More safely handle reading files

This replaces readIfFileExists, which has a race-condition and may raise
an exception, with safeReadFile, which safely reads a file and includes
the IOException if something goes wrong.

src/Unused/Cache/DirectoryFingerprint.hs

@@ -10,7 +10,7 @@ import qualified System.Directory as D
 import qualified Data.Char as C
 import Data.Maybe (fromMaybe)
 import Unused.Cache.FindArgsFromIgnoredPaths
-import Unused.Util (safeHead, readIfFileExists)
+import Unused.Util (safeHead, safeReadFile)
 
 type MD5Config = ReaderT String IO
 
@@ -43,7 +43,7 @@ md5Result r = do
     liftIO $ readProcess md5exec [] r
 
 ignoredPaths :: IO [String]
-ignoredPaths = fromMaybe [] <$> (fmap lines <$> readIfFileExists ".gitignore")
+ignoredPaths = either (const []) id <$> (fmap lines <$> safeReadFile ".gitignore")
 
 md5Executable :: IO (Maybe String)
 md5Executable =

src/Unused/ResultsClassifier/Config.hs

@@ -12,7 +12,7 @@ import System.FilePath ((</>))
 import System.Directory (getHomeDirectory, doesFileExist)
 import Paths_unused (getDataFileName)
 import Unused.ResultsClassifier.Types (LanguageConfiguration, ParseConfigError(..))
-import Unused.Util (readIfFileExists)
+import Unused.Util (safeReadFile)
 
 loadConfig :: IO (Either String [LanguageConfiguration])
 loadConfig = do
@@ -39,10 +39,10 @@ loadAllConfigurations = do
 
 loadConfigFromFile :: String -> IO (Either ParseConfigError [LanguageConfiguration])
 loadConfigFromFile path = do
-    file <- fmap C.pack <$> readIfFileExists path
+    file <- fmap C.pack <$> safeReadFile path
     return $ case file of
-        Nothing -> Right []
-        Just body -> addSourceToLeft path $ Y.decodeEither body
+        Left _ -> Right []
+        Right body -> addSourceToLeft path $ Y.decodeEither body
 
 addSourceToLeft :: String -> Either String c -> Either ParseConfigError c
 addSourceToLeft source = B.first (ParseConfigError source)

src/Unused/Util.hs

@@ -2,11 +2,11 @@ module Unused.Util
     ( groupBy
     , stringToInt
     , safeHead
-    , readIfFileExists
+    , safeReadFile
     ) where
 
-import System.Directory (doesFileExist)
 import Control.Arrow ((&&&))
+import qualified Control.Exception as E
 import qualified Data.List as L
 import Data.Function
 import Data.Char (digitToInt, isDigit)
@@ -27,10 +27,5 @@ stringToInt xs
   where
     loop = foldl (\acc x -> acc * 10 + digitToInt x)
 
-readIfFileExists :: String -> IO (Maybe String)
-readIfFileExists path = do
-    exists <- doesFileExist path
-
-    if exists
-        then Just <$> readFile path
-        else return Nothing
+safeReadFile :: FilePath -> IO (Either E.IOException String)
+safeReadFile = E.try . readFile