Add readVerbatimFile

- Have readVerbatimFile read contents strictly

cabal-testsuite/PackageTests/ProjectImport/ParseErrorProvenance/cabal.test.hs

@@ -3,14 +3,10 @@ import System.Directory
 
 main = cabalTest . recordMode RecordMarked $ do
   let log = recordHeader . pure
-  cwd <- liftIO getCurrentDirectory
-  env <- getTestEnv
-  let testDir = testCurrentDir env
-  liftIO . putStrLn $ "Current working directory: " ++ cwd
-  msg <- liftIO . readFile $ testDir </> "msg.txt"
 
   outElse <- fails $ cabal' "v2-build" [ "all", "--dry-run", "--project-file=else.project" ]
 
+  msg <- readVerbatimFile "msg.expect.txt"
   let msgSingle = lineBreaksToSpaces msg
 
   log "Multiline string marking:"

cabal-testsuite/src/Test/Cabal/Prelude.hs

@@ -1271,3 +1271,32 @@ findDependencyInStore pkgName = do
                       [] -> error $ "Could not find " <> pkgName' <> " when searching for " <> pkgName' <> " in\n" <> show packageDirs
                       (dir:_) -> dir
       pure (storeDir </> storeDirForGhcVersion </> libDir)
+
+-- | It can easier to paste expected output verbatim into a text file,
+-- especially if it is a multiline string, rather than encoding it as a multiline
+-- string in Haskell source code.
+--
+-- With `-XMultilineStrings` triple quoted strings with line breaks will be
+-- easier to write in source code but then this will only work with ghc-9.12.1
+-- and later, in which case we'd have to use CPP with test scripts to support
+-- older GHC versions. CPP doesn't play nicely with multiline strings using
+-- string gaps. None of our test script import other modules. That might be a
+-- way to avoid CPP in a module that uses multiline strings.
+--
+-- In summary, it is easier to read multiline strings from a file. That is what
+-- this function facilitates.
+--
+-- The contents of the file are read strictly to avoid problems seen on Windows
+-- deleting the file:
+--
+-- > cabal.test.hs:
+-- > C:\Users\<username>\AppData\Local\Temp\cabal-testsuite-8376\errors.expect.txt:
+-- > removePathForcibly:DeleteFile
+-- > "\\\\?\\C:\\Users\\<username>\\AppData\\Local\\Temp\\cabal-testsuite-8376\\errors.expect.txt":
+-- > permission denied (The process cannot access the file because it is being
+-- > used by another process.)
+readVerbatimFile :: FilePath -> TestM String
+readVerbatimFile filename = do
+  testDir <- testCurrentDir <$> getTestEnv
+  s <- liftIO . readFile $ testDir </> filename
+  length s `seq` return s