Reflect ulidtko's suggestions

Also adds a comment to explain the purpose of the random suffix

Author: mpilgrem

src/Stack/Build/Execute.hs

@@ -1928,12 +1928,15 @@ singleTest topts testsToRun ac ee task installedMap = do
                 -- see e.g. https://github.com/doctest/issues/119
                 -- also we set HASKELL_DIST_DIR to a package dist directory so
                 -- doctest will be able to load modules autogenerated by Cabal
-                ghcEnvRandomId <- liftIO (randomIO :: IO Int)
                 let setEnv f pc = modifyEnvVars pc $ \envVars ->
                       Map.insert "HASKELL_DIST_DIR" (T.pack $ toFilePath buildDir) $
                       Map.insert "GHC_ENVIRONMENT" (T.pack f) envVars
-                    fp = toFilePath (eeTempDir ee </> testGhcEnvRelFile) <> show ghcEnvRandomId
-                    snapDBPath = toFilePathNoTrailingSep (bcoSnapDB $ eeBaseConfigOpts ee)
+                    fp' = eeTempDir ee </> testGhcEnvRelFile
+                -- Add a random suffix to avoid conflicts between parallel jobs
+                -- See https://github.com/commercialhaskell/stack/issues/5024
+                randomSuffix <- ("-"<>) . show <$> liftIO (randomIO :: IO Int)
+                fp <- toFilePath <$> addExtension randomSuffix fp'
+                let snapDBPath = toFilePathNoTrailingSep (bcoSnapDB $ eeBaseConfigOpts ee)
                     localDBPath = toFilePathNoTrailingSep (bcoLocalDB $ eeBaseConfigOpts ee)
                     ghcEnv =
                         "clear-package-db\n" <>