Do correct path checking in precompiled cache.

The commit ed9ccc0 changed the behavior
of the precompiled cache to store relative instead of absolute paths.
That seems to have broken the ability to properly check if a library
exists, thereby defeating precompiled caching.

The issue that discovered this is #3431, and ascribed this to extensible
snapshots. I _think_, though I'm not 100% certain, that this bug reaches
much farther than extensible snapshots, and in fact breaks all
precompiled caches.

ChangeLog.md

@@ -148,6 +148,8 @@ Bug fixes:
 * `stack build --only-dependencies` no longer builds local project packages
   that are depended on. See
   [#3476](https://github.com/commercialhaskell/stack/issues/3476).
+* Properly handle relative paths stored in the precompiled cache files. See
+  [#3431](https://github.com/commercialhaskell/stack/issues/3431).
 
 
 ## 1.5.1

src/Stack/Build/Cache.hs

@@ -34,6 +34,7 @@ module Stack.Build.Cache
 
 import           Stack.Prelude
 import           Crypto.Hash (hashWith, SHA256(..))
+import           Control.Monad.Trans.Maybe
 import qualified Data.ByteArray as Mem (convert)
 import qualified Data.ByteString.Base64.URL as B64URL
 import qualified Data.ByteString as B
@@ -55,6 +56,7 @@ import           Stack.Types.GhcPkgId
 import           Stack.Types.Package
 import           Stack.Types.PackageIdentifier
 import           Stack.Types.Version
+import qualified System.FilePath as FP
 
 -- | Directory containing files to mark an executable as installed
 exeInstalledDir :: (MonadReader env m, HasEnvConfig env, MonadThrow m)
@@ -313,11 +315,27 @@ writePrecompiledCache baseConfigOpts loc copts depIDs mghcPkgId exes = do
 
 -- | Check the cache for a precompiled package matching the given
 -- configuration.
-readPrecompiledCache :: (MonadThrow m, MonadReader env m, HasEnvConfig env, MonadUnliftIO m, MonadLogger m)
+readPrecompiledCache :: forall env. HasEnvConfig env
                      => PackageLocationIndex FilePath -- ^ target package
                      -> ConfigureOpts
                      -> Set GhcPkgId -- ^ dependencies
-                     -> m (Maybe PrecompiledCache)
-readPrecompiledCache loc copts depIDs =
-    precompiledCacheFile loc copts depIDs >>=
-    maybe (return Nothing) $(versionedDecodeFile precompiledCacheVC)
+                     -> RIO env (Maybe PrecompiledCache)
+readPrecompiledCache loc copts depIDs = runMaybeT $
+    MaybeT (precompiledCacheFile loc copts depIDs) >>=
+    MaybeT . $(versionedDecodeFile precompiledCacheVC) >>=
+    lift . mkAbs
+  where
+    -- Since commit ed9ccc08f327bad68dd2d09a1851ce0d055c0422,
+    -- pcLibrary paths are stored as relative to the stack
+    -- root. Therefore, we need to prepend the stack root when
+    -- checking that the file exists. For the older cached paths, the
+    -- file will contain an absolute path, which will make `stackRoot
+    -- </>` a no-op.
+    mkAbs :: PrecompiledCache -> RIO env PrecompiledCache
+    mkAbs pc0 = do
+      stackRoot <- view stackRootL
+      let mkAbs' = (toFilePath stackRoot FP.</>)
+      return PrecompiledCache
+        { pcLibrary = mkAbs' <$> pcLibrary pc0
+        , pcExes = mkAbs' <$> pcExes pc0
+        }