race condition: access to lastStamp outside of lock

lastStamp was assigned to stamp outside the lock, which could have meant
that stamp was assigned the same value if multiple threads were creating
Var objects simultaneously. Moved it into a function to ensure access
was limited to Interlocked function.

Author: manofstick

src/fsharp/FSharp.Core/quotations.fs

@@ -91,14 +91,16 @@ open Helpers
 [<CompiledName("FSharpVar")>]
 [<System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage","CA2218:OverrideGetHashCodeOnOverridingEquals",Justification="Equals override does not equate further objects, so default GetHashCode is still valid")>]
 type Var(name: string, typ:Type, ?isMutable: bool) =
-
     inherit obj()
-    static let mutable lastStamp = 0L
+
+    static let getStamp =
+        let mutable lastStamp = -1 // first value retrieved will be 0
+        fun () -> System.Threading.Interlocked.Increment &lastStamp
+
     static let globals = new Dictionary<(string*Type),Var>(11)
 
-    let stamp = lastStamp    
+    let stamp = getStamp ()
     let isMutable = defaultArg isMutable false
-    do lock globals (fun () -> lastStamp <- lastStamp + 1L)
 
     member v.Name = name
     member v.IsMutable = isMutable