rework master-to-worker signaling to use a pipe

Signaling using normal kill(2) is preserved, but the master now
prefers to signal workers using a pipe rather than kill(2).
Non-graceful signals (:TERM/:KILL) are still sent using kill(2),
as they ask for immediate shutdown.

This change is necessary to avoid triggering the ubf (unblocking
function) for rb_thread_call_without_gvl (and similar) functions
extensions.  Most notably, this fixes compatibility with newer
versions of the 'pg' gem which will cancel a running DB query if
signaled[1].

This also has the nice side-effect of allowing a premature
master death (assuming preload_app didn't cause the master to
spawn off rogue child daemons).

Note: users should also refrain from using "killall" if using the
'pg' gem or something like it.

Unfortunately, this increases FD usage in the master as the writable
end of the pipe is preserved in the master.  This limit the number
of worker processes the master may run to the open file limit of the
master process.  Increasing the open file limit of the master
process may be needed.  However, the FD use on the workers is
reduced by one as the internal self-pipe is no longer used.  Thus,
overall pipe allocation for the kernel remains unchanged.

[1] - pg is correct to cancel a query, as it cannot know if
      the signal was for a) graceful unicorn shutdown or
      b) oh-noes-I-started-a-bad-query-ABORT-ABORT-ABORT!!

1 files changed, 6 insertions, 0 deletions

diff --git a/SIGNALS b/SIGNALS
index 48c651e..ef0b0d9 100644
--- a/SIGNALS
+++ b/SIGNALS
@@ -45,6 +45,10 @@ http://unicorn.bogomips.org/examples/init.sh
 
 === Worker Processes
 
+Note: as of unicorn 4.8, the master uses a pipe to signal workers
+instead of kill(2) for most cases.  Using signals still (and works and
+remains supported for external tools/libraries), however.
+
 Sending signals directly to the worker processes should not normally be
 needed.  If the master process is running, any exited worker will be
 automatically respawned.
@@ -52,6 +56,8 @@ automatically respawned.
 * INT/TERM - Quick shutdown, immediately exit.
   Unless WINCH has been sent to the master (or the master is killed),
   the master process will respawn a worker to replace this one.
+  Immediate shutdown is still triggered using kill(2) and not the
+  internal pipe as of unicorn 4.8
 
 * QUIT - Gracefully exit after finishing the current request.
   Unless WINCH has been sent to the master (or the master is killed),