[GHC] #10017: signal handlers are invoked multiple times when the threaded rts is used

24 Jan 2015

      #10017: signal handlers are invoked multiple times when the threaded rts is used
-------------------------------------+-------------------------------------
              Reporter:  redneb      |             Owner:  simonmar
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Runtime     |           Version:  7.10.1-rc1
  System                             |  Operating System:  Linux
              Keywords:              |   Type of failure:  Incorrect result
          Architecture:              |  at runtime
  Unknown/Multiple                   |        Blocked By:
             Test Case:              |   Related Tickets:  #9423
              Blocking:              |
Differential Revisions:              |
-------------------------------------+-------------------------------------
 When you install a custom signal handler and the threaded rts is being
 used, then the signal handler will be invoked multiple times. Here's a
 program that the demonstrates this:

 {{{#!hs
 import Control.Concurrent
 import System.Posix.Signals

 main :: IO ()
 main = do
     _ <- flip (installHandler sig) Nothing $ Catch $
         putStrLn $ "Received signal " ++ show sig
     raiseSignal sig
     threadDelay 100000
   where
     sig = sigUSR2
 }}}

 If you compile this with the `-threaded` flag and then run it with say
 `+RTS -N4` then it produces the following output:

 {{{
 Received signal 12
 Received signal 12
 Received signal 12
 Received signal 12
 Received signal 12
 }}}

 In general the signal handler is invoked `n_capabilities+1` times. This
 also happens with all other signals.

 This regression was introduced by f9f89b7884ccc8ee5047cf4fffdf2b36df6832df
 (which was later [changeset:4748f5936fe72d96edfa17b153dbfd84f2c4c053
 reverted] but then [changeset:7e658bc14e2dd6baf208deebbdab9e1285ce4c72 re-
 added]), a commit addressing #9423.

 The cause of the problem is
 [source:/rts/posix/Signals.c@f44bbc83bab62f9a2d25e69d87c2b4af25318d52#L256
 this] loop. I don't understand why we need to write an event about the
 signal received in the per capability control pipe introduced by the
 aforementioned commit. Aren't these control pipes supposed only to be used
 to shutdown the capabilities (which happens
 [source:/rts/posix/Signals.c@f44bbc83bab62f9a2d25e69d87c2b4af25318d52#L183
 here])?

 Removing the loop seems to solve the issue, but I don't know if it makes
 #9423 reappear. I cannot test this on a Mac OS X right now.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10017
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