attempt to fix #4640

lib/system/excpt.nim

@@ -11,7 +11,8 @@
 # use the heap (and nor exceptions) do not include the GC or memory allocator.
 
 var
-  errorMessageWriter*: (proc(msg: string) {.tags: [WriteIOEffect], benign.})
+  errorMessageWriter*: (proc(msg: string) {.tags: [WriteIOEffect], benign,
+                                            nimcall.})
     ## Function that will be called
     ## instead of stdmsg.write when printing stacktrace.
     ## Unstable API.
@@ -26,7 +27,7 @@ else:
   proc writeToStdErr(msg: cstring) =
     discard MessageBoxA(0, msg, nil, 0)
 
-proc showErrorMessage(data: cstring) =
+proc showErrorMessage(data: cstring) {.gcsafe.} =
   if errorMessageWriter != nil:
     errorMessageWriter($data)
   else:

lib/system/threads.nim

@@ -331,7 +331,9 @@ else:
     when TArg is void:
       thrd.dataFn()
     else:
-      thrd.dataFn(thrd.data)
+      var x: TArg
+      deepCopy(x, thrd.data)
+      thrd.dataFn(x)
 
 proc threadProcWrapStackFrame[TArg](thrd: ptr Thread[TArg]) =
   when defined(boehmgc):