PackageIndex.hs

{-# LANGUAGE DeriveGeneric #-}

-----------------------------------------------------------------------------
-- |
-- Module      :  Distribution.Simple.PackageIndex
-- Copyright   :  (c) David Himmelstrup 2005,
--                    Bjorn Bringert 2007,
--                    Duncan Coutts 2008-2009
--
-- Maintainer  :  cabal-devel@haskell.org
-- Portability :  portable
--
-- An index of packages whose primary key is 'UnitId'.  Public libraries
-- are additionally indexed by 'PackageName' and 'Version'.
-- Technically, these are an index of *units* (so we should eventually
-- rename it to 'UnitIndex'); but in the absence of internal libraries
-- or Backpack each unit is equivalent to a package.
--
-- 'PackageIndex' is parametric over what it actually records, and it
-- is used in two ways:
--
--      * The 'InstalledPackageIndex' (defined here) contains a graph of
--        'InstalledPackageInfo's representing the packages in a
--        package database stack.  It is used in a variety of ways:
--
--          * The primary use to let Cabal access the same installed
--            package database which is used by GHC during compilation.
--            For example, this data structure is used by 'ghc-pkg'
--            and 'Cabal' to do consistency checks on the database
--            (are the references closed).
--
--          * Given a set of dependencies, we can compute the transitive
--            closure of dependencies.  This is to check if the versions
--            of packages are consistent, and also needed by multiple
--            tools (Haddock must be explicitly told about the every
--            transitive package to do cross-package linking;
--            preprocessors must know about the include paths of all
--            transitive dependencies.)
--
--      * The 'PlanIndex' (defined in 'Distribution.Client.InstallPlan'),
--        contains a graph of 'GenericPlanPackage'.  Ignoring its type
--        parameters for a moment, a 'PlanIndex' is an extension of the
--        'InstalledPackageIndex' to also record nodes for packages
--        which are *planned* to be installed, but not actually
--        installed yet.  A 'PlanIndex' containing only 'PreExisting'
--        packages is essentially a 'PackageIndex'.
--
--        'PlanIndex'es actually require some auxiliary information, so
--        most users interact with a 'GenericInstallPlan'.  This type is
--        specialized as an 'ElaboratedInstallPlan' (for @cabal
--        new-build@) or an 'InstallPlan' (for @cabal install@).
--
-- This 'PackageIndex' is NOT to be confused with
-- 'Distribution.Client.PackageIndex', which indexes packages only by
-- 'PackageName' (this makes it suitable for indexing source packages,
-- for which we don't know 'UnitId's.)
--
module Distribution.Simple.PackageIndex (
  -- * Package index data type
  InstalledPackageIndex,
  PackageIndex,

  -- * Creating an index
  fromList,

  -- * Updates
  merge,

  insert,

  deleteUnitId,
  deleteSourcePackageId,
  deletePackageName,
--  deleteDependency,

  -- * Queries

  -- ** Precise lookups
  lookupUnitId,
  lookupComponentId,
  lookupSourcePackageId,
  lookupPackageId,
  lookupPackageName,
  lookupDependency,

  -- ** Case-insensitive searches
  searchByName,
  SearchResult(..),
  searchByNameSubstring,

  -- ** Bulk queries
  allPackages,
  allPackagesByName,
  allPackagesBySourcePackageId,

  -- ** Special queries
  brokenPackages,
  dependencyClosure,
  reverseDependencyClosure,
  topologicalOrder,
  reverseTopologicalOrder,
  dependencyInconsistencies,
  dependencyCycles,
  dependencyGraph,
  moduleNameIndex,

  -- * Backwards compatibility
  deleteInstalledPackageId,
  lookupInstalledPackageId,
  ) where

import Prelude ()
import Distribution.Compat.Prelude hiding (lookup)

import Distribution.Package
import Distribution.Backpack
import Distribution.ModuleName
import qualified Distribution.InstalledPackageInfo as IPI
import Distribution.Version
import Distribution.Simple.Utils
import Distribution.Types.Dependency

import Control.Exception (assert)
import Data.Array ((!))
import qualified Data.Array as Array
import qualified Data.Graph as Graph
import Data.List as List ( groupBy,  deleteBy, deleteFirstsBy )
import qualified Data.Map as Map
import qualified Data.Tree  as Tree

