[RFC] emulate sd_listen_fds for systemd support

[Eric Wong <e@80x24.org>]

Unfortunately, testing this is tricky because FD=3
(SD_LISTEN_FDS_START) tends to be grabbed by (MRI) Ruby 1.9.3
and onwards for the internal self-pipe.

So for now, I've manually tested this with a systemd installation

Disclaimer: this is also my first experience using systemd
---
  Eric Wong <e@80x24.org> wrote:
  > OK, I'll probably add LISTEN_FDS and LISTEN_PID support to unicorn
  > directly so the wrapper is unnecessary.

 lib/unicorn/http_server.rb | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/lib/unicorn/http_server.rb b/lib/unicorn/http_server.rb
index 297b9f9..0f97516 100644
--- a/lib/unicorn/http_server.rb
+++ b/lib/unicorn/http_server.rb
@@ -766,12 +766,22 @@ def redirect_io(io, path)
   def inherit_listeners!
     # inherit sockets from parents, they need to be plain Socket objects
     # before they become Kgio::UNIXServer or Kgio::TCPServer
-    inherited = ENV['UNICORN_FD'].to_s.split(',').map do |fd|
-      io = Socket.for_fd(fd.to_i)
+    inherited = ENV['UNICORN_FD'].to_s.split(',')
+
+    # emulate sd_listen_fds() for systemd
+    sd_pid, sd_fds = ENV.values_at('LISTEN_PID', 'LISTEN_FDS')
+    if sd_pid && sd_pid.to_i == $$
+      # 3 = SD_LISTEN_FDS_START
+      inherited.concat((3...(3 + sd_fds.to_i)).map { |fd| Socket.for_fd(fd) })
+    end
+    # to ease debugging, we will not unset LISTEN_PID and LISTEN_FDS
+
+    inherited.map! do |fd|
+      io = String === fd ? Socket.for_fd(fd.to_i) : fd
       io.autoclose = false
       io = server_cast(io)
       set_server_sockopt(io, listener_opts[sock_name(io)])
-      logger.info "inherited addr=#{sock_name(io)} fd=#{fd}"
+      logger.info "inherited addr=#{sock_name(io)} fd=#{io.fileno}"
       io
     end
 
-- 
EW