hadrian: Fix leakage of GHC in PATH into build

Previously hadrian would use GHC on PATH when configuring packages (or
fail if there is no such GHC). Fix this. Unfortunately this runs into
another bug in Cabal which we workaround.

1 files changed, 7 insertions, 1 deletions

diff --git a/hadrian/src/Hadrian/Oracles/Cabal/Rules.hs b/hadrian/src/Hadrian/Oracles/Cabal/Rules.hs
index a8fe8c773a..546406ecd2 100644
--- a/hadrian/src/Hadrian/Oracles/Cabal/Rules.hs
+++ b/hadrian/src/Hadrian/Oracles/Cabal/Rules.hs
@@ -15,6 +15,7 @@ import Control.Monad
 import Data.Maybe
 import Development.Shake
 import Distribution.Simple.GHC
+import Distribution.Simple.Program.Builtin
 import Distribution.Simple.Program.Db
 import Distribution.Verbosity
 
@@ -58,8 +59,13 @@ cabalOracle = do
                ++ quote (pkgName pkg) ++ " (" ++ show stage ++ ")..."
         -- Configure the package with the GHC corresponding to the given stage
         hcPath <- builderPath (Ghc CompileHs stage)
+        let progDb = userSpecifyPath "ghc" hcPath
+                     $ addKnownProgram ghcProgram emptyProgramDb
         (compiler, maybePlatform, _pkgdb) <- liftIO $
-            configure silent (Just hcPath) Nothing emptyProgramDb
+            -- N.B. the hcPath parameter of `configure` is broken when given an
+            -- empty ProgramDb. To work around this we manually construct an
+            -- appropriate ProgramDb.
+            configure silent Nothing Nothing progDb
         let platform = fromMaybe (error msg) maybePlatform
             msg      = "PackageConfiguration oracle: cannot detect platform"
         return $ PackageConfiguration (compiler, platform)