Make clear when pkg-config is not present

When no pkg-config program was found, cabal would
claim that the package is not in the db, instead
of telling clearly that no pkg-config was found at
all.

cabal-install-solver/src/Distribution/Solver/Modular/Message.hs

@@ -279,6 +279,7 @@ showFR :: ConflictSet -> FailReason -> String
 showFR _ (UnsupportedExtension ext)       = " (conflict: requires " ++ showUnsupportedExtension ext ++ ")"
 showFR _ (UnsupportedLanguage lang)       = " (conflict: requires " ++ showUnsupportedLanguage lang ++ ")"
 showFR _ (MissingPkgconfigPackage pn vr)  = " (conflict: pkg-config package " ++ prettyShow pn ++ prettyShow vr ++ ", not found in the pkg-config database)"
+showFR _ MissingPkgconfigProgram          = " (pkg-config packages are needed but no pkg-config program was found)"
 showFR _ (NewPackageDoesNotMatchExistingConstraint d) = " (conflict: " ++ showConflictingDep d ++ ")"
 showFR _ (ConflictingConstraints d1 d2)   = " (conflict: " ++ L.intercalate ", " (L.map showConflictingDep [d1, d2]) ++ ")"
 showFR _ (NewPackageIsMissingRequiredComponent comp dr) = " (does not contain " ++ showExposedComponent comp ++ ", which is required by " ++ showDependencyReason dr ++ ")"

cabal-install-solver/src/Distribution/Solver/Modular/Tree.hs

@@ -102,6 +102,7 @@ data POption = POption I (Maybe PackagePath)
 data FailReason = UnsupportedExtension Extension
                 | UnsupportedLanguage Language
                 | MissingPkgconfigPackage PkgconfigName PkgconfigVersionRange
+                | MissingPkgconfigProgram
                 | NewPackageDoesNotMatchExistingConstraint ConflictingDep
                 | ConflictingConstraints ConflictingDep ConflictingDep
                 | NewPackageIsMissingRequiredComponent ExposedComponent (DependencyReason QPN)

cabal-install-solver/src/Distribution/Solver/Modular/Validate.hs

@@ -90,7 +90,7 @@ import Distribution.Types.PkgconfigVersionRange
 data ValidateState = VS {
   supportedExt        :: Extension -> Bool,
   supportedLang       :: Language  -> Bool,
-  presentPkgs         :: PkgconfigName -> PkgconfigVersionRange  -> Bool,
+  presentPkgs         :: PkgconfigName -> PkgconfigVersionRange  -> Maybe Bool,
   index               :: Index,
 
   -- Saved, scoped, dependencies. Every time 'validate' makes a package choice,
@@ -383,7 +383,7 @@ extractNewDeps v b fa sa = go
 -- or the successfully extended assignment.
 extend :: (Extension -> Bool)            -- ^ is a given extension supported
        -> (Language  -> Bool)            -- ^ is a given language supported
-       -> (PkgconfigName -> PkgconfigVersionRange -> Bool) -- ^ is a given pkg-config requirement satisfiable
+       -> (PkgconfigName -> PkgconfigVersionRange -> Maybe Bool) -- ^ is a given pkg-config requirement satisfiable
        -> [LDep QPN]
        -> PPreAssignment
        -> Either Conflict PPreAssignment
@@ -398,8 +398,10 @@ extend extSupported langSupported pkgPresent newactives ppa = foldM extendSingle
       if langSupported lang then Right a
                             else Left (dependencyReasonToConflictSet dr, UnsupportedLanguage lang)
     extendSingle a (LDep dr (Pkg pn vr))  =
-      if pkgPresent pn vr then Right a
-                          else Left (dependencyReasonToConflictSet dr, MissingPkgconfigPackage pn vr)
+      case pkgPresent pn vr of
+        Just True -> Right a
+        Just False -> Left (dependencyReasonToConflictSet dr, MissingPkgconfigPackage pn vr)
+        Nothing -> Left (dependencyReasonToConflictSet dr, MissingPkgconfigProgram)
     extendSingle a (LDep dr (Dep dep@(PkgComponent qpn _) ci)) =
       let mergedDep = M.findWithDefault (MergedDepConstrained []) qpn a
       in  case (\ x -> M.insert qpn x a) <$> merge mergedDep (PkgDep dr dep ci) of

cabal-install-solver/src/Distribution/Solver/Types/PkgConfigDb.hs

@@ -156,17 +156,17 @@ pkgConfigDbFromList pairs = (PkgConfigDb . M.fromList . map convert) pairs
 
 -- | Check whether a given package range is satisfiable in the given
 -- @pkg-config@ database.
-pkgConfigPkgIsPresent :: PkgConfigDb -> PkgconfigName -> PkgconfigVersionRange -> Bool
+pkgConfigPkgIsPresent :: PkgConfigDb -> PkgconfigName -> PkgconfigVersionRange -> Maybe Bool
 pkgConfigPkgIsPresent (PkgConfigDb db) pn vr =
     case M.lookup pn db of
-      Nothing       -> False    -- Package not present in the DB.
-      Just Nothing  -> True     -- Package present, but version unknown.
-      Just (Just v) -> withinPkgconfigVersionRange v vr
+      Nothing       -> Just False    -- Package not present in the DB.
+      Just Nothing  -> Just True     -- Package present, but version unknown.
+      Just (Just v) -> Just $ withinPkgconfigVersionRange v vr
 -- If we could not read the pkg-config database successfully we fail.
 -- The plan found by the solver can't be executed later, because pkg-config itself
 -- is going to be called in the build phase to get the library location for linking
 -- so even if there is a library, it would need to be passed manual flags anyway.
-pkgConfigPkgIsPresent NoPkgConfigDb _ _ = False
+pkgConfigPkgIsPresent NoPkgConfigDb _ _ = Nothing
 
 
 

cabal-testsuite/PackageTests/ExtraProgPath/setup.out

@@ -5,6 +5,6 @@ Resolving dependencies...
 Error: [Cabal-7107]
 Could not resolve dependencies:
 [__0] next goal: CheckExtraProgPath (user goal)
-[__0] rejecting: CheckExtraProgPath-0.1 (conflict: pkg-config package zlib-any, not found in the pkg-config database)
+[__0] rejecting: CheckExtraProgPath-0.1 (pkg-config packages are needed but no pkg-config program was found)
 [__0] fail (backjumping, conflict set: CheckExtraProgPath)
 After searching the rest of the dependency tree exhaustively, these were the goals I've had most trouble fulfilling: CheckExtraProgPath (2)