmsg_nonblock(dlen, flags, clen, scm_rights, exception)
   end
+
+  # Linux-specific optimizations to avoid fcntl for IO#read_nonblock
+  # and IO#write_nonblock using MSG_DONTWAIT
+  # Do other platforms suport MSG_DONTWAIT reliably?
+  if RUBY_PLATFORM =~ /linux/ && Socket.const_defined?(:MSG_DONTWAIT)
+    def read_nonblock(len, str = nil, exception: true) # :nodoc:
+      case rv = __recv_nonblock(len, 0, str, exception)
+      when '' # recv_nonblock returns empty string on EOF
+        exception ? raise(EOFError, 'end of file reached') : nil
+      else
+        rv
+      end
+    end
+
+    def write_nonblock(buf, exception: true) # :nodoc:
+      __sendmsg_nonblock(buf, 0, nil, nil, exception)
+    end
+  end
 end
 
 class Socket < BasicSocket