-- | The collection of information about packages from one or more 'PackageDB's.
-- These packages generally should have an instance of 'PackageInstalled'
--
-- Packages are uniquely identified in by their 'UnitId', they can
-- also be efficiently looked up by package name or by name and version.
--
data PackageIndex a = PackageIndex {
  -- The primary index. Each InstalledPackageInfo record is uniquely identified
  -- by its UnitId.
  --
  unitIdIndex :: !(Map UnitId a),

  -- This auxiliary index maps package names (case-sensitively) to all the
  -- versions and instances of that package. This allows us to find all
  -- versions satisfying a dependency.
  --
  -- It is a three-level index. The first level is the package name,
  -- the second is the package version and the final level is instances
  -- of the same package version. These are unique by UnitId
  -- and are kept in preference order.
  --
  -- FIXME: Clarify what "preference order" means. Check that this invariant is
  -- preserved. See #1463 for discussion.
  packageIdIndex :: !(Map PackageName (Map Version [a]))

  } deriving (Eq, Generic, Show, Read)

instance Binary a => Binary (PackageIndex a)

-- | The default package index which contains 'InstalledPackageInfo'.  Normally
-- use this.
type InstalledPackageIndex = PackageIndex IPI.InstalledPackageInfo

instance HasUnitId a => Monoid (PackageIndex a) where
  mempty  = PackageIndex Map.empty Map.empty
  mappend = (<>)
  --save one mappend with empty in the common case:
  mconcat [] = mempty
  mconcat xs = foldr1 mappend xs

instance HasUnitId a => Semigroup (PackageIndex a) where
  (<>) = merge

invariant :: HasUnitId a => PackageIndex a -> Bool
invariant (PackageIndex pids pnames) =
     map installedUnitId (Map.elems pids)
  == sort
     [ assert pinstOk (installedUnitId pinst)
     | (pname, pvers)  <- Map.toList pnames
     , let pversOk = not (Map.null pvers)
     , (pver,  pinsts) <- assert pversOk $ Map.toList pvers
     , let pinsts'  = sortBy (comparing installedUnitId) pinsts
           pinstsOk = all (\g -> length g == 1)
                          (groupBy (equating installedUnitId) pinsts')
     , pinst           <- assert pinstsOk $ pinsts'
     , let pinstOk = packageName    pinst == pname
                  && packageVersion pinst == pver
     ]
  -- If you see this invariant failing (ie the assert in mkPackageIndex below)
  -- then one thing to check is if it is happening in fromList. Check if the
  -- second list above (the sort [...] bit) is ending up with duplicates. This
  -- has been observed in practice once due to a messed up ghc-pkg db. How/why
  -- it became messed up was not discovered.


--
-- * Internal helpers
--

mkPackageIndex :: HasUnitId a
               => Map UnitId a
               -> Map PackageName (Map Version [a])
               -> PackageIndex a
mkPackageIndex pids pnames = assert (invariant index) index
  where index = PackageIndex pids pnames


--
-- * Construction
--

-- | Build an index out of a bunch of packages.
--
-- If there are duplicates by 'UnitId' then later ones mask earlier
-- ones.
--
fromList :: HasUnitId a => [a] -> PackageIndex a
fromList pkgs = mkPackageIndex pids pnames
  where
    pids      = Map.fromList [ (installedUnitId pkg, pkg) | pkg <- pkgs ]
    pnames    =
      Map.fromList
        [ (packageName (head pkgsN), pvers)
        | pkgsN <- groupBy (equating  packageName)
                 . sortBy  (comparing packageId)
                 $ pkgs
        , let pvers =
                Map.fromList
                [ (packageVersion (head pkgsNV),
                   nubBy (equating installedUnitId) (reverse pkgsNV))
                | pkgsNV <- groupBy (equating packageVersion) pkgsN
                ]
        ]

--
-- * Updates
--

-- | Merge two indexes.
--
-- Packages from the second mask packages from the first if they have the exact
-- same 'UnitId'.
--
-- For packages with the same source 'PackageId', packages from the second are
-- \"preferred\" over those from the first. Being preferred means they are top
-- result when we do a lookup by source 'PackageId'. This is the mechanism we
-- use to prefer user packages over global packages.
--
merge :: HasUnitId a => PackageIndex a -> PackageIndex a
      -> PackageIndex a
merge (PackageIndex pids1 pnames1) (PackageIndex pids2 pnames2) =
  mkPackageIndex (Map.unionWith (\_ y -> y) pids1 pids2)
                 (Map.unionWith (Map.unionWith mergeBuckets) pnames1 pnames2)
  where
    -- Packages in the second list mask those in the first, however preferred
    -- packages go first in the list.
    mergeBuckets xs ys = ys ++ (xs \\ ys)
    (\\) = deleteFirstsBy (equating installedUnitId)


