From e8902f32bd73d6728c917722143187f3e92c9bae Mon Sep 17 00:00:00 2001
From: Pepe Iborra <pepeiborra@gmail.com>
Date: Sun, 18 Apr 2021 13:41:44 +0100
Subject: [PATCH] Handle LSP Initialize request synchronously

The LSP spec. has strong restrictions around what messages are allowed
in between the Initialize request and response. Handling the message
synchronously goes a long way towards satisfying those invariants.
---
 ghcide/src/Development/IDE/LSP/LanguageServer.hs |  4 +++-
 ghcide/src/Development/IDE/LSP/Server.hs         | 10 ++++++++--
 2 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/ghcide/src/Development/IDE/LSP/LanguageServer.hs b/ghcide/src/Development/IDE/LSP/LanguageServer.hs
index 9fcc520db2..556e6351af 100644
--- a/ghcide/src/Development/IDE/LSP/LanguageServer.hs
+++ b/ghcide/src/Development/IDE/LSP/LanguageServer.hs
@@ -154,7 +154,9 @@ runLanguageServer options inH outH getHieDbLoc defaultConfig onConfigurationChan
                         logError (ideLogger ide) $ T.pack $
                           "Unexpected exception on notification, please report!\n" ++
                           "Exception: " ++ show e
-                    ReactorRequest _id act k -> void $ async $
+                    ReactorRequestSync _id act k ->
+                      checkCancelled ide clearReqId waitForCancel _id act k
+                    ReactorRequestAsync _id act k -> void $ async $
                       checkCancelled ide clearReqId waitForCancel _id act k
             pure $ Right (env,ide)
 
diff --git a/ghcide/src/Development/IDE/LSP/Server.hs b/ghcide/src/Development/IDE/LSP/Server.hs
index f2edc004db..c72532cc89 100644
--- a/ghcide/src/Development/IDE/LSP/Server.hs
+++ b/ghcide/src/Development/IDE/LSP/Server.hs
@@ -26,7 +26,8 @@ import           UnliftIO.Chan
 
 data ReactorMessage
   = ReactorNotification (IO ())
-  | ReactorRequest SomeLspId (IO ()) (ResponseError -> IO ())
+  | ReactorRequestSync SomeLspId (IO ()) (ResponseError -> IO ())
+  | ReactorRequestAsync SomeLspId (IO ()) (ResponseError -> IO ())
 
 type ReactorChan = Chan ReactorMessage
 type ServerM c = ReaderT (ReactorChan, IdeState) (LspM c)
@@ -43,7 +44,12 @@ requestHandler m k = LSP.requestHandler m $ \RequestMessage{_method,_id,_params}
       trace x = otTracedHandler "Request" (show _method) $ \sp -> do
         traceWithSpan sp _params
         x
-  writeChan chan $ ReactorRequest (SomeLspId _id) (trace $ LSP.runLspT env $ resp' =<< k ide _params) (LSP.runLspT env . resp' . Left)
+      item = case _method of
+          SInitialize ->
+            ReactorRequestSync  (SomeLspId _id) (trace $ LSP.runLspT env $ resp' =<< k ide _params) (LSP.runLspT env . resp' . Left)
+          _other ->
+            ReactorRequestAsync (SomeLspId _id) (trace $ LSP.runLspT env $ resp' =<< k ide _params) (LSP.runLspT env . resp' . Left)
+  writeChan chan item
 
 notificationHandler
   :: forall (m :: Method FromClient Notification) c. (HasTracing (MessageParams m)) =>