Make the paths stored in precompiled-cache Stack-root relative

Because these paths always point inside the Stack root, better
make them relative to the Stack root, making the Stack root
more easily relocatable, in the future.

This change is backward-compatible.

src/Stack/Build/Cache.hs

@@ -66,6 +66,7 @@ import           Path
 import           Path.IO
 import           Stack.Constants
 import           Stack.Types
+import qualified System.FilePath as FilePath
 
 -- | Directory containing files to mark an executable as installed
 exeInstalledDir :: (MonadReader env m, HasEnvConfig env, MonadThrow m)
@@ -306,15 +307,19 @@ writePrecompiledCache :: (MonadThrow m, MonadReader env m, HasEnvConfig env, Mon
 writePrecompiledCache baseConfigOpts pkgident copts depIDs mghcPkgId exes = do
     (file, _) <- precompiledCacheFile pkgident copts depIDs
     ensureDir (parent file)
+    ec <- asks getEnvConfig
+    let stackRootRelative = makeRelative (getStackRoot ec)
     mlibpath <-
         case mghcPkgId of
             Executable _ -> return Nothing
             Library _ ipid -> liftM Just $ do
                 ipid' <- parseRelFile $ ghcPkgIdString ipid ++ ".conf"
-                return $ toFilePath $ bcoSnapDB baseConfigOpts </> ipid'
+                relPath <- stackRootRelative $ bcoSnapDB baseConfigOpts </> ipid'
+                return $ toFilePath $ relPath
     exes' <- forM (Set.toList exes) $ \exe -> do
         name <- parseRelFile $ T.unpack exe
-        return $ toFilePath $ bcoSnapInstallRoot baseConfigOpts </> bindirSuffix </> name
+        relPath <- stackRootRelative $ bcoSnapInstallRoot baseConfigOpts </> bindirSuffix </> name
+        return $ toFilePath $ relPath
     taggedEncodeFile file PrecompiledCache
         { pcLibrary = mlibpath
         , pcExes = exes'
@@ -328,14 +333,25 @@ readPrecompiledCache :: (MonadThrow m, MonadReader env m, HasEnvConfig env, Mona
                      -> Set GhcPkgId -- ^ dependencies
                      -> m (Maybe PrecompiledCache)
 readPrecompiledCache pkgident copts depIDs = do
+    ec <- asks getEnvConfig
+    let toAbsPath path = do
+          if FilePath.isAbsolute path
+              then path -- Only older version store absolute path
+              else toFilePath (getStackRoot ec) FilePath.</> path
+    let toAbsPC pc =
+            PrecompiledCache
+                  { pcLibrary = fmap toAbsPath (pcLibrary pc)
+                  , pcExes = map toAbsPath (pcExes pc)
+                  }
+
     (file, getOldFile) <- precompiledCacheFile pkgident copts depIDs
     mres <- decodeFileMaybe file
     case mres of
-        Just res -> return (Just res)
+        Just res -> return (Just $ toAbsPC res)
         Nothing -> do
             -- Fallback on trying the old binary format.
             oldFile <- getOldFile
-            mpc <- binaryDecodeFileOrFailDeep oldFile
+            mpc <- fmap (fmap toAbsPC) $ binaryDecodeFileOrFailDeep oldFile
             -- Write out file in new format. Keep old file around for
             -- the benefit of older stack versions.
             forM_ mpc (taggedEncodeFile file)