Diagnostics don't always refresh on edits.

[AndreasPK]

Your environment

Windows/VSCode/ghc-9.10 currently but I think I've seen this issue with ghc-9.12 too.

Steps to reproduce

The Problem

Sometimes HLS doesn't refresh the diagnostics after the file changed.

I think this is some race condition between updating diagnostics and saving the file.
That is a rapid succession of edit->save events makes this happen.

[crtschin]

This is unfortunately quite easy to repro. The following test case fails pretty quickly running on repeat. It rapidly changes _ for different values generating diagnostics, but settling on 0 which should clear them:

foo :: Int
foo = _

Cause could be in ghcide or deeper in hls-graph.

patch containing a flaky repro

diff --git a/ghcide-test/exe/DiagnosticTests.hs b/ghcide-test/exe/DiagnosticTests.hs
index 99f2191f3..7a2e99bb8 100644
--- a/ghcide-test/exe/DiagnosticTests.hs
+++ b/ghcide-test/exe/DiagnosticTests.hs
@@ -15,7 +15,8 @@ import           Development.IDE.Test            (diagnostic,
                                                   expectDiagnostics,
                                                   expectDiagnosticsWithTags,
                                                   expectNoMoreDiagnostics,
-                                                  flushMessages, waitForAction)
+                                                  flushMessages, waitForAction,
+                                                  waitForBuildQueue)
 import           Development.IDE.Types.Location
 import qualified Language.LSP.Protocol.Lens      as L
 import           Language.LSP.Protocol.Message
@@ -39,6 +40,7 @@ import           Test.Hls                        (TestConfig (testConfigCaps, te
                                                   waitForProgressBegin)
 import           Test.Hls.FileSystem
 import           Test.Tasty
+import           Test.Tasty.ExpectedFailure      (ignoreTestBecause)
 import           Test.Tasty.HUnit
 
 tests :: TestTree
@@ -54,6 +56,21 @@ tests = testGroup "diagnostics"
               }
       changeDoc doc [change]
       expectDiagnostics [("Testing.hs", [])]
+  , testWithDummyPluginEmpty "rapid edits then save does not strand a stale diagnostic" $ do
+      let v rhs = T.unlines ["module Testing where", "foo :: Int", "foo = " <> rhs]
+          whole rhs = TextDocumentContentChangeEvent . InR . TextDocumentContentChangeWholeDocument $ v rhs
+      doc <- createDoc "Testing.hs" "haskell" (v "()")
+      expectDiagnostics [("Testing.hs", [(DiagnosticSeverity_Error, (2, 6), "Couldn't match expected type 'Int' with actual type '()'", Just "GHC-83865")])]
+      changeDoc doc [whole "()"]
+      changeDoc doc [whole "'a'"]
+      changeDoc doc [whole "True"]
+      changeDoc doc [whole "0"]
+      sendNotification SMethod_TextDocumentDidSave (DidSaveTextDocumentParams doc Nothing)
+      waitForBuildQueue
+      liftIO $ sleep 0.2
+      flushMessages
+      diags <- getCurrentDiagnostics doc
+      liftIO $ assertBool ("stale diagnostic survived: " ++ show diags) (null diags)
   , testWithDummyPluginEmpty "introduce syntax error" $ do
       let content = T.unlines [ "module Testing where" ]
       doc <- createDoc "Testing.hs" "haskell" content