Break command args into chunks under system limit like what xargs does (

#63)

Closes #62

We removed the depedency on xargs to for better windows compatibility.
However, xargs was mainly introduced to break shell call into multiple
ones to avoid hitting the system limit when we have tens of thousands
files. We need to simulate this behavior.

Retrie/Options.hs

@@ -5,6 +5,7 @@
 --
 {-# LANGUAGE ApplicativeDo #-}
 {-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE NumericUnderscores #-}
 {-# LANGUAGE RecordWildCards #-}
 {-# LANGUAGE TupleSections #-}
 module Retrie.Options
@@ -487,13 +488,28 @@ runGrepChain targetDir verbosity GrepCommands{..} = foldM (commandStep targetDir
 
 -- | run a command with a list of files as quoted arguments
 commandStep :: FilePath -> Verbosity -> [FilePath]-> CommandLine -> IO [FilePath]
-commandStep targetDir verbosity files cmd = doCmd targetDir verbosity (cmd <> formatPaths files)
- where
-    formatPaths [] = ""
-    formatPaths xs = " " <> unwords (map quotePath xs)
+commandStep targetDir verbosity files cmd =
+  fmap concat $ forM (chunkifyArgs $ map quotePath files) $ \args ->
+    doCmd targetDir verbosity $ cmd <> " " <> unwords args
+
 quotePath :: FilePath -> FilePath
 quotePath x = "'" <> x <> "'"
 
+-- Split arguments into chunks whose size does not exceed 'argMax'.
+-- Note that @chunkifyArgs []@ returns @[[]]@ intentionally.
+chunkifyArgs :: [String] -> [[String]]
+chunkifyArgs = go 0 []
+  where
+    go _ acc [] = [reverse acc]
+    go n acc (arg: args)
+      | n <= 0 || n + len <= argMax = go (n + len) (arg: acc) args
+      | otherwise = reverse acc: go len [arg] args
+      where
+        len = length arg + 1
+
+-- | Hardcoded default @--max-args=max-args@ used by @xargs@.
+argMax :: Int
+argMax = 128 * 1024
 
 doCmd :: FilePath -> Verbosity -> String -> IO [FilePath]
 doCmd targetDir verbosity shellCmd = do