-- | Inserts a single package into the index.
--
-- This is equivalent to (but slightly quicker than) using 'mappend' or
-- 'merge' with a singleton index.
--
insert :: HasUnitId a => a -> PackageIndex a -> PackageIndex a
insert pkg (PackageIndex pids pnames) =
    mkPackageIndex pids' pnames'

  where
    pids'   = Map.insert (installedUnitId pkg) pkg pids
    pnames' = insertPackageName pnames
    insertPackageName =
      Map.insertWith' (\_ -> insertPackageVersion)
                     (packageName pkg)
                     (Map.singleton (packageVersion pkg) [pkg])

    insertPackageVersion =
      Map.insertWith' (\_ -> insertPackageInstance)
                     (packageVersion pkg) [pkg]

    insertPackageInstance pkgs =
      pkg : deleteBy (equating installedUnitId) pkg pkgs


-- | Removes a single installed package from the index.
--
deleteUnitId :: HasUnitId a
             => UnitId -> PackageIndex a
             -> PackageIndex a
deleteUnitId ipkgid original@(PackageIndex pids pnames) =
  case Map.updateLookupWithKey (\_ _ -> Nothing) ipkgid pids of
    (Nothing,     _)     -> original
    (Just spkgid, pids') -> mkPackageIndex pids'
                                          (deletePkgName spkgid pnames)

  where
    deletePkgName spkgid =
      Map.update (deletePkgVersion spkgid) (packageName spkgid)

    deletePkgVersion spkgid =
        (\m -> if Map.null m then Nothing else Just m)
      . Map.update deletePkgInstance (packageVersion spkgid)

    deletePkgInstance =
        (\xs -> if null xs then Nothing else Just xs)
      . List.deleteBy (\_ pkg -> installedUnitId pkg == ipkgid) undefined

-- | Backwards compatibility wrapper for Cabal pre-1.24.
{-# DEPRECATED deleteInstalledPackageId "Use deleteUnitId instead" #-}
deleteInstalledPackageId :: HasUnitId a
                         => UnitId -> PackageIndex a
                         -> PackageIndex a
deleteInstalledPackageId = deleteUnitId

-- | Removes all packages with this source 'PackageId' from the index.
--
deleteSourcePackageId :: HasUnitId a => PackageId -> PackageIndex a
                      -> PackageIndex a
deleteSourcePackageId pkgid original@(PackageIndex pids pnames) =
  case Map.lookup (packageName pkgid) pnames of
    Nothing     -> original
    Just pvers  -> case Map.lookup (packageVersion pkgid) pvers of
      Nothing   -> original
      Just pkgs -> mkPackageIndex
                     (foldl' (flip (Map.delete . installedUnitId)) pids pkgs)
                     (deletePkgName pnames)
  where
    deletePkgName =
      Map.update deletePkgVersion (packageName pkgid)

    deletePkgVersion =
        (\m -> if Map.null m then Nothing else Just m)
      . Map.delete (packageVersion pkgid)


-- | Removes all packages with this (case-sensitive) name from the index.
--
deletePackageName :: HasUnitId a => PackageName -> PackageIndex a
                  -> PackageIndex a
deletePackageName name original@(PackageIndex pids pnames) =
  case Map.lookup name pnames of
    Nothing     -> original
    Just pvers  -> mkPackageIndex
                     (foldl' (flip (Map.delete . installedUnitId)) pids
                             (concat (Map.elems pvers)))
                     (Map.delete name pnames)

{-
-- | Removes all packages satisfying this dependency from the index.
--
deleteDependency :: Dependency -> PackageIndex -> PackageIndex
deleteDependency (Dependency name verstionRange) =
  delete' name (\pkg -> packageVersion pkg `withinRange` verstionRange)
-}

--
-- * Bulk queries
--

-- | Get all the packages from the index.
--
allPackages :: PackageIndex a -> [a]
allPackages = Map.elems . unitIdIndex

