From 4413e08252092b760effadaefce5e44dd91fdfd3 Mon Sep 17 00:00:00 2001
From: Curtis Chin Jen Sem <csochinjensem@gmail.com>
Date: Fri, 14 Aug 2026 01:29:13 +0200
Subject: [PATCH] Use a unique id for each file watcher registration

---
 ghcide-test/exe/WatchedFileTests.hs          | 19 +++++++++++++++++++
 ghcide/src/Development/IDE/Core/FileStore.hs | 19 +++++++++++++++----
 2 files changed, 34 insertions(+), 4 deletions(-)

diff --git a/ghcide-test/exe/WatchedFileTests.hs b/ghcide-test/exe/WatchedFileTests.hs
index 18299109f0..be61de81fc 100644
--- a/ghcide-test/exe/WatchedFileTests.hs
+++ b/ghcide-test/exe/WatchedFileTests.hs
@@ -9,6 +9,7 @@ import           Config                          (mkIdeTestFs,
 import           Control.Applicative.Combinators
 import           Control.Monad.IO.Class          (liftIO)
 import qualified Data.Aeson                      as A
+import           Data.List                       (nub)
 import qualified Data.Text                       as T
 import qualified Data.Text.IO                    as T
 import           Development.IDE.Plugin.Test     (WaitForIdeRuleResult (..))
@@ -51,6 +52,15 @@ tests = testGroup "watched files"
         -- Expect 2 subscriptions: one for all .hs files and one for the hie.yaml cradle
         liftIO $ length watchedFileRegs @?= 2
 
+    , testWithDummyPluginEmpty' "distinct registration ids" $ \sessionDir -> do
+        liftIO $ atomicFileWriteString (sessionDir </> "hie.yaml") "cradle: {direct: {arguments: [\"-isrc\", \"A\", \"WatchedFilesMissingModule\"]}}"
+        _doc <- createDoc "A.hs" "haskell" "{-#LANGUAGE NoImplicitPrelude #-}\nmodule A where\nimport WatchedFilesMissingModule"
+        setIgnoringRegistrationRequests False
+        ids <- getWatchedFilesRegistrationIdsUntil SMethod_TextDocumentPublishDiagnostics
+
+        liftIO $ length ids @?= 2
+        liftIO $ assertEqual "registration ids must be distinct" (nub ids) ids
+
     -- TODO add a test for didChangeWorkspaceFolder
     ]
   , testGroup "Changes"
@@ -190,6 +200,15 @@ tests = testGroup "watched files"
     ]
   ]
 
+getWatchedFilesRegistrationIdsUntil :: forall m. SServerMethod m -> Session [T.Text]
+getWatchedFilesRegistrationIdsUntil m = do
+      msgs <- manyTill (Just <$> message SMethod_ClientRegisterCapability <|> Nothing <$ anyMessage) (message m)
+      return
+            [ _id
+            | Just TRequestMessage{_params = RegistrationParams regs} <- msgs
+            , Registration _id "workspace/didChangeWatchedFiles" _ <- regs
+            ]
+
 getWatchedFilesSubscriptionsUntil :: forall m. SServerMethod m -> Session [DidChangeWatchedFilesRegistrationOptions]
 getWatchedFilesSubscriptionsUntil m = do
       msgs <- manyTill (Just <$> message SMethod_ClientRegisterCapability <|> Nothing <$ anyMessage) (message m)
diff --git a/ghcide/src/Development/IDE/Core/FileStore.hs b/ghcide/src/Development/IDE/Core/FileStore.hs
index 7d253131d6..1bdd55d7f3 100644
--- a/ghcide/src/Development/IDE/Core/FileStore.hs
+++ b/ghcide/src/Development/IDE/Core/FileStore.hs
@@ -307,6 +307,18 @@ setSomethingModified vfs state reason actionBetweenSession = do
     atomically $ writeTaskQueue (indexQueue $ hiedbWriter $ shakeExtras state) (\withHieDb -> withHieDb deleteMissingRealFiles)
     void $ restartShakeSession (shakeExtras state) vfs reason [] actionBetweenSession
 
+{- Note [Unique file watcher registration ids]
+   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+We send more than one 'client/registerCapability' for watched files: one for
+the global source globs, plus one foreach cradle dependency that the globs
+do not already cover, see 'addWatchedFileRule'. These are logically distinct
+registrations, so they must not share an id.
+
+LSP leaves the meaning of a repeated id undefined. emacs' eglot unregisters the
+old watches before re-registering, while neovim and vscode accumulate. We derive
+the id from the globs to keep them distinct and idempotent, while supporting
+potentially de-registering in the future.
+-}
 registerFileWatches :: [String] -> LSP.LspT Config IO Bool
 registerFileWatches globs = do
       watchSupported <- isWatchSupported
@@ -314,12 +326,11 @@ registerFileWatches globs = do
       then do
         let
           regParams    = LSP.RegistrationParams  [toUntypedRegistration registration]
-          -- The registration ID is arbitrary and is only used in case we want to deregister (which we won't).
-          -- We could also use something like a random UUID, as some other servers do, but this works for
-          -- our purposes.
-          registration = LSP.TRegistration { _id ="globalFileWatches"
+          -- See Note [Unique file watcher registration ids]
+          registration = LSP.TRegistration { _id = registrationId
                                            , _method = LSP.SMethod_WorkspaceDidChangeWatchedFiles
                                            , _registerOptions = Just regOptions}
+          registrationId = "hls-file-watches:" <> Text.intercalate "," (map Text.pack globs)
           regOptions =
             DidChangeWatchedFilesRegistrationOptions { _watchers = watchers }
           -- See Note [File existence cache and LSP file watchers] for why this exists, and the choice of watch kind