Improve migration strategy for the XDG compliance change to the GHC application

directory.

We want to always use the old path (~/.ghc/..) if it exists.
But we never want to create the old path.
This ensures that the migration can eventually be completed once older GHC
versions are no longer in circulation.

Fixes #20684, #20669, #20660

1 files changed, 19 insertions, 3 deletions

diff --git a/compiler/GHC/Driver/Session.hs b/compiler/GHC/Driver/Session.hs
index 2e7574837f..b17b0c8d3d 100644
--- a/compiler/GHC/Driver/Session.hs
+++ b/compiler/GHC/Driver/Session.hs
@@ -1,6 +1,7 @@
 {-# LANGUAGE CPP #-}
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE RankNTypes #-}
+{-# LANGUAGE LambdaCase #-}
 
 -------------------------------------------------------------------------------
 --
@@ -887,13 +888,28 @@ opt_i                 :: DynFlags -> [String]
 opt_i dflags= toolSettings_opt_i $ toolSettings dflags
 
 -- | The directory for this version of ghc in the user's app directory
--- (typically something like @~/.ghc/x86_64-linux-7.6.3@)
+-- The appdir used to be in ~/.ghc but to respect the XDG specification
+-- we want to move it under $XDG_DATA_HOME/
+-- However, old tooling (like cabal) might still write package environments
+-- to the old directory, so we prefer that if a subdirectory of ~/.ghc
+-- with the correct target and GHC version suffix exists.
 --
+-- i.e. if ~/.ghc/$UNIQUE_SUBDIR exists we use that
+-- otherwise we use $XDG_DATA_HOME/$UNIQUE_SUBDIR
+--
+-- UNIQUE_SUBDIR is typically a combination of the target platform and GHC version
 versionedAppDir :: String -> ArchOS -> MaybeT IO FilePath
 versionedAppDir appname platform = do
   -- Make sure we handle the case the HOME isn't set (see #11678)
-  appdir <- tryMaybeT $ getXdgDirectory XdgData appname
-  return $ appdir </> versionedFilePath platform
+  -- We need to fallback to the old scheme if the subdirectory exists.
+  msum $ map (checkIfExists <=< fmap (</> versionedFilePath platform))
+       [ tryMaybeT $ getAppUserDataDirectory appname  -- this is ~/.ghc/
+       , tryMaybeT $ getXdgDirectory XdgData appname -- this is $XDG_DATA_HOME/
+       ]
+  where
+    checkIfExists dir = tryMaybeT (doesDirectoryExist dir) >>= \case
+      True -> pure dir
+      False -> MaybeT (pure Nothing)
 
 versionedFilePath :: ArchOS -> FilePath
 versionedFilePath platform = uniqueSubdir platform