setNumCapabilities call breaks eventlog events

The problem was found when i tried to eventlog ghc --make itself. I've missinterpreted is as a threadscope bug: https://github.com/haskell/ThreadScope/issues/37

Here is small program to reliably reproduce the bug:

module Main where

import qualified Data.List as L
import qualified System.Environment as E
import Control.Monad
import qualified Control.Concurrent as CC
import qualified Control.Concurrent.MVar as CC

slow_and_silly :: Int -> IO Int
slow_and_silly i = return $ length $ L.foldl' (\a v -> a ++ [v]) [] [1..i]

-- build as:
-- $ ghc --make a -O2 -threaded -eventlog

-- valid eventlog:
-- $ ./a 2 7000 +RTS -ls -N2
-- $ ghc-events validate threads a.eventlog
-- Valid eventlog:
-- ...

-- invalid eventlog
-- $ ./a 2 7000 +RTS -ls
-- $ ghc-events validate threads a.eventlog
-- Invalid eventlog:
-- ...

main = do
    [caps, count] <- E.getArgs
    let n_caps :: Int
        n_caps = read caps
        max_n :: Int
        max_n = read count

    CC.setNumCapabilities n_caps

    waits <- replicateM n_caps $ CC.newEmptyMVar

    forM_ waits $ \w -> CC.forkIO $ do
        slow_and_silly max_n >>= print
        CC.putMVar w ()

    forM_ waits $ \w -> CC.takeMVar w

How to reproduce (comments have ghc-events version):

$ ghc --make a -O2 -threaded -eventlog
$ ./a 2 7000 +RTS -ls -N2
$ threadscope a.eventlog # works
$ ./a 2 7000 +RTS -ls
$ threadscope a.eventlog # crashes

Trac metadata

Trac field	Value
Version	7.8.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Profiling
Test case
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