Implement --mergetool mode

Inspired by oxij's #277, but
implemented in Haskell, therefore faster (less process spawning) and
trivially integrated into the build and distribution.

Author: infinisil

main/Main.hs

@@ -1,4 +1,5 @@
 {-# LANGUAGE DeriveDataTypeable #-}
+{-# LANGUAGE LambdaCase #-}
 {-# LANGUAGE MultiWayIf #-}
 {-# LANGUAGE NamedFieldPuns #-}
 {-# LANGUAGE TemplateHaskell #-}
@@ -14,7 +15,7 @@ import Data.FileEmbed
 import Data.List (isSuffixOf)
 import Data.Maybe (fromMaybe)
 import Data.Text (Text)
-import qualified Data.Text.IO as TextIO (getContents, hPutStr, putStr)
+import qualified Data.Text.IO as TextIO (getContents, hGetContents, hPutStr, putStr)
 import Data.Version (showVersion)
 import GHC.IO.Encoding (utf8)
 import qualified Nixfmt
@@ -31,13 +32,15 @@ import System.Console.CmdArgs (
   (&=),
  )
 import System.Directory (doesDirectoryExist, listDirectory)
+import System.Environment (getEnv)
 import System.Exit (ExitCode (..), exitFailure, exitSuccess)
 import System.FilePath ((</>))
-import System.IO (hPutStrLn, hSetEncoding, stderr)
+import System.IO (Handle, hGetContents, hPutStrLn, hSetEncoding, stderr)
 import System.IO.Atomic (withOutputFile)
 import System.IO.Utf8 (readFileUtf8, withUtf8StdHandles)
 import System.Posix.Process (exitImmediately)
 import System.Posix.Signals (Handler (..), installHandler, keyboardSignal)
+import System.Process (CreateProcess (std_err, std_out), StdStream (CreatePipe), createProcess, proc, waitForProcess)
 
 type Result = Either String ()
 
@@ -47,6 +50,7 @@ data Nixfmt = Nixfmt
   { files :: [FilePath],
     width :: Width,
     check :: Bool,
+    mergetool :: Bool,
     quiet :: Bool,
     strict :: Bool,
     verify :: Bool,
@@ -70,6 +74,7 @@ options =
           defaultWidth
             &= help (addDefaultHint defaultWidth "Maximum width in characters"),
         check = False &= help "Check whether files are formatted without modifying them",
+        mergetool = False &= help "Whether to run in git mergetool mode, see TODO for more info",
         quiet = False &= help "Do not report errors",
         strict = False &= help "Enable a stricter formatting mode that isn't influenced as much by how the input is formatted",
         verify =
@@ -156,6 +161,14 @@ fileTarget path = Target (readFileUtf8 path) path atomicWriteFile
     -- Don't do anything if the file is already formatted
     atomicWriteFile False _ = mempty
 
+-- | Writes to a (potentially non-existent) file path, but reads from a potentially separate handle
+copyTarget :: Handle -> FilePath -> Target
+copyTarget from to = Target (TextIO.hGetContents from) to atomicWriteFile
+  where
+    atomicWriteFile _ t = withOutputFile to $ \h -> do
+      hSetEncoding h utf8
+      TextIO.hPutStr h t
+
 checkFileTarget :: FilePath -> Target
 checkFileTarget path = Target (readFileUtf8 path) path (const $ const $ pure ())
 
@@ -183,8 +196,41 @@ toWriteError :: Nixfmt -> String -> IO ()
 toWriteError Nixfmt{quiet = False} = hPutStrLn stderr
 toWriteError Nixfmt{quiet = True} = const $ return ()
 
+mergeToolJob :: Nixfmt -> IO Result
+mergeToolJob Nixfmt{mergetool = False} = return $ Right ()
+mergeToolJob opts = do
+  let formatter = toFormatter opts
+      oneByOne [] = return $ Right ()
+      oneByOne (y : ys) =
+        y >>= \case
+          Left err -> return $ Left err
+          Right () -> oneByOne ys
+
+  base <- getEnv "BASE"
+  local <- getEnv "LOCAL"
+  remote <- getEnv "REMOTE"
+  merged <- getEnv "MERGED"
+
+  if not (".nix" `isSuffixOf` merged)
+    then return $ Left ("Skipping non-Nix file " ++ merged)
+    else do
+      inputResult <- oneByOne $ map (formatTarget formatter . fileTarget) [base, local, remote]
+      case inputResult of
+        Left err -> return $ Left (err <> "pre-formatting with `nixfmt` failed\n")
+        Right () -> do
+          (_, Just out, Just err, process) <-
+            createProcess
+              (proc "git" ["merge-file", "--stdout", base, local, remote])
+                { std_out = CreatePipe,
+                  std_err = CreatePipe
+                }
+
+          waitForProcess process >>= \case
+            ExitFailure _ -> Left . (<> "`git merge-file` failed\n") <$> hGetContents err
+            ExitSuccess -> formatTarget formatter (copyTarget out merged)
+
 toJobs :: Nixfmt -> IO [IO Result]
-toJobs opts = map (toOperation opts $ toFormatter opts) <$> toTargets opts
+toJobs opts = (mergeToolJob opts :) . map (toOperation opts $ toFormatter opts) <$> toTargets opts
 
 writeErrorBundle :: (String -> IO ()) -> Result -> IO Result
 writeErrorBundle doWrite result = do

nixfmt.cabal

@@ -38,6 +38,7 @@ executable nixfmt
     , unix             >= 2.7.2 && < 2.9
     , text             >= 1.2.3 && < 2.2
     , transformers
+    , process
 
     -- for System.IO.Atomic
     , directory        >= 1.3.3 && < 1.4