[GHC] #15594: --abi-hash with Backpack incorrectly loads modules from dependent packages

[GHC]

#15594: --abi-hash with Backpack incorrectly loads modules from dependent packages
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        Reporter:  ezyang            |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:  backpack
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D5123
       Wiki Page:                    |
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Comment (by Edward Z. Yang <ezyang@…>):

 In [changeset:"13ff0b7ced097286e0d7b054f050871effe07f86/ghc"
 13ff0b7c/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="13ff0b7ced097286e0d7b054f050871effe07f86"
 Fix #15594 (--abi-hash with Backpack sometimes fails)

 Summary:
 For holes, its necessary to "see through" the instantiation
 of the hole to get accurate family instance dependencies.
 For example, if B imports <A>, and <A> is instantiated with
 F, we must grab and include all of the dep_finsts from
 F to have an accurate transitive dep_finsts list.

 However, we MUST NOT do this for regular modules.
 First, for efficiency reasons, doing this
 bloats the the dep_finsts list, because we *already* had
 those modules in the list (it wasn't a hole module, after
 all). But there's a second, more important correctness
 consideration: we perform module renaming when running
 --abi-hash.  In this case, GHC's contract to the user is that
 it will NOT go and read out interfaces of any dependencies
 (https://github.com/haskell/cabal/issues/3633); the point of
 --abi-hash is just to get a hash of the on-disk interfaces
 for this *specific* package.  If we go off and tug on the
 interface for /everything/ in dep_finsts, we're gonna have a
 bad time.  (It's safe to do do this for hole modules, though,
 because the hmap for --abi-hash is always trivial, so the
 interface we request is local.  Though, maybe we ought
 not to do it in this case either...)

 Signed-off-by: Edward Z. Yang <ezyang at fb.com>

 Test Plan: validate

 Reviewers: alexbiehl, goldfire, bgamari

 Subscribers: ppk, shlevy, rwbarton, carter

 GHC Trac Issues: #15594

 Differential Revision: https://phabricator.haskell.org/D5123
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15594#comment:4>
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