[PATCH] support SO_REUSEPORT on new listeners (:reuseport)

[Eric Wong <normalperson@yhbt.net>]

This allows users to start an independent instance of unicorn on
a the same port as a running unicorn (as long as both instances
use :reuseport).

ref: https://lwn.net/Articles/542629/
---
 lib/unicorn/configurator.rb     | 19 +++++++++++++++++++
 lib/unicorn/socket_helper.rb    | 30 ++++++++++++++++++++++--------
 test/unit/test_socket_helper.rb |  8 ++++++++
 3 files changed, 49 insertions(+), 8 deletions(-)

diff --git a/lib/unicorn/configurator.rb b/lib/unicorn/configurator.rb
index 0d0eac7..fc3405a 100644
--- a/lib/unicorn/configurator.rb
+++ b/lib/unicorn/configurator.rb
@@ -319,6 +319,25 @@ class Unicorn::Configurator
   #
   #   Default: Operating-system dependent
   #
+  # [:reuseport => true or false]
+  #
+  #   This enables multiple, independently-started unicorn instances to
+  #   bind to the same port (as long as all the processes enable this).
+  #
+  #   This option must be used when unicorn first binds the listen socket.
+  #   It cannot be enabled when a socket is inherited via SIGUSR2
+  #   (but it will remain on if inherited), and it cannot be enabled
+  #   directly via SIGHUP.
+  #
+  #   Note: there is a chance of connections being dropped if
+  #   one of the unicorn instances is stopped while using this.
+  #
+  #   This is supported on *BSD systems and Linux 3.9 or later.
+  #
+  #   ref: https://lwn.net/Articles/542629/
+  #
+  #   Default: false (unset)
+  #
   # [:tries => Integer]
   #
   #   Times to retry binding a socket if it is already in use
diff --git a/lib/unicorn/socket_helper.rb b/lib/unicorn/socket_helper.rb
index 18b0be7..2701d58 100644
--- a/lib/unicorn/socket_helper.rb
+++ b/lib/unicorn/socket_helper.rb
@@ -41,6 +41,15 @@ module Unicorn
 
       # do not send out partial frames (Linux)
       TCP_CORK = 3 unless defined?(TCP_CORK)
+
+      # Linux got SO_REUSEPORT in 3.9, BSDs have had it for ages
+      unless defined?(SO_REUSEPORT)
+        if RUBY_PLATFORM =~ /(?:alpha|mips|parisc|sparc)/
+          SO_REUSEPORT = 0x0200 # untested
+        else
+          SO_REUSEPORT = 15 # only tested on x86_64 and i686
+        end
+      end
     when /freebsd/
       # do not send out partial frames (FreeBSD)
       TCP_NOPUSH = 4 unless defined?(TCP_NOPUSH)
@@ -142,9 +151,9 @@ module Unicorn
           File.umask(old_umask)
         end
       elsif /\A\[([a-fA-F0-9:]+)\]:(\d+)\z/ =~ address
-        new_ipv6_server($1, $2.to_i, opt)
+        new_tcp_server($1, $2.to_i, opt.merge(:ipv6=>true))
       elsif /\A(\d+\.\d+\.\d+\.\d+):(\d+)\z/ =~ address
-        Kgio::TCPServer.new($1, $2.to_i)
+        new_tcp_server($1, $2.to_i, opt)
       else
         raise ArgumentError, "Don't know how to bind: #{address}"
       end
@@ -152,13 +161,18 @@ module Unicorn
       sock
     end
 
-    def new_ipv6_server(addr, port, opt)
-      opt.key?(:ipv6only) or return Kgio::TCPServer.new(addr, port)
-      defined?(IPV6_V6ONLY) or
-        abort "Socket::IPV6_V6ONLY not defined, upgrade Ruby and/or your OS"
-      sock = Socket.new(AF_INET6, SOCK_STREAM, 0)
-      sock.setsockopt(IPPROTO_IPV6, IPV6_V6ONLY, opt[:ipv6only] ? 1 : 0)
+    def new_tcp_server(addr, port, opt)
+      # n.b. we set FD_CLOEXEC in the workers
+      sock = Socket.new(opt[:ipv6] ? AF_INET6 : AF_INET, SOCK_STREAM, 0)
+      if opt.key?(:ipv6only)
+        defined?(IPV6_V6ONLY) or
+          abort "Socket::IPV6_V6ONLY not defined, upgrade Ruby and/or your OS"
+        sock.setsockopt(IPPROTO_IPV6, IPV6_V6ONLY, opt[:ipv6only] ? 1 : 0)
+      end
       sock.setsockopt(SOL_SOCKET, SO_REUSEADDR, 1)
+      if defined?(SO_REUSEPORT) && opt[:reuseport]
+        sock.setsockopt(SOL_SOCKET, SO_REUSEPORT, 1)
+      end
       sock.bind(Socket.pack_sockaddr_in(port, addr))
       IO_PURGATORY << sock
       Kgio::TCPServer.for_fd(sock.fileno)
diff --git a/test/unit/test_socket_helper.rb b/test/unit/test_socket_helper.rb
index a38082c..abc177b 100644
--- a/test/unit/test_socket_helper.rb
+++ b/test/unit/test_socket_helper.rb
@@ -184,4 +184,12 @@ class TestSocketHelper < Test::Unit::TestCase
     assert_equal 1, cur
     rescue Errno::EAFNOSUPPORT
   end if RUBY_VERSION >= "1.9.2"
+
+  def test_reuseport
+    port = unused_port @test_addr
+    name = "#@test_addr:#{port}"
+    sock = bind_listen(name, :reuseport => true)
+    cur = sock.getsockopt(Socket::SOL_SOCKET, SO_REUSEPORT).unpack('i')[0]
+    assert_equal 1, cur
+  end if defined?(SO_REUSEPORT)
 end
-- 
Eric Wong
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