-- | Get all the packages from the index.
--
-- They are grouped by package name (case-sensitively).
--
allPackagesByName :: PackageIndex a -> [(PackageName, [a])]
allPackagesByName index =
  [ (pkgname, concat (Map.elems pvers))
  | (pkgname, pvers) <- Map.toList (packageIdIndex index) ]

-- | Get all the packages from the index.
--
-- They are grouped by source package id (package name and version).
--
allPackagesBySourcePackageId :: HasUnitId a => PackageIndex a
                             -> [(PackageId, [a])]
allPackagesBySourcePackageId index =
  [ (packageId ipkg, ipkgs)
  | pvers <- Map.elems (packageIdIndex index)
  , ipkgs@(ipkg:_) <- Map.elems pvers ]

--
-- * Lookups
--

-- | Does a lookup by unit identifier.
--
-- Since multiple package DBs mask each other by 'UnitId',
-- then we get back at most one package.
--
lookupUnitId :: PackageIndex a -> UnitId
             -> Maybe a
lookupUnitId index uid = Map.lookup uid (unitIdIndex index)

-- | Does a lookup by component identifier.  In the absence
-- of Backpack, this is just a 'lookupUnitId'.
--
lookupComponentId :: PackageIndex a -> ComponentId
                  -> Maybe a
lookupComponentId index cid =
    Map.lookup (newSimpleUnitId cid) (unitIdIndex index)

-- | Backwards compatibility for Cabal pre-1.24.
{-# DEPRECATED lookupInstalledPackageId "Use lookupUnitId instead" #-}
lookupInstalledPackageId :: PackageIndex a -> UnitId
                         -> Maybe a
lookupInstalledPackageId = lookupUnitId


-- | Does a lookup by source package id (name & version).
--
-- There can be multiple installed packages with the same source 'PackageId'
-- but different 'UnitId'. They are returned in order of
-- preference, with the most preferred first.
--
lookupSourcePackageId :: PackageIndex a -> PackageId -> [a]
lookupSourcePackageId index pkgid =
  case Map.lookup (packageName pkgid) (packageIdIndex index) of
    Nothing     -> []
    Just pvers  -> case Map.lookup (packageVersion pkgid) pvers of
      Nothing   -> []
      Just pkgs -> pkgs -- in preference order

-- | Convenient alias of 'lookupSourcePackageId', but assuming only
-- one package per package ID.
lookupPackageId :: PackageIndex a -> PackageId -> Maybe a
lookupPackageId index pkgid = case lookupSourcePackageId index pkgid  of
    []    -> Nothing
    [pkg] -> Just pkg
    _     -> error "Distribution.Simple.PackageIndex: multiple matches found"

-- | Does a lookup by source package name.
--
lookupPackageName :: PackageIndex a -> PackageName
                  -> [(Version, [a])]
lookupPackageName index name =
  case Map.lookup name (packageIdIndex index) of
    Nothing     -> []
    Just pvers  -> Map.toList pvers


-- | Does a lookup by source package name and a range of versions.
--
-- We get back any number of versions of the specified package name, all
-- satisfying the version range constraint.
--
-- INVARIANT: List of eligible 'IPI.InstalledPackageInfo' is non-empty.
--
lookupDependency :: InstalledPackageIndex -> Dependency
                 -> [(Version, [IPI.InstalledPackageInfo])]
lookupDependency index (Dependency name versionRange) =
  case Map.lookup name (packageIdIndex index) of
    Nothing    -> []
    Just pvers -> [ (ver, pkgs')
                  | (ver, pkgs) <- Map.toList pvers
                  , ver `withinRange` versionRange
                  , let pkgs' = filter eligible pkgs
                  -- Enforce the invariant
                  , not (null pkgs')
                  ]
 where
  -- When we select for dependencies, we ONLY want to pick up indefinite
  -- packages, or packages with no instantiations.  We'll do mix-in
  -- linking to improve any such package into an instantiated one
  -- later.
  eligible pkg = IPI.indefinite pkg || null (IPI.instantiatedWith pkg)

