llbsolver: take copy of history record to avoid marshal race

For active builds these records get stored in the memory
and marshalled on user request. This leads to possibility that
the original record may be updated while the marshalling is
happening, leading to a crash.

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

solver/llbsolver/history.go

@@ -680,6 +680,8 @@ func (h *HistoryQueue) Update(ctx context.Context, e *controlapi.BuildHistoryEve
 	h.mu.Lock()
 	defer h.mu.Unlock()
 
+	e = e.CloneVT()
+
 	if e.Type == controlapi.BuildHistoryEventType_STARTED {
 		h.active[e.Record.Ref] = e.Record
 		h.ps.Send(e)