[GHC] #9395: Debug.Trace should not use %s for format string

[GHC]

#9395: Debug.Trace should not use %s for format string
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              Reporter:  ezyang      |            Owner:  ezyang
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:
             Component:              |          Version:  7.8.2
  libraries/base                     |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:  Incorrect   |  Related Tickets:
  result at runtime                  |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by Edward Z. Yang <ezyang@…>):

 In [changeset:"d360d440b994c03d645603c50f25ef208700db02/ghc"]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="d360d440b994c03d645603c50f25ef208700db02"
 Filter out null bytes from trace, and warn accordingly, fixing #9395.

 Summary:
 Previously, if you ran trace "foo\0bar", the output was truncated so
 that everything after the null byte was omitted.  This was terrible.
 Now we filter out null bytes, and emit an extra trace saying that
 null bytes were filtered out.

 NB: we CANNOT fix debugBelch, because all printf variants *always*
 respect null bytes, even if you're using string precision such as
 %.*s.  The alternative would have been to introduce a new function
 debugRawBelch which did not use format strings and took an explicit
 string length, but I decided we generally should avoid putting null
 bytes in our trace messages, and warn the user.

 Signed-off-by: Edward Z. Yang <ezyang at cs.stanford.edu>

 Test Plan: validate

 Reviewers: hvr, austin

 Subscribers: simonmar, relrod, ezyang, carter

 Differential Revision: https://phabricator.haskell.org/D121

 GHC Trac Issues: #9395
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9395#comment:3>
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