Use a separate finder cache for each typecheck call (#148)

* Use a separate finder cache for each typecheck call

On a large DAML project, we occasionally saw error about missing
modules during typechecking during concurrent compilations. This was
caused by the fact that we modified the IORef in the HscEnv which is
shared between concurrent compilations.

Author: cocreature

azure-pipelines.yml

@@ -35,7 +35,9 @@ jobs:
         displayName: "HLint via ./fmt.sh"
       - bash: |
           sudo apt-get install -y g++ gcc libc6-dev libffi-dev libgmp-dev make zlib1g-dev
-          curl -sSL https://get.haskellstack.org/ | sh
+          if ! which stack >/dev/null 2>&1; then
+              curl -sSL https://get.haskellstack.org/ | sh
+          fi
         displayName: 'Install Stack'
       - bash: stack setup
         displayName: 'stack setup'
@@ -79,7 +81,9 @@ jobs:
         displayName: "HLint via ./fmt.sh"
       - bash: |
           sudo apt-get install -y g++ gcc libc6-dev libffi-dev libgmp-dev make zlib1g-dev
-          curl -sSL https://get.haskellstack.org/ | sh
+          if ! which stack >/dev/null 2>&1; then
+              curl -sSL https://get.haskellstack.org/ | sh
+          fi
         displayName: 'Install Stack'
       - bash: stack setup --stack-yaml=stack84.yaml
         displayName: 'stack setup --stack-yaml=stack84.yaml'
@@ -123,7 +127,9 @@ jobs:
         displayName: "HLint via ./fmt.sh"
       - bash: |
           sudo apt-get install -y g++ gcc libc6-dev libffi-dev libgmp-dev make zlib1g-dev
-          curl -sSL https://get.haskellstack.org/ | sh
+          if ! which stack >/dev/null 2>&1; then
+              curl -sSL https://get.haskellstack.org/ | sh
+          fi
         displayName: 'Install Stack'
       - bash: stack setup --stack-yaml=stack88.yaml
         displayName: 'stack setup --stack-yaml=stack88.yaml'
@@ -167,7 +173,9 @@ jobs:
         displayName: "HLint via ./fmt.sh"
       - bash: |
           sudo apt-get install -y g++ gcc libc6-dev libffi-dev libgmp-dev make zlib1g-dev
-          curl -sSL https://get.haskellstack.org/ | sh
+          if ! which stack >/dev/null 2>&1; then
+              curl -sSL https://get.haskellstack.org/ | sh
+          fi
         displayName: 'Install Stack'
       - bash: stack setup --stack-yaml=stack-ghc-lib.yaml
         displayName: 'stack setup --stack-yaml=stack-ghc-lib.yaml'

src/Development/IDE/Core/Compile.hs

@@ -196,9 +196,15 @@ setupEnv tmsIn = do
     -- by putting them in the finder cache.
     let ims  = map (InstalledModule (thisInstalledUnitId $ hsc_dflags session) . moduleName . ms_mod) mss
         ifrs = zipWith (\ms -> InstalledFound (ms_location ms)) mss ims
-    liftIO $ modifyIORef (hsc_FC session) $ \fc ->
-        foldl' (\fc (im, ifr) -> GHC.extendInstalledModuleEnv fc im ifr) fc
-            $ zip ims ifrs
+    -- We have to create a new IORef here instead of modifying the existing IORef as
+    -- it is shared between concurrent compilations.
+    prevFinderCache <- liftIO $ readIORef $ hsc_FC session
+    let newFinderCache =
+            foldl'
+                (\fc (im, ifr) -> GHC.extendInstalledModuleEnv fc im ifr) prevFinderCache
+                $ zip ims ifrs
+    newFinderCacheVar <- liftIO $ newIORef $! newFinderCache
+    modifySession $ \s -> s { hsc_FC = newFinderCacheVar }
 
     -- load dependent modules, which must be in topological order.
     mapM_ loadModuleHome tms