[GHC] #12733: Orphan hashes are not being propagated correctly

18 Oct 2016

      #12733: Orphan hashes are not being propagated correctly
-------------------------------------+-------------------------------------
           Reporter:  ezyang         |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Driver         |           Version:  8.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 In Phab:D2607, simonpj asked why we didn't also put class instances in the
 export hash. Why indeed. In the course of investigating, I discovered
 another bug in how we handle orphan recompilation.

 Unfortunately, this bug needs two packages to manifest.

 {{{
 -- p/P.hs
 module P where
 instance Show (a -> b) where
 --instance Show (IO a) where

 -- q/Q.hs
 module Q where
 import P

 -- q/Q2.hs
 module Q2 where
 import Q
 }}}

 Put in some default Cabal files, with q depending on p, and then build the
 entire kaboodle. Then edit `p/P.hs` to uncomment the second orphan
 instance. Rebuild.

 {{{
 ezyang@sabre:~/Dev/labs/orph$ cabal new-build q -w ghc-head
 In order, the following will be built (use -v for more details):
  - p-0.1.0.0 (lib) (file P.hs changed)
  - q-0.1.0.0 (lib) (dependency rebuilt)
 Preprocessing library p-0.1.0.0...
 [1 of 1] Compiling P                ( P.hs, /srv/code/labs/orph/dist-
 newstyle/build/x86_64-linux/ghc-8.1.20161016/p-0.1.0.0/build/P.o )

 P.hs:2:10: warning: [-Wmissing-methods]
     • No explicit implementation for
         either ‘showsPrec’ or ‘show’
     • In the instance declaration for ‘Show (a -> b)’

 P.hs:3:10: warning: [-Wmissing-methods]
     • No explicit implementation for
         either ‘showsPrec’ or ‘show’
     • In the instance declaration for ‘Show (IO a)’
 module Q2 where
 Preprocessing library q-0.1.0.0...
 [1 of 2] Compiling Q                ( Q.hs, /srv/code/labs/orph/dist-
 newstyle/build/x86_64-linux/ghc-8.1.20161016/q-0.1.0.0/build/Q.o ) [P
 changed]
 ezyang@sabre:~/Dev/labs/orph$
 }}}

 Notice: Q2 never got rebuilt. This is wrong wrong wrong: P changed, and we
 need to rebuild Q2!

 Here are the hashes for Q.hi before and after:

 {{{
 -- Before
 interface Q 80120161016
   interface hash: 2ffc06a1552565a73c888110ab4cf8df
   ABI hash: b2768862866252ea5a63401d2aa05aaa
   export-list hash: 092acc280445a8bcc96904afb8b766b0
   orphan hash: 693e9af84d3dfcc71e640e005bdc5e2e
   flag hash: f01bb78d3c47152bc5253f256b2a60d3

 -- After
 interface Q 80120161016
   interface hash: 009db9e5ecbe49e36b7bf38e02087e75
   ABI hash: b2768862866252ea5a63401d2aa05aaa
   export-list hash: 092acc280445a8bcc96904afb8b766b0
   orphan hash: 693e9af84d3dfcc71e640e005bdc5e2e
   flag hash: f01bb78d3c47152bc5253f256b2a60d3
 }}}

 The orphan hash didn't get updated.

 At the moment, when we compute the orphan hash, we only consider the
 orphan hashes from the home package. Here's the comment justifying it:

 {{{
    -- the export hash of a module depends on the orphan hashes of the
    -- orphan modules below us in the dependency tree.  This is the way
    -- that changes in orphans get propagated all the way up the
    -- dependency tree.  We only care about orphan modules in the current
    -- package, because changes to orphans outside this package will be
    -- tracked by the usage on the ABI hash of package modules that we
 import.
 }}}

 It is true that we know to rebuild Q.hs when P.hs changes. But that is not
 much solace for Q2.hs...

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12733
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