only allow one GetProjectOptionsFromScript at a time

ionide/ionide-vscode-fsharp#2005

src/FsAutoComplete.Core/CompilerServiceInterface.fs

@@ -12,6 +12,7 @@ open FSharp.Compiler.Symbols
 open Microsoft.Extensions.Caching.Memory
 open System
 open FsToolkit.ErrorHandling
+open System.Threading
 
 
 
@@ -34,6 +35,10 @@ type FSharpCompilerServiceChecker(hasAnalyzers, typecheckCacheSize, parallelRefe
 
   let entityCache = EntityCache()
 
+  // FCS can't seem to handle parallel project restores for script files
+  // https://github.com/ionide/ionide-vscode-fsharp/issues/2005
+  let scriptLocker = new SemaphoreSlim(1, 1)
+
   // This is used to hold previous check results for autocompletion.
   // We can't seem to rely on the checker for previous cached versions
   let memoryCache () =
@@ -211,9 +216,16 @@ type FSharpCompilerServiceChecker(hasAnalyzers, typecheckCacheSize, parallelRefe
     }
 
   member self.GetProjectOptionsFromScript(file: string<LocalPath>, source, tfm) =
-    match tfm with
-    | FSIRefs.TFM.NetFx -> self.GetNetFxScriptOptions(file, source)
-    | FSIRefs.TFM.NetCore -> self.GetNetCoreScriptOptions(file, source)
+    async {
+      try
+        do! scriptLocker.WaitAsync() |> Async.AwaitTask
+
+        match tfm with
+        | FSIRefs.TFM.NetFx -> return! self.GetNetFxScriptOptions(file, source)
+        | FSIRefs.TFM.NetCore -> return! self.GetNetCoreScriptOptions(file, source)
+      finally
+        scriptLocker.Release() |> ignore<int>
+    }
 
 
   member __.ScriptTypecheckRequirementsChanged =