--
-- * Case insensitive name lookups
--

-- | Does a case-insensitive search by package name.
--
-- If there is only one package that compares case-insensitively to this name
-- then the search is unambiguous and we get back all versions of that package.
-- If several match case-insensitively but one matches exactly then it is also
-- unambiguous.
--
-- If however several match case-insensitively and none match exactly then we
-- have an ambiguous result, and we get back all the versions of all the
-- packages. The list of ambiguous results is split by exact package name. So
-- it is a non-empty list of non-empty lists.
--
searchByName :: PackageIndex a -> String -> SearchResult [a]
searchByName index name =
  case [ pkgs | pkgs@(pname,_) <- Map.toList (packageIdIndex index)
              , lowercase (unPackageName pname) == lname ] of
    []               -> None
    [(_,pvers)]      -> Unambiguous (concat (Map.elems pvers))
    pkgss            -> case find ((mkPackageName name ==) . fst) pkgss of
      Just (_,pvers) -> Unambiguous (concat (Map.elems pvers))
      Nothing        -> Ambiguous (map (concat . Map.elems . snd) pkgss)
  where lname = lowercase name

data SearchResult a = None | Unambiguous a | Ambiguous [a]

-- | Does a case-insensitive substring search by package name.
--
-- That is, all packages that contain the given string in their name.
--
searchByNameSubstring :: PackageIndex a -> String -> [a]
searchByNameSubstring index searchterm =
  [ pkg
  | (pname, pvers) <- Map.toList (packageIdIndex index)
  , lsearchterm `isInfixOf` lowercase (unPackageName pname)
  , pkgs <- Map.elems pvers
  , pkg <- pkgs ]
  where lsearchterm = lowercase searchterm


--
-- * Special queries
--

-- None of the stuff below depends on the internal representation of the index.
--

-- | Find if there are any cycles in the dependency graph. If there are no
-- cycles the result is @[]@.
--
-- This actually computes the strongly connected components. So it gives us a
-- list of groups of packages where within each group they all depend on each
-- other, directly or indirectly.
--
dependencyCycles :: PackageInstalled a => PackageIndex a -> [[a]]
dependencyCycles index =
  [ vs | Graph.CyclicSCC vs <- Graph.stronglyConnComp adjacencyList ]
  where
    adjacencyList = [ (pkg, installedUnitId pkg, installedDepends pkg)
                    | pkg <- allPackages index ]


-- | All packages that have immediate dependencies that are not in the index.
--
-- Returns such packages along with the dependencies that they're missing.
--
brokenPackages :: PackageInstalled a => PackageIndex a
               -> [(a, [UnitId])]
brokenPackages index =
  [ (pkg, missing)
  | pkg  <- allPackages index
  , let missing = [ pkg' | pkg' <- installedDepends pkg
                         , isNothing (lookupUnitId index pkg') ]
  , not (null missing) ]

-- | Tries to take the transitive closure of the package dependencies.
--
-- If the transitive closure is complete then it returns that subset of the
-- index. Otherwise it returns the broken packages as in 'brokenPackages'.
--
-- * Note that if the result is @Right []@ it is because at least one of
-- the original given 'PackageId's do not occur in the index.
--
dependencyClosure :: PackageInstalled a => PackageIndex a
                  -> [UnitId]
                  -> Either (PackageIndex a)
                            [(a, [UnitId])]
dependencyClosure index pkgids0 = case closure mempty [] pkgids0 of
  (completed, []) -> Left completed
  (completed, _)  -> Right (brokenPackages completed)
 where
    closure completed failed []             = (completed, failed)
    closure completed failed (pkgid:pkgids) = case lookupUnitId index pkgid of
      Nothing   -> closure completed (pkgid:failed) pkgids
      Just pkg  -> case lookupUnitId completed (installedUnitId pkg) of
        Just _  -> closure completed  failed pkgids
        Nothing -> closure completed' failed pkgids'
          where completed' = insert pkg completed
                pkgids'    = installedDepends pkg ++ pkgids

-- | Takes the transitive closure of the packages reverse dependencies.
--
-- * The given 'PackageId's must be in the index.
--
reverseDependencyClosure :: PackageInstalled a => PackageIndex a
                         -> [UnitId]
                         -> [a]
reverseDependencyClosure index =
    map vertexToPkg
  . concatMap Tree.flatten
  . Graph.dfs reverseDepGraph
  . map (fromMaybe noSuchPkgId . pkgIdToVertex)

  where
    (depGraph, vertexToPkg, pkgIdToVertex) = dependencyGraph index
    reverseDepGraph = Graph.transposeG depGraph
    noSuchPkgId = error "reverseDependencyClosure: package is not in the graph"

topologicalOrder :: PackageInstalled a => PackageIndex a -> [a]
topologicalOrder index = map toPkgId
                       . Graph.topSort
                       $ graph
  where (graph, toPkgId, _) = dependencyGraph index

reverseTopologicalOrder :: PackageInstalled a => PackageIndex a -> [a]
reverseTopologicalOrder index = map toPkgId
                              . Graph.topSort
                              . Graph.transposeG
                              $ graph
  where (graph, toPkgId, _) = dependencyGraph index

