MapCache: Invalidating a key doesn't invalidate outstanding requests

[creisman]

Repro:

map.get('test', ifAbsent: (k) => longLoad());
map.invalidate('test');
// longLoad() completes
expect(await map.get('test'), isNull);  // FAILS

Things to consider:
Should the original call to get() that passed ifAbsent return the value or null?

[cbracken]

Should the original call to get() that passed ifAbsent return the value or null?

The two most obvious candidates would be the value or to complete with a Future.error. The latter doesn't offer particularly good ergonomics though. Given we don't have a mechanism to kill an async task in flight, I suspect the value is the right answer here.

Thanks for including repro steps.

[caiboyang-google]

Have we considered the option of only removing the outstanding value without failing the existing thread?

If I have two threads, the first thread is obtaining the value from the RPC, and the second thread wants to invalidate all the cached values and reload the value from the current timestamp. The second thread cannot accomplish this by simply invalidating the cache.