tests: make UserConfig unit tests concurrency-safe with unique tempiles

cabal-install/src/Distribution/Client/Config.hs

@@ -114,6 +114,7 @@ import Distribution.Utils.NubList
   )
 
 import qualified Data.ByteString as BS
+import qualified Data.ByteString.Lazy as LBS
 import qualified Data.Map as M
 import Distribution.Client.Errors
 import Distribution.Client.HttpUtils
@@ -208,6 +209,7 @@ import Distribution.Simple.Utils
   , notice
   , toUTF8BS
   , warn
+  , writeFileAtomic
   )
 import Distribution.Solver.Types.ConstraintSource
 import Distribution.Utils.Path (getSymbolicPath, unsafeMakeSymbolicPath)
@@ -233,7 +235,6 @@ import System.Directory
 import System.FilePath
   ( normalise
   , takeDirectory
-  , (<.>)
   , (</>)
   )
 import System.IO.Error
@@ -1078,11 +1079,10 @@ createDefaultConfigFile verbosity extraLines filePath = do
 
 writeConfigFile :: FilePath -> SavedConfig -> SavedConfig -> IO ()
 writeConfigFile file comments vals = do
-  let tmpFile = file <.> "tmp"
   createDirectoryIfMissing True (takeDirectory file)
-  writeFile tmpFile $
-    explanation ++ showConfigWithComments comments vals ++ "\n"
-  renameFile tmpFile file
+  writeFileAtomic file $
+    LBS.fromStrict . toUTF8BS $
+      explanation ++ showConfigWithComments comments vals ++ "\n"
   where
     explanation =
       unlines

cabal-install/tests/UnitTests/Distribution/Client/UserConfig.hs

@@ -13,6 +13,7 @@ import System.Directory
   , getTemporaryDirectory
   )
 import System.FilePath ((</>))
+import System.IO (hClose, openTempFile)
 
 import Test.Tasty
 import Test.Tasty.HUnit
@@ -99,7 +100,11 @@ bracketTest =
   bracket testSetup testTearDown
   where
     testSetup :: IO FilePath
-    testSetup = fmap (</> "test-user-config") getCurrentDirectory
+    testSetup = do
+      cwd <- getCurrentDirectory
+      (configFile, h) <- openTempFile cwd "test-user-config"
+      hClose h
+      pure configFile
 
     testTearDown :: FilePath -> IO ()
     testTearDown configFile =