[commit: ghc] master: compareByPreference: handle the integer-gmp vs -simple case (86ee74d)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/86ee74dc999db6030874ddaf5d10adec23108c02/ghc

>---------------------------------------------------------------

commit 86ee74dc999db6030874ddaf5d10adec23108c02
Author: Alp Mestanogullari <alp at well-typed.com>
Date:   Mon Nov 12 15:38:46 2018 +0100

    compareByPreference: handle the integer-gmp vs -simple case
    
    Currently, it assumes the package names are identical and this
    breaks in the case where integer-gmp is in one package db and
    integer-simple in another. This became a problem with
    the commit: fc2ff6dd7496a33bf68165b28f37f40b7d647418.
    
    Instead of following the precedence information, leading to
    the right choice, the current code would compare the
    integer-gmp and integer-simple versions and pick integer-gmp
    because it happened to have a greater version, despite having
    a lower precedence. See
    https://github.com/snowleopard/hadrian/issues/702 for
    a comprehensive report about the problem.
    
    This effectively un-breaks integer-simple builds with hadrian.
    
    Test Plan: hadrian/build.sh --integer-simple
    
    Reviewers: snowleopard, bgamari
    
    Reviewed By: bgamari
    
    Subscribers: snowleopard, rwbarton, carter
    
    Differential Revision: https://phabricator.haskell.org/D5266


>---------------------------------------------------------------

86ee74dc999db6030874ddaf5d10adec23108c02
 compiler/main/Packages.hs | 25 ++++++++++++++++++++++---
 1 file changed, 22 insertions(+), 3 deletions(-)

diff --git a/compiler/main/Packages.hs b/compiler/main/Packages.hs
index fadcd31..78d5961 100644
--- a/compiler/main/Packages.hs
+++ b/compiler/main/Packages.hs
@@ -891,15 +891,28 @@ sortByPreference prec_map = sortBy (flip (compareByPreference prec_map))
 --
 -- Pursuant to #12518, we could change this policy to, for example, remove
 -- the version preference, meaning that we would always prefer the packages
--- in alter package database.
+-- in later package database.
 --
+-- Instead, we use that preference based policy only when one of the packages
+-- is integer-gmp and the other is integer-simple.
+-- This currently only happens when we're looking up which concrete
+-- package to use in place of @integer-wired-in@ and that two different
+-- package databases supply a different integer library. For more about
+-- the fake @integer-wired-in@ package, see Note [The integer library]
+-- in the @PrelNames@ module.
 compareByPreference
     :: PackagePrecedenceIndex
     -> PackageConfig
     -> PackageConfig
     -> Ordering
-compareByPreference prec_map pkg pkg' =
-    case comparing packageVersion pkg pkg' of
+compareByPreference prec_map pkg pkg'
+  | Just prec  <- Map.lookup (unitId pkg)  prec_map
+  , Just prec' <- Map.lookup (unitId pkg') prec_map
+  , differentIntegerPkgs pkg pkg'
+  = compare prec prec'
+
+  | otherwise
+  = case comparing packageVersion pkg pkg' of
         GT -> GT
         EQ | Just prec  <- Map.lookup (unitId pkg)  prec_map
            , Just prec' <- Map.lookup (unitId pkg') prec_map
@@ -910,6 +923,12 @@ compareByPreference prec_map pkg pkg' =
            -> EQ
         LT -> LT
 
+  where isIntegerPkg p = packageNameString p `elem`
+          ["integer-simple", "integer-gmp"]
+        differentIntegerPkgs p p' =
+          isIntegerPkg p && isIntegerPkg p' &&
+          (packageName p /= packageName p')
+
 comparing :: Ord a => (t -> a) -> t -> t -> Ordering
 comparing f a b = f a `compare` f b