Refine the fix for requiring Cabal > 1.20 for Setup.hs

Going along with the existing approach of using a constraint rather than
altering the deps of custom Setup.hs scripts, but make the constraint
only apply to Cabal instances that are dependencies of Setup.hs scripts
not all instances of Cabal.

Also rearrange things a little with a dedicated solver policy function
for adding a min dep on Cabal versions for setup scripts.

Author: dcoutts

cabal-install/Distribution/Client/Dependency.hs

@@ -56,6 +56,7 @@ module Distribution.Client.Dependency (
     removeLowerBounds,
     removeUpperBounds,
     addDefaultSetupDependencies,
+    addSetupCabalMinVersionConstraint,
   ) where
 
 import Distribution.Solver.Modular
@@ -84,9 +85,9 @@ import Distribution.PackageDescription.Configuration
 import Distribution.Client.PackageUtils
          ( externalBuildDepends )
 import Distribution.Version
-         ( mkVersion, VersionRange, anyVersion, thisVersion, orLaterVersion
-         , withinRange, simplifyVersionRange
-         , removeLowerBound, removeUpperBound )
+         ( Version, mkVersion
+         , VersionRange, anyVersion, thisVersion, orLaterVersion, withinRange
+         , simplifyVersionRange, removeLowerBound, removeUpperBound )
 import Distribution.Compiler
          ( CompilerInfo(..) )
 import Distribution.System
@@ -460,6 +461,22 @@ addDefaultSetupDependencies defaultSetupDeps params =
         gpkgdesc = packageDescription srcpkg
         pkgdesc  = PD.packageDescription gpkgdesc
 
+-- | There If a package has a custom setup then we need to add a setup-depends
+-- on Cabal. For now it's easier to add this unconditionally.  Once
+-- qualified constraints land we can turn this into a custom setup
+-- only constraint.
+--
+addSetupCabalMinVersionConstraint :: Version
+                                  -> DepResolverParams -> DepResolverParams
+addSetupCabalMinVersionConstraint minVersion =
+    addConstraints
+      [ LabeledPackageConstraint
+          (PackageConstraintVersion cabalPkgname (orLaterVersion minVersion))
+          ConstraintSetupCabalMinVersion
+      ]
+  where
+    cabalPkgname = mkPackageName "Cabal"
+
 
 upgradeDependencies :: DepResolverParams -> DepResolverParams
 upgradeDependencies = setPreferenceDefault PreferAllLatest

cabal-install/Distribution/Client/ProjectPlanning.hs

@@ -946,23 +946,25 @@ planPackages comp platform solver SolverSettings{..}
                                    . PD.packageDescription
                                    . packageDescription)
 
+      . addSetupCabalMinVersionConstraint (mkVersion [1,20])
+          -- While we can talk to older Cabal versions (we need to be able to
+          -- do so for custom Setup scripts that require older Cabal lib
+          -- versions), we have problems talking to some older versions that
+          -- don't support certain features.
+          --
+          -- For example, Cabal-1.16 and older do not know about build targets.
+          -- Even worse, 1.18 and older only supported the --constraint flag
+          -- with source package ids, not --dependency with installed package
+          -- ids. That is bad because we cannot reliably select the right
+          -- dependencies in the presence of multiple instances (i.e. the
+          -- store). See issue #3932. So we require Cabal 1.20 as a minimum.
+
       . addPreferences
           -- preferences from the config file or command line
           [ PackageVersionPreference name ver
           | Dependency name ver <- solverSettingPreferences ]
 
