Add more diagnostics to isolateEnvironment.setEnvs

tests/Util.hs

@@ -144,6 +144,17 @@ withNewDirectory keep dir action = do
   where cleanup dir' | keep      = return ()
                      | otherwise = removePathForcibly dir'
 
+diffAscAssoc
+  :: (Ord a, Eq b) => (b -> c) -> [(a, b)] -> [(a, b)] -> [(a, Either c c)]
+diffAscAssoc _ [] [] = []
+diffAscAssoc f ((k, v) : kvs) [] = (k, Left (f v)) : diffAscAssoc f kvs []
+diffAscAssoc f [] ((k, v) : kvs) = (k, Right (f v)) : diffAscAssoc f [] kvs
+diffAscAssoc f ((k, v) : kvs) ((k', v') : kvs')
+  | k < k' = (k, Left (f v)) : diffAscAssoc f kvs ((k', v') : kvs')
+  | k > k' = (k', Right (f v')) : diffAscAssoc f ((k, v) : kvs) kvs'
+  | v /= v' = (k, Left (f v)) : (k', Right (f v')) : diffAscAssoc f kvs kvs'
+  | otherwise = diffAscAssoc f kvs kvs'
+
 isolateEnvironment :: IO a -> IO a
 isolateEnvironment = bracket getEnvs setEnvs . const
   where
@@ -153,8 +164,9 @@ isolateEnvironment = bracket getEnvs setEnvs . const
       updateEnvs current target
       new <- getEnvs
       when (target /= new) $ do
-        -- Environment variables may be sensitive, so don't log them.
-        throwIO (userError "isolateEnvironment.setEnvs failed")
+        let diff = diffAscAssoc length target new
+        -- Environment variables may be sensitive, so don't log their values.
+        throwIO (userError ("isolateEnvironment.setEnvs failed" <> show diff))
     updateEnvs kvs1@((k1, v1) : kvs1') kvs2@((k2, v2) : kvs2') =
       case compare k1 k2 of
         LT -> unsetEnv k1 *> updateEnvs kvs1' kvs2