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 pn vr)  = " (pkg-config package " ++ prettyShow pn ++ prettyShow vr ++ " is needed but no pkg-config executable was found or querying it failed)"
 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 PkgconfigName PkgconfigVersionRange
                 | 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         :: Maybe (PkgconfigName -> PkgconfigVersionRange  -> 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
+       -> Maybe (PkgconfigName -> PkgconfigVersionRange -> 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 (\f -> f pn vr) <$> pkgPresent of
+        Just True -> Right a
+        Just False -> Left (dependencyReasonToConflictSet dr, MissingPkgconfigPackage pn vr)
+        Nothing -> Left (dependencyReasonToConflictSet dr, MissingPkgconfigProgram pn vr)
     extendSingle a (LDep dr (Dep dep@(PkgComponent qpn _) ci)) =
       let mergedDep = M.findWithDefault (MergedDepConstrained []) qpn a
       in  case (\ x -> M.insert qpn x a) <$> merge mergedDep (PkgDep dr dep ci) of
@@ -569,7 +571,7 @@ validateTree cinfo idx pkgConfigDb t = runValidate (validate t) VS {
   , supportedLang       = maybe (const True)
                                 (flip L.elem) -- use list lookup because language list is small and no Ord instance
                                 (compilerInfoLanguages  cinfo)
-  , presentPkgs         = maybe (\_pn _pvr -> False) pkgConfigPkgIsPresent pkgConfigDb
+  , presentPkgs         = pkgConfigPkgIsPresent <$> pkgConfigDb
   , index               = idx
   , saved               = M.empty
   , pa                  = PA M.empty M.empty M.empty

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

@@ -46,21 +46,15 @@ import Distribution.Types.PkgconfigVersion
 import Distribution.Types.PkgconfigVersionRange
 import Distribution.Verbosity                   (Verbosity)
 
+-- | The list of packages installed in the system visible to
+-- @pkg-config@.
+--
+-- If an entry is `Nothing`, this means that the package seems to be present,
+-- but we don't know the exact version (because parsing of the version number
+-- failed).
 newtype PkgConfigDb = PkgConfigDb (M.Map PkgconfigName (Maybe PkgconfigVersion))
      deriving (Show, Generic, Typeable)
 
--- -- | The list of packages installed in the system visible to
--- -- @pkg-config@. This is an opaque datatype, to be constructed with
--- -- `readPkgConfigDb` and queried with `pkgConfigPkgPresent`.
--- data PkgConfigDb =  PkgConfigDb (M.Map PkgconfigName (Maybe PkgconfigVersion))
---                  -- ^ If an entry is `Nothing`, this means that the
---                  -- package seems to be present, but we don't know the
---                  -- exact version (because parsing of the version
---                  -- number failed).
---                  | NoPkgConfigDb
---                  -- ^ For when we could not run pkg-config successfully.
---      deriving (Show, Generic, Typeable)
-
 instance Binary PkgConfigDb
 instance Structured PkgConfigDb
 
@@ -71,7 +65,7 @@ readPkgConfigDb :: Verbosity -> ProgramDb -> IO (Maybe PkgConfigDb)
 readPkgConfigDb verbosity progdb = handle ioErrorHandler $ do
     mpkgConfig <- needProgram verbosity pkgConfigProgram progdb
     case mpkgConfig of
-      Nothing             -> noPkgConfig "Cannot find pkg-config program"
+      Nothing             -> noPkgConfig "cannot find pkg-config program"
       Just (pkgConfig, _) -> do
         -- To prevent malformed Unicode in the descriptions from crashing cabal,
         -- read without interpreting any encoding first. (#9608)
@@ -124,8 +118,8 @@ readPkgConfigDb verbosity progdb = handle ioErrorHandler $ do
     -- For when pkg-config invocation fails (possibly because of a
     -- too long command line).
     noPkgConfig extra = do
-        info verbosity ("Failed to query pkg-config, Cabal will continue"
-                        ++ " without solving for pkg-config constraints: "
+        info verbosity ("Warning: Failed to query pkg-config, Cabal will backtrack "
+                        ++ "if a package from pkg-config is requested. Error message: "
                         ++ extra)
         return 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 package zlib-any is needed but no pkg-config executable was found or querying it failed)
 [__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)

changelog.d/pr-10122

@@ -0,0 +1,10 @@
+synopsis: Clarify error message when pkg-config is not found
+packages: cabal-install-solver
+prs: #10122
+
+description: {
+
+- The error message when pkg-config is not found or querying it fails will no
+longer incorrectly claim that the package is missing in the database.
+
+}