       . addConstraints
-
-          -- If a package has a custom setup then we need to add a setup-depends
-          -- on Cabal. For now it's easier to add this unconditionally.  Once
-          -- qualified constraints land we can turn this into a custom setup
-          -- only constraint.
-          --
-          -- TODO: use a qualified constraint
-            [ LabeledPackageConstraint (PackageConstraintVersion cabalPkgname
-                                       (orLaterVersion (mkVersion [1,20])))
-                                       ConstraintNewBuildCustomSetupLowerBoundCabal
-                                       ] . addConstraints
           -- version constraints from the config file or command line
             [ LabeledPackageConstraint (userToPackageConstraint pc) src
             | (pc, src) <- solverSettingConstraints ]
@@ -2383,6 +2385,10 @@ packageSetupScriptStyle pkg
 -- we still need to distinguish the case of explicit and implict setup deps.
 -- See 'rememberImplicitSetupDeps'.
 --
+-- Note in addition to adding default setup deps, we also use
+-- 'addSetupCabalMinVersionConstraint' (in 'planPackages') to require
+-- @Cabal >= 1.20@ for Setup scripts.
+--
 defaultSetupDeps :: Compiler -> Platform
                  -> PD.PackageDescription
                  -> Maybe [Dependency]
@@ -2401,35 +2407,19 @@ defaultSetupDeps compiler platform pkg =
         where
           -- The Cabal dep is slightly special:
           -- * We omit the dep for the Cabal lib itself, since it bootstraps.
-          -- * We constrain it to be >= 1.18 < 2
+          -- * We constrain it to be < 1.25
           --
-          -- Note: cabalCompatMinVer only gets applied WHEN WE ARE ADDING a
-          -- default setup build info, i.e., when there is no custom-setup
-          -- stanza. If there is a custom-setup stanza, this codepath never gets
-          -- invoked (that's why there's an error case for
-          -- SetupCustomExplicitDeps).
+          -- Note: we also add a global constraint to require Cabal >= 1.20
+          -- for Setup scripts (see use addSetupCabalMinVersionConstraint).
           --
-          -- One way we could solve this problem is by also modifying
-          -- custom-setup stanzas when they exist, but we're going to take a
-          -- different approach: add an extra constraint on Cabal globally to
-          -- make sure the solver respects it regardless of whether or not there
-          -- is an explicit setup build info or not. See planPackages.
-          cabalConstraint   = orLaterVersion cabalCompatMinVer
-                                `intersectVersionRanges`
-                              orLaterVersion (PD.specVersion pkg)
+          cabalConstraint   = orLaterVersion (PD.specVersion pkg)
                                 `intersectVersionRanges`
                               earlierVersion cabalCompatMaxVer
           -- The idea here is that at some point we will make significant
           -- breaking changes to the Cabal API that Setup.hs scripts use.
           -- So for old custom Setup scripts that do not specify explicit
           -- constraints, we constrain them to use a compatible Cabal version.
           cabalCompatMaxVer = mkVersion [1,25]
-          -- In principle we can talk to any old Cabal version, and we need to
-          -- be able to do that for custom Setup scripts that require older
-          -- Cabal lib versions. However in practice we have currently have
-          -- problems with Cabal-1.16. (1.16 does not know about build targets)
-          -- If this is fixed we can relax this constraint.
-          cabalCompatMinVer = mkVersion [1,18]
 
       -- For other build types (like Simple) if we still need to compile an
       -- external Setup.hs, it'll be one of the simple ones that only depends
@@ -2444,7 +2434,7 @@ defaultSetupDeps compiler platform pkg =
       SetupNonCustomInternalLib -> Just []
 
       -- This case gets ruled out by the caller, planPackages, see the note
-      -- above in the SetupCustomIplicitDeps case.
+      -- above in the SetupCustomImplicitDeps case.
       SetupCustomExplicitDeps ->
         error $ "defaultSetupDeps: called for a package with explicit "
              ++ "setup deps: " ++ display (packageId pkg)

cabal-install/Distribution/Solver/Modular/Package.hs

@@ -11,6 +11,7 @@ module Distribution.Solver.Modular.Package
   , instI
   , makeIndependent
   , primaryPP
+  , setupPP
   , showI
   , showPI
   , unPN
@@ -91,6 +92,14 @@ primaryPP (PackagePath _ns q) = go q
     go (Setup _)   = False
     go (Exe _ _)   = False
 
+-- | Is the package a dependency of a setup script. This is used establish
+-- whether or not certain constraints should apply to this dependency
+-- (grep 'setupPP' to see the use sites).
+--
+setupPP :: PackagePath -> Bool
+setupPP (PackagePath _ns (Setup _)) = True
+setupPP (PackagePath _ns _)         = False
+
 -- | Create artificial parents for each of the package names, making
 -- them all independent.
 makeIndependent :: [PN] -> [QPN]

cabal-install/Distribution/Solver/Modular/Preference.hs

@@ -156,6 +156,9 @@ processPackageConstraintP pp _ _ (LabeledPackageConstraint _ src) r
   | src == ConstraintSourceUserTarget && not (primaryPP pp)         = r
     -- the constraints arising from targets, like "foo-1.0" only apply to
     -- the main packages in the solution, they don't constrain setup deps
+  | src == ConstraintSetupCabalMinVersion && not (setupPP pp)       = r
+    -- the internal constraints on the Setup.hs CLI version don't apply to
+    -- the main packages in the solution, they only constrain setup deps
 
 processPackageConstraintP _ c i (LabeledPackageConstraint pc src) r = go i pc
   where

cabal-install/Distribution/Solver/Types/ConstraintSource.hs

@@ -46,8 +46,9 @@ data ConstraintSource =
   -- | The source of the constraint is not specified.
   | ConstraintSourceUnknown
 
-  -- | Custom setup requires a minimum lower bound on Cabal
-  | ConstraintNewBuildCustomSetupLowerBoundCabal
+  -- | An internal constraint due to compatability issues with the Setup.hs
+  -- command line interface requires a minimum lower bound on Cabal
+  | ConstraintSetupCabalMinVersion
   deriving (Eq, Show, Generic)
 
 instance Binary ConstraintSource
@@ -71,4 +72,5 @@ showConstraintSource ConstraintSourceFreeze = "cabal freeze"
 showConstraintSource ConstraintSourceConfigFlagOrTarget =
     "config file, command line flag, or user target"
 showConstraintSource ConstraintSourceUnknown = "unknown source"
-showConstraintSource ConstraintNewBuildCustomSetupLowerBoundCabal = "new-build's support of Custom Setup (issue #3932)"
+showConstraintSource ConstraintSetupCabalMinVersion =
+    "minimum version of Cabal used by Setup.hs"