[GHC] #8316: GHCi debugger panics when trying force a certain variable

[GHC]

#8316: GHCi debugger panics when trying force a certain variable
-------------------------------------+-------------------------------------
        Reporter:  guest             |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:
       Component:  GHCi              |              Version:  7.6.3
      Resolution:                    |             Keywords:  debugger
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  GHCi crash        |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D4535,
       Wiki Page:                    |  Phab:D5179
-------------------------------------+-------------------------------------

Comment (by Ben Gamari <ben@…>):

 In [changeset:"45ed4619fd5cfe785bbf1142b9d16e4f3c5148ce/ghc" 45ed461/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="45ed4619fd5cfe785bbf1142b9d16e4f3c5148ce"
 Fix BLACKHOLE inspection in RtClosureInspect

 When inspecing a BLACKHOLE if the BLACKHOLE points to a TSO or a
 BLOCKING_QUEUE we should return a suspension to the BLACKHOLE itself
 (instead of returning a suspension to the indirectee). The reason is
 because in the debugger when we want to evaluate this term we need to
 enter the BLACKHOLE and not to the TSO or BLOCKING_QUEUE. See the
 runtime panic caused by this in #8316.

 Note that while with this patch we do the right thing to evaluate
 thunks in GHCi, evaluating thunks that are owned by the evaluator thread
 in a breakpoint will cause a deadlock as we don't release the breakMVar,
 which is what blocks the evaluator thread from continuing with
 evaluation. So the GHCi thread will enter the BLACKHOLE, but owner of
 the BLACKHOLE is also blocked.

 Reviewers: simonmar, hvr, bgamari

 Reviewed By: bgamari

 Subscribers: rwbarton, carter

 GHC Trac Issues: #8316

 Differential Revision: https://phabricator.haskell.org/D5179
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8316#comment:21>
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