Add Haskell script trying to reproduce the build

We add a Haskell script in charge of reproducing the build.

We currently handle quite poorly the progress update: stderr isn't
streamed to the console. Knowing that a GHC build takes ~30 minutes,
it'll be good to at least show some kind of progress to the user while
building.

ReproTest.hs

@@ -0,0 +1,68 @@
+#!/usr/bin/env nix-shell
+#!nix-shell -i runhaskell -p "haskellPackages.ghcWithPackages(p: with p; [ optparse-applicative text process ])" -p nix -p diffoscope
+
+import Options.Applicative
+import Data.Text hiding (filter)
+import System.Exit
+import System.Process
+
+
+data Config = Config
+  { pkgName :: Text,
+    ghcPath :: Text }
+
+type ReproPaths = [Text]
+type StorePath  = Text
+
+main :: IO ()
+main = do
+  c <- execParser opts
+  mdp <- repro c
+  print mdp
+  pure ()
+  where
+    opts = info (configParser <**> helper)
+       ( fullDesc
+       <> header "ReproTest.hs - Test the GHC binary reproducibility for a hackage package"
+       )
+
+repro :: Config -> IO (Maybe ReproPaths)
+repro c = do
+  -- 1. Nix instantiate derivation
+  (eci, drvPathDirty, err) <- readProcessWithExitCode "nix-instantiate"
+    [
+      "--arg", "hs-pkg-name", "\"" ++ unpack (pkgName c) ++ "\"",
+      "--arg", "ghcSrc", unpack $ ghcPath c,
+      "./repro.nix"
+    ] ""
+  let drvPath =  filter (/= '\n') drvPathDirty
+  print err
+  print drvPath
+  print eci
+  -- 2. nix-store --realize /nix/store/xxx.drv
+  (ecr1, p1, err1) <- readProcessWithExitCode "nix-store"
+    [ "--realise", "--quiet", filter (/= '\n') drvPath ] ""
+  print ecr1
+  print p1
+  print err1
+  -- 3. nix-store --realize --check /nix/store/xxx.drv
+  (ecr2, p2, err2) <- readProcessWithExitCode "nix-store"
+    [ "--realise", "--quiet", "-K", "--check", drvPath ] ""
+  print ecr2
+  print p2
+  print ecr2
+  -- 4. If exit code > 1 => Just two repro paths
+  if ecr2 == ExitSuccess
+    then pure Nothing
+    else pure $ Just [ pack p1, pack p2 ]
+
+runDiffoscope :: ReproPaths -> IO StorePath
+runDiffoscope = undefined
+  -- Take the two different paths.
+  -- Run diffoscope on them.
+
+configParser :: Parser Config
+configParser = Config
+  <$> strOption (long "package-name" <> short 'p' <> help "Hackage package to test")
+  <*> (strOption (long "ghc-path" <> short 'g' <> help "Path to the local GHC checkpoint")
+       <|> strOption (long "ghc-nixexpr-path" <> short 'e' <> help "Path to the nix expression generating the GHC binary"))

default.nix

@@ -2,12 +2,5 @@
   hs-pkg-name ? "wcwidth",
   ghcSrc ? /home/ninjatrappeur/Code/perso/haskell/ghc
 }:
-let
-  ghcOverlay =
-    import ./overlays/ghc.nix {
-      inherit ghcSrc pkgs;
-    };
-  nixpkgsConfig = { overlays = [ ghcOverlay ]; };
-  nixpkgsSrc = (import ./nix/sources.nix).nixpkgs;
-  pkgs = import nixpkgsSrc nixpkgsConfig;
-in pkgs.customGhcHEAD."${hs-pkg-name}"
+
+import ./repro.nix { inherit hs-pkg-name ghcSrc; }