Have you ever seen anything on what Hitler’s plans were if the Russians hadn’t agreed to the pact? Would he have invaded Poland anyway even if the Russian didn’t agree he could?
More important, had the Soviets not entered the war against Poland, could Poland have held out long enough in the east, to allow for Britain and France to come to her aid?
Hitler ordered the invasion of Poland months before talks even began with Stalin. And Stalin was so disgusted by British and French appeasement of Hitler, he wouldn't trust them for anything.
Remember that Stalin's intelligence services were excellent, and he knew pretty much everything Hitler was up to. So, what Stalin gained by his 1939 nonaggression pact was at least the following:
Point is: Stalin was just as eager for the agreement as Hitler. And the western allies had nothing of any interest to offer Stalin -- only bargaining chips in his negotiations with Hitler.
Here's the bottom line: had there been no 1939 Stalin-Hitler nonaggression pact, the Germans would have started their invasion of the Soviet Union in 1941 that much closer to Moscow, with little or no resistance in the Baltic countries. The result would have been Hitler's victory over Stalin in 1941, and most probable victory in the war.
So Stalin was the ultimate survivor. But he paid a huge, huge price in other people's blood.