[GHC] #3874: GHC.Exts.traceEvent segfaults

[GHC]

#3874: GHC.Exts.traceEvent segfaults
-----------------------------------+--------------------------------
        Reporter:  cjs             |         Owner:  igloo
            Type:  merge           |        Status:  closed
        Priority:  high            |     Milestone:  6.12.2
       Component:  Runtime System  |       Version:  6.12.1
      Resolution:  fixed           |      Keywords:  event, eventlog
Operating System:  Linux           |  Architecture:  x86_64 (amd64)
 Type of failure:  Runtime crash   |     Test Case:
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Comment (by Austin Seipp <austin@…>):

 In [changeset:"a82364c9410d35fa9cb5031d553212267c3628c5/ghc"]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="a82364c9410d35fa9cb5031d553212267c3628c5"
 Don't truncate traceEvents to 512 bytes (#8309)

 Summary:
 Don't call postLogMsg to post a user msg, because it truncates messages to
 512 bytes.

 Rename traceCap_stderr and trace_stderr to vtraceCap_stderr and
 trace_stderr,
 to signal that they take a va_list (similar to vdebugBelch vs debugBelch).

 See #3874 for the original reason behind traceFormatUserMsg.

 See the commit msg in #9395 (d360d440) for a discussion about using
 null-terminated strings vs strings with an explicit length.

 Test Plan:
 Run `cabal install ghc-events` and inspect the result of `ghc-events show`
 on
 an eventlog file created with `ghc -eventlog Test.hs` and `./Test +RTS
 -l`,
 where Test.hs contains:

 ```
 import Debug.Trace
 main = traceEvent (replicate 510 'a' ++ "bcd") $ return ()
 ```

 Depends on D655.

 Reviewers: austin

 Reviewed By: austin

 Subscribers: thomie

 Differential Revision: https://phabricator.haskell.org/D656

 GHC Trac Issues: #8309
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/3874#comment:9>
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