Add Haskell script trying to reproduce the build

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

ReproTest.hs

@@ -2,6 +2,7 @@
 #!nix-shell -i runhaskell -p "haskellPackages.ghcWithPackages(p: with p; [ relude optparse-applicative process ])" -p nix -p diffoscope
 {-# LANGUAGE NoImplicitPrelude #-}
 {-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE BangPatterns #-}
 
 import Relude
 
@@ -18,8 +19,11 @@ import System.Process
 
 
 data Config = Config
-  { pkgName :: Text,
-    ghcPath :: Text }
+  { pkgName     :: Text,
+    ghcPath     :: Text,
+    diffOutDir  :: Text,
+    nbRepro     :: Int
+  }
 data ReproStatus = ReproStatus IsReproducible ReproPaths
 
 type IsReproducible = Bool
@@ -33,8 +37,11 @@ main = do
   rs <- runExceptT $ repro c
   case rs of
     Left err -> T.putStrLn err >> exitFailure
-    Right (ReproStatus True xs)  -> T.putStrLn "SUCCESS" >> exitSuccess
-    Right (ReproStatus False xs) -> T.putStr $ "Cannot Reproduce build:\n" <> unwords xs
+    Right (ReproStatus True xs)  -> T.putStrLn "The build seems to be reproducible" >> exitSuccess
+    Right (ReproStatus False xs) -> do
+      runDiffoscope c xs
+      T.putStrLn $ "Cannot Reproduce build.\n" <> unwords xs
+      T.putStrLn $ "See diffoscope report for more details: " <> diffOutDir c
   where
     opts = info (configParser <**> helper)
        ( fullDesc
@@ -43,8 +50,10 @@ main = do
 
 configParser :: Parser Config
 configParser = Config
-  <$> strOption (long "package-name" <> short 'p' <> help "Hackage package to test")
+  <$> strOption (long "package-name" <> short 'p' <> help "Hackage package to test.")
   <*> strOption (long "ghc-nixexpr-path" <> short 'e' <> help "Path to the nix derivation retrieving the custom GHC checkpout you want to use.")
+  <*> strOption (long "report-dir" <> short 'o' <> help "Directory in which you want to store the diff output in case of a failing test." <> showDefault <> value "./diff-out")
+  <*> option auto (long "nb-repro" <> short 'n' <> help "How much reproduction iterations we want to perform." <> showDefault <> value 3)
 
 repro :: (MonadIO m) => Config -> ExceptT NixError m ReproStatus
 repro c = do
@@ -59,15 +68,23 @@ repro c = do
   (_, p1) <- streamRunCmd "nix-store"
     ["--realise", "--quiet", drvPath] True
   -- 3. nix-store --realize --check /nix/store/xxx.drv
-  (ecr2, p2) <- streamRunCmd "nix-store"
-    ["--realise", "--quiet", "-K", "--check", drvPath] False
+  (ecr2, p2) <- repeatRepro c drvPath
   -- 4. If exit code > 1 => Just two repro paths
   pure $ ReproStatus (ecr2 == E.ExitSuccess) [p1, p2]
 
-runDiffoscope :: ReproPaths -> IO StorePath
-runDiffoscope = undefined
-  -- Take the two different paths.
-  -- Run diffoscope on them.
+repeatRepro :: (MonadIO m) => Config -> Text -> ExceptT NixError m (E.ExitCode, StorePath)
+repeatRepro c drv = go $ nbRepro c
+  where
+    go (!i) = do
+      (ecr2, p2) <- streamRunCmd "nix-store"
+          ["--realise", "--quiet", "-K", "--check", drv] False
+      if ecr2 /= E.ExitSuccess || i < 1
+        then pure (ecr2, p2)
+        else go $ i - 1
+
+runDiffoscope :: Config -> ReproPaths -> IO ()
+runDiffoscope c rp = callProcess "diffoscope"
+                       $ unpack <$> [ "--html-dir", diffOutDir c ] <> rp
 
 -- Partially vendored from Shask
 -- TODO:

examples/ghc-src.nix

@@ -0,0 +1,7 @@
+let
+  pkgs = import (import ../nix/sources.nix).nixpkgs {};
+in pkgs.fetchgit {
+    url = "https://gitlab.haskell.org/ghc/ghc.git/";
+    rev = "40c71c2cf38b4e134d81b7184a4d5e02949ae70c";
+    sha256 = "04h9rcyzm9w3an1z00hjs062dp7dl19b8pkyxjsypr7a2i9dmvkb";
+}

repro.nix

@@ -0,0 +1,14 @@
+{
+  hs-pkg-name ? "wcwidth",
+  ghcSrcDrv ? ./examples/ghc.nix
+}:
+let
+  ghcSrc = import ghcSrcDrv;
+  ghcOverlay =
+    import ./overlays/ghc.nix {
+      inherit ghcSrc pkgs;
+    };
+  nixpkgsConfig = { overlays = [ ghcOverlay ]; };
+  nixpkgsSrc = (import ./nix/sources.nix).nixpkgs;
+  pkgs = import nixpkgsSrc nixpkgsConfig;
+in pkgs.haskellPackages."${hs-pkg-name}"