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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. 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.  - 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!!
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1 parent fa17da9 commit 6f6e4115b4bb03e5e7c55def91527799190566f2 Eric Wong committed with Eric Wong
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