Improve isolation of build artefacts of test runs

Even though we copy test files into temporary directories, we used to
reuse the same cache directory for build artefacts, hiedb and
compilation artefacts. While there is practially no chance this causes
any issues for the test runs themselves, it litters the cache directory
with a lot of files. So, we create one main directory in the temporary
directory, and generate all caches and in there. This makes it trivial
to delete all test caches, without risking deleting the cache that is
still used.

hls-test-utils/hls-test-utils.cabal

@@ -46,6 +46,7 @@ library
     , lens
     , lsp-test                ^>=0.17
     , lsp-types               ^>=2.1
+    , safe-exceptions
     , tasty
     , tasty-expected-failure
     , tasty-golden

hls-test-utils/src/Test/Hls.hs

@@ -63,9 +63,9 @@ where
 import           Control.Applicative.Combinators
 import           Control.Concurrent.Async           (async, cancel, wait)
 import           Control.Concurrent.Extra
-import           Control.Exception.Base
+import           Control.Exception.Safe
 import           Control.Lens.Extras                (is)
-import           Control.Monad                      (guard, unless, void)
+import           Control.Monad                      (guard, unless, void, when)
 import           Control.Monad.Extra                (forM)
 import           Control.Monad.IO.Class
 import           Data.Aeson                         (Result (Success),
@@ -106,11 +106,13 @@ import           Language.LSP.Protocol.Message
 import           Language.LSP.Protocol.Types        hiding (Null)
 import           Language.LSP.Test
 import           Prelude                            hiding (log)
-import           System.Directory                   (getCurrentDirectory,
+import           System.Directory                   (createDirectoryIfMissing,
+                                                     getCurrentDirectory,
+                                                     getTemporaryDirectory,
                                                      setCurrentDirectory)
-import           System.Environment                 (lookupEnv)
+import           System.Environment                 (lookupEnv, setEnv)
 import           System.FilePath
-import           System.IO.Extra                    (newTempDir, withTempDir)
+import           System.IO.Extra                    (newTempDirWithin)
 import           System.IO.Unsafe                   (unsafePerformIO)
 import           System.Process.Extra               (createPipe)
 import           System.Time.Extra
@@ -423,22 +425,24 @@ runSessionWithServerInTmpDir' ::
   Session a ->
   IO a
 runSessionWithServerInTmpDir' plugins conf sessConf caps tree act = withLock lockForTempDirs $ do
+  testRoot <- setupTestEnvironment
   (recorder, _) <- initialiseTestRecorder
     ["LSP_TEST_LOG_STDERR", "HLS_TEST_HARNESS_STDERR", "HLS_TEST_LOG_STDERR"]
 
   -- Do not clean up the temporary directory if this variable is set to anything but '0'.
   -- Aids debugging.
   cleanupTempDir <- lookupEnv "HLS_TEST_HARNESS_NO_TESTDIR_CLEANUP"
-  let runTestInDir = case cleanupTempDir of
+  let runTestInDir action = case cleanupTempDir of
         Just val
-          | val /= "0" -> \action -> do
-            (tempDir, _) <- newTempDir
+          | val /= "0" -> do
+            (tempDir, _) <- newTempDirWithin testRoot
             a <- action tempDir
             logWith recorder Debug LogNoCleanup
             pure a
 
-        _ -> \action -> do
-          a <- withTempDir action
+        _ -> do
+          (tempDir, cleanup) <- newTempDirWithin testRoot
+          a <- action tempDir `finally` cleanup
           logWith recorder Debug LogCleanup
           pure a
 
@@ -447,6 +451,25 @@ runSessionWithServerInTmpDir' plugins conf sessConf caps tree act = withLock loc
     _fs <- FS.materialiseVFT tmpDir tree
     runSessionWithServer' plugins conf sessConf caps tmpDir act
 
+-- | Setup the test environment for isolated tests.
+--
+-- This creates a directory in the temporary directory that will be
+-- reused for running isolated tests.
+-- It returns the root to the testing directory that tests should use.
+-- This directory is not fully cleaned between reruns.
+-- However, it is totally safe to delete the directory between runs.
+--
+-- Additionally, this overwrites the 'XDG_CACHE_HOME' variable to isolate
+-- the tests from existing caches. 'hie-bios' and 'ghcide' honour the
+-- 'XDG_CACHE_HOME' environment variable and generate their caches there.
+setupTestEnvironment :: IO FilePath
+setupTestEnvironment = do
+  tmpDirRoot <- getTemporaryDirectory
+  let testRoot = tmpDirRoot </> "hls-test-root"
+      testCacheDir = testRoot </> ".cache"
+  createDirectoryIfMissing True testCacheDir
+  setEnv "XDG_CACHE_HOME" testCacheDir
+  pure testRoot
 goldenWithHaskellDocFormatter
   :: Pretty b
   => Config