Decouple the session loader into reader and writer over the cache

ghcide-test/data/multi-unit/a-1.0.0-inplace

@@ -16,6 +16,3 @@ base
 text
 -XHaskell98
 A
-+RTS
--A32M
--RTS

ghcide-test/data/multi-unit/c-1.0.0-inplace

@@ -17,5 +17,3 @@ a-1.0.0-inplace
 base
 -XHaskell98
 C
-+RTS
--A32M

ghcide/ghcide.cabal

@@ -179,7 +179,9 @@ library
     Development.IDE.Plugin.Test
     Development.IDE.Plugin.TypeLenses
     Development.IDE.Session
+    Development.IDE.Session.Dependency
     Development.IDE.Session.Diagnostics
+    Development.IDE.Session.Ghc
     Development.IDE.Session.Implicit
     Development.IDE.Spans.AtPoint
     Development.IDE.Spans.Common
@@ -202,6 +204,7 @@ library
     Development.IDE.GHC.CPP
     Development.IDE.GHC.Warnings
     Development.IDE.Types.Action
+    Development.IDE.Session.OrderedSet
 
   if flag(pedantic)
     ghc-options:

ghcide/session-loader/Development/IDE/Session/Dependency.hs

@@ -0,0 +1,35 @@
+module Development.IDE.Session.Dependency where
+
+import           Control.Exception.Safe as Safe
+import           Data.Either.Extra
+import qualified Data.Map.Strict        as Map
+import           Data.Time.Clock
+import           System.Directory
+
+type DependencyInfo = Map.Map FilePath (Maybe UTCTime)
+
+-- | Check if any dependency has been modified lately.
+checkDependencyInfo :: DependencyInfo -> IO Bool
+checkDependencyInfo old_di = do
+  di <- getDependencyInfo (Map.keys old_di)
+  return (di == old_di)
+
+-- Note [Multi Cradle Dependency Info]
+-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+-- Why do we implement our own file modification tracking here?
+-- The primary reason is that the custom caching logic is quite complicated and going into shake
+-- adds even more complexity and more indirection. I did try for about 5 hours to work out how to
+-- use shake rules rather than IO but eventually gave up.
+
+-- | Computes a mapping from a filepath to its latest modification date.
+-- See Note [Multi Cradle Dependency Info] why we do this ourselves instead
+-- of letting shake take care of it.
+getDependencyInfo :: [FilePath] -> IO DependencyInfo
+getDependencyInfo fs = Map.fromList <$> mapM do_one fs
+
+  where
+    safeTryIO :: IO a -> IO (Either IOException a)
+    safeTryIO = Safe.try
+
+    do_one :: FilePath -> IO (FilePath, Maybe UTCTime)
+    do_one fp = (fp,) . eitherToMaybe <$> safeTryIO (getModificationTime fp)