-- | Builds a graph of the package dependencies.
--
-- Dependencies on other packages that are not in the index are discarded.
-- You can check if there are any such dependencies with 'brokenPackages'.
--
dependencyGraph :: PackageInstalled a => PackageIndex a
                -> (Graph.Graph,
                    Graph.Vertex -> a,
                    UnitId -> Maybe Graph.Vertex)
dependencyGraph index = (graph, vertex_to_pkg, id_to_vertex)
  where
    graph = Array.listArray bounds
              [ [ v | Just v <- map id_to_vertex (installedDepends pkg) ]
              | pkg <- pkgs ]

    pkgs             = sortBy (comparing packageId) (allPackages index)
    vertices         = zip (map installedUnitId pkgs) [0..]
    vertex_map       = Map.fromList vertices
    id_to_vertex pid = Map.lookup pid vertex_map

    vertex_to_pkg vertex = pkgTable ! vertex

    pkgTable   = Array.listArray bounds pkgs
    topBound = length pkgs - 1
    bounds = (0, topBound)

-- | Given a package index where we assume we want to use all the packages
-- (use 'dependencyClosure' if you need to get such a index subset) find out
-- if the dependencies within it use consistent versions of each package.
-- Return all cases where multiple packages depend on different versions of
-- some other package.
--
-- Each element in the result is a package name along with the packages that
-- depend on it and the versions they require. These are guaranteed to be
-- distinct.
--
dependencyInconsistencies :: PackageInstalled a => PackageIndex a
                          -> [(PackageName, [(PackageId, Version)])]
dependencyInconsistencies index =
  [ (name, [ (pid,packageVersion dep) | (dep,pids) <- uses, pid <- pids])
  | (name, ipid_map) <- Map.toList inverseIndex
  , let uses = Map.elems ipid_map
  , reallyIsInconsistent (map fst uses) ]

  where -- for each PackageName,
        --   for each package with that name,
        --     the InstalledPackageInfo and the package Ids of packages
        --     that depend on it.
        inverseIndex = Map.fromListWith (Map.unionWith (\(a,b) (_,b') -> (a,b++b')))
          [ (packageName dep,
             Map.fromList [(ipid,(dep,[packageId pkg]))])
          | pkg <- allPackages index
          , ipid <- installedDepends pkg
          , Just dep <- [lookupUnitId index ipid]
          ]

        -- Added in 991e52a474e2b8280432257c1771dc474a320a30,
        -- this is a special case to handle the base 3 compatibility
        -- package which shipped with GHC 6.10 and GHC 6.12
        -- (it was removed in GHC 7.0).  Remove this when GHC 6.12
        -- goes out of our support window.
        reallyIsInconsistent :: PackageInstalled a => [a] -> Bool
        reallyIsInconsistent []       = False
        reallyIsInconsistent [_p]     = False
        reallyIsInconsistent [p1, p2] =
          let pid1 = installedUnitId p1
              pid2 = installedUnitId p2
          in pid1 `notElem` installedDepends p2
          && pid2 `notElem` installedDepends p1
        reallyIsInconsistent _ = True

-- | A rough approximation of GHC's module finder, takes a
-- 'InstalledPackageIndex' and turns it into a map from module names to their
-- source packages.  It's used to initialize the @build-deps@ field in @cabal
-- init@.
moduleNameIndex :: InstalledPackageIndex -> Map ModuleName [IPI.InstalledPackageInfo]
moduleNameIndex index =
  Map.fromListWith (++) $ do
    pkg <- allPackages index
    IPI.ExposedModule m reexport <- IPI.exposedModules pkg
    case reexport of
        Nothing -> return (m, [pkg])
        Just (OpenModuleVar _) -> []
        Just (OpenModule _ m') | m == m'   -> []
                                | otherwise -> return (m', [pkg])
        -- The heuristic is this: we want to prefer the original package
        -- which originally exported a module.  However, if a reexport
        -- also *renamed* the module (m /= m'), then we have to use the
        -- downstream package, since the upstream package has the wrong
